# Django 2.2.9.dev20191202104323 文档

# django.db.models.expressions 源代码

    import copy
    import datetime
    import inspect
    from decimal import Decimal

    from django.core.exceptions import EmptyResultSet, FieldError
    from django.db import connection
    from django.db.models import fields
    from django.db.models.query_utils import Q
    from django.utils.deconstruct import deconstructible
    from django.utils.functional import cached_property
    from django.utils.hashable import make_hashable

    class SQLiteNumericMixin:
        """
        Some expressions with output_field=DecimalField() must be cast to
        numeric to be properly filtered.
        """
        def as_sqlite(self, compiler, connection, **extra_context):
            sql, params = self.as_sql(compiler, connection, **extra_context)
            try:
                if self.output_field.get_internal_type() == 'DecimalField':
                    sql = 'CAST(%s AS NUMERIC)' % sql
            except FieldError:
                pass
            return sql, params

    class Combinable:
        """
        Provide the ability to combine one or two objects with
        some connector. For example F('foo') + F('bar').
        """

        # Arithmetic connectors
        ADD = '+'
        SUB = '-'
        MUL = '*'
        DIV = '/'
        POW = '^'
        # The following is a quoted % operator - it is quoted because it can be
        # used in strings that also have parameter substitution.
        MOD = '%%'

        # Bitwise operators - note that these are generated by .bitand()
        # and .bitor(), the '&' and '|' are reserved for boolean operator
        # usage.
        BITAND = '&'
        BITOR = '|'
        BITLEFTSHIFT = '<<'
        BITRIGHTSHIFT = '>>'

        def _combine(self, other, connector, reversed):
            if not hasattr(other, 'resolve_expression'):
                # everything must be resolvable to an expression
                if isinstance(other, datetime.timedelta):
                    other = DurationValue(other, output_field=fields.DurationField())
                else:
                    other = Value(other)

            if reversed:
                return CombinedExpression(other, connector, self)
            return CombinedExpression(self, connector, other)

        #############
        # OPERATORS #
        #############

        def __neg__(self):
            return self._combine(-1, self.MUL, False)

        def __add__(self, other):
            return self._combine(other, self.ADD, False)

        def __sub__(self, other):
            return self._combine(other, self.SUB, False)

        def __mul__(self, other):
            return self._combine(other, self.MUL, False)

        def __truediv__(self, other):
            return self._combine(other, self.DIV, False)

        def __mod__(self, other):
            return self._combine(other, self.MOD, False)

        def __pow__(self, other):
            return self._combine(other, self.POW, False)

        def __and__(self, other):
            raise NotImplementedError(
                "Use .bitand() and .bitor() for bitwise logical operations."
            )

        def bitand(self, other):
            return self._combine(other, self.BITAND, False)

        def bitleftshift(self, other):
            return self._combine(other, self.BITLEFTSHIFT, False)

        def bitrightshift(self, other):
            return self._combine(other, self.BITRIGHTSHIFT, False)

        def __or__(self, other):
            raise NotImplementedError(
                "Use .bitand() and .bitor() for bitwise logical operations."
            )

        def bitor(self, other):
            return self._combine(other, self.BITOR, False)

        def __radd__(self, other):
            return self._combine(other, self.ADD, True)

        def __rsub__(self, other):
            return self._combine(other, self.SUB, True)

        def __rmul__(self, other):
            return self._combine(other, self.MUL, True)

        def __rtruediv__(self, other):
            return self._combine(other, self.DIV, True)

        def __rmod__(self, other):
            return self._combine(other, self.MOD, True)

        def __rpow__(self, other):
            return self._combine(other, self.POW, True)

        def __rand__(self, other):
            raise NotImplementedError(
                "Use .bitand() and .bitor() for bitwise logical operations."
            )

        def __ror__(self, other):
            raise NotImplementedError(
                "Use .bitand() and .bitor() for bitwise logical operations."
            )

    @deconstructible
    class BaseExpression:
        """Base class for all query expressions."""

        # aggregate specific fields
        is_summary = False
        _output_field_resolved_to_none = False
        # Can the expression be used in a WHERE clause?
        filterable = True
        # Can the expression can be used as a source expression in Window?
        window_compatible = False

        def __init__(self, output_field=None):
            if output_field is not None:
                self.output_field = output_field

        def __getstate__(self):
            state = self.__dict__.copy()
            state.pop('convert_value', None)
            return state

        def get_db_converters(self, connection):
            return (
                []
                if self.convert_value is self._convert_value_noop else
                [self.convert_value]
            ) + self.output_field.get_db_converters(connection)

        def get_source_expressions(self):
            return []

        def set_source_expressions(self, exprs):
            assert not exprs

        def _parse_expressions(self, *expressions):
            return [
                arg if hasattr(arg, 'resolve_expression') else (
                    F(arg) if isinstance(arg, str) else Value(arg)
                ) for arg in expressions
            ]

        def as_sql(self, compiler, connection):
            """
            Responsible for returning a (sql, [params]) tuple to be included
            in the current query.

            Different backends can provide their own implementation, by
            providing an `as_{vendor}` method and patching the Expression:

            ```
            def override_as_sql(self, compiler, connection):
                # custom logic
                return super().as_sql(compiler, connection)
            setattr(Expression, 'as_' + connection.vendor, override_as_sql)
            ```

            Arguments:
             * compiler: the query compiler responsible for generating the query.
               Must have a compile method, returning a (sql, [params]) tuple.
               Calling compiler(value) will return a quoted `value`.

             * connection: the database connection used for the current query.

            Return: (sql, params)
              Where `sql` is a string containing ordered sql parameters to be
              replaced with the elements of the list `params`.
            """
            raise NotImplementedError("Subclasses must implement as_sql()")

        @cached_property
        def contains_aggregate(self):
            return any(expr and expr.contains_aggregate for expr in self.get_source_expressions())

        @cached_property
        def contains_over_clause(self):
            return any(expr and expr.contains_over_clause for expr in self.get_source_expressions())

        @cached_property
        def contains_column_references(self):
            return any(expr and expr.contains_column_references for expr in self.get_source_expressions())

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            """
            Provide the chance to do any preprocessing or validation before being
            added to the query.

            Arguments:
             * query: the backend query implementation
             * allow_joins: boolean allowing or denying use of joins
               in this query
             * reuse: a set of reusable joins for multijoins
             * summarize: a terminal aggregate clause
             * for_save: whether this expression about to be used in a save or update

            Return: an Expression to be added to the query.
            """
            c = self.copy()
            c.is_summary = summarize
            c.set_source_expressions([
                expr.resolve_expression(query, allow_joins, reuse, summarize)
                if expr else None
                for expr in c.get_source_expressions()
            ])
            return c

        def _prepare(self, field):
            """Hook used by Lookup.get_prep_lookup() to do custom preparation."""
            return self

        @property
        def field(self):
            return self.output_field

        @cached_property
        def output_field(self):
            """Return the output type of this expressions."""
            output_field = self._resolve_output_field()
            if output_field is None:
                self._output_field_resolved_to_none = True
                raise FieldError('Cannot resolve expression type, unknown output_field')
            return output_field

        @cached_property
        def _output_field_or_none(self):
            """
            Return the output field of this expression, or None if
            _resolve_output_field() didn't return an output type.
            """
            try:
                return self.output_field
            except FieldError:
                if not self._output_field_resolved_to_none:
                    raise

        def _resolve_output_field(self):
            """
            Attempt to infer the output type of the expression. If the output
            fields of all source fields match then, simply infer the same type
            here. This isn't always correct, but it makes sense most of the time.

            Consider the difference between `2 + 2` and `2 / 3`. Inferring
            the type here is a convenience for the common case. The user should
            supply their own output_field with more complex computations.

            If a source's output field resolves to None, exclude it from this check.
            If all sources are None, then an error is raised higher up the stack in
            the output_field property.
            """
            sources_iter = (source for source in self.get_source_fields() if source is not None)
            for output_field in sources_iter:
                if any(not isinstance(output_field, source.__class__) for source in sources_iter):
                    raise FieldError('Expression contains mixed types. You must set output_field.')
                return output_field

        @staticmethod
        def _convert_value_noop(value, expression, connection):
            return value

        @cached_property
        def convert_value(self):
            """
            Expressions provide their own converters because users have the option
            of manually specifying the output_field which may be a different type
            from the one the database returns.
            """
            field = self.output_field
            internal_type = field.get_internal_type()
            if internal_type == 'FloatField':
                return lambda value, expression, connection: None if value is None else float(value)
            elif internal_type.endswith('IntegerField'):
                return lambda value, expression, connection: None if value is None else int(value)
            elif internal_type == 'DecimalField':
                return lambda value, expression, connection: None if value is None else Decimal(value)
            return self._convert_value_noop

        def get_lookup(self, lookup):
            return self.output_field.get_lookup(lookup)

        def get_transform(self, name):
            return self.output_field.get_transform(name)

        def relabeled_clone(self, change_map):
            clone = self.copy()
            clone.set_source_expressions([
                e.relabeled_clone(change_map) if e is not None else None
                for e in self.get_source_expressions()
            ])
            return clone

        def copy(self):
            return copy.copy(self)

        def get_group_by_cols(self):
            if not self.contains_aggregate:
                return [self]
            cols = []
            for source in self.get_source_expressions():
                cols.extend(source.get_group_by_cols())
            return cols

        def get_source_fields(self):
            """Return the underlying field types used by this aggregate."""
            return [e._output_field_or_none for e in self.get_source_expressions()]

        def asc(self, **kwargs):
            return OrderBy(self, **kwargs)

        def desc(self, **kwargs):
            return OrderBy(self, descending=True, **kwargs)

        def reverse_ordering(self):
            return self

        def flatten(self):
            """
            Recursively yield this expression and all subexpressions, in
            depth-first order.
            """
            yield self
            for expr in self.get_source_expressions():
                if expr:
                    yield from expr.flatten()

        @cached_property
        def identity(self):
            constructor_signature = inspect.signature(self.__init__)
            args, kwargs = self._constructor_args
            signature = constructor_signature.bind_partial(*args, **kwargs)
            signature.apply_defaults()
            arguments = signature.arguments.items()
            identity = [self.__class__]
            for arg, value in arguments:
                if isinstance(value, fields.Field):
                    if value.name and value.model:
                        value = (value.model._meta.label, value.name)
                    else:
                        value = type(value)
                else:
                    value = make_hashable(value)
                identity.append((arg, value))
            return tuple(identity)

        def __eq__(self, other):
            return isinstance(other, BaseExpression) and other.identity == self.identity

        def __hash__(self):
            return hash(self.identity)

    class Expression(BaseExpression, Combinable):
        """An expression that can be combined with other expressions."""
        pass

    class CombinedExpression(SQLiteNumericMixin, Expression):

        def __init__(self, lhs, connector, rhs, output_field=None):
            super().__init__(output_field=output_field)
            self.connector = connector
            self.lhs = lhs
            self.rhs = rhs

        def __repr__(self):
            return "<{}: {}>".format(self.__class__.__name__, self)

        def __str__(self):
            return "{} {} {}".format(self.lhs, self.connector, self.rhs)

        def get_source_expressions(self):
            return [self.lhs, self.rhs]

        def set_source_expressions(self, exprs):
            self.lhs, self.rhs = exprs

        def as_sql(self, compiler, connection):
            try:
                lhs_output = self.lhs.output_field
            except FieldError:
                lhs_output = None
            try:
                rhs_output = self.rhs.output_field
            except FieldError:
                rhs_output = None
            if (not connection.features.has_native_duration_field and
                    ((lhs_output and lhs_output.get_internal_type() == 'DurationField') or
                     (rhs_output and rhs_output.get_internal_type() == 'DurationField'))):
                return DurationExpression(self.lhs, self.connector, self.rhs).as_sql(compiler, connection)
            if (lhs_output and rhs_output and self.connector == self.SUB and
                lhs_output.get_internal_type() in {'DateField', 'DateTimeField', 'TimeField'} and
                    lhs_output.get_internal_type() == rhs_output.get_internal_type()):
                return TemporalSubtraction(self.lhs, self.rhs).as_sql(compiler, connection)
            expressions = []
            expression_params = []
            sql, params = compiler.compile(self.lhs)
            expressions.append(sql)
            expression_params.extend(params)
            sql, params = compiler.compile(self.rhs)
            expressions.append(sql)
            expression_params.extend(params)
            # order of precedence
            expression_wrapper = '(%s)'
            sql = connection.ops.combine_expression(self.connector, expressions)
            return expression_wrapper % sql, expression_params

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            c = self.copy()
            c.is_summary = summarize
            c.lhs = c.lhs.resolve_expression(query, allow_joins, reuse, summarize, for_save)
            c.rhs = c.rhs.resolve_expression(query, allow_joins, reuse, summarize, for_save)
            return c

    class DurationExpression(CombinedExpression):
        def compile(self, side, compiler, connection):
            if not isinstance(side, DurationValue):
                try:
                    output = side.output_field
                except FieldError:
                    pass
                else:
                    if output.get_internal_type() == 'DurationField':
                        sql, params = compiler.compile(side)
                        return connection.ops.format_for_duration_arithmetic(sql), params
            return compiler.compile(side)

        def as_sql(self, compiler, connection):
            connection.ops.check_expression_support(self)
            expressions = []
            expression_params = []
            sql, params = self.compile(self.lhs, compiler, connection)
            expressions.append(sql)
            expression_params.extend(params)
            sql, params = self.compile(self.rhs, compiler, connection)
            expressions.append(sql)
            expression_params.extend(params)
            # order of precedence
            expression_wrapper = '(%s)'
            sql = connection.ops.combine_duration_expression(self.connector, expressions)
            return expression_wrapper % sql, expression_params

    class TemporalSubtraction(CombinedExpression):
        output_field = fields.DurationField()

        def __init__(self, lhs, rhs):
            super().__init__(lhs, self.SUB, rhs)

        def as_sql(self, compiler, connection):
            connection.ops.check_expression_support(self)
            lhs = compiler.compile(self.lhs, connection)
            rhs = compiler.compile(self.rhs, connection)
            return connection.ops.subtract_temporals(self.lhs.output_field.get_internal_type(), lhs, rhs)

    @deconstructible
    class F(Combinable):
        """An object capable of resolving references to existing query objects."""
        # Can the expression be used in a WHERE clause?
        filterable = True

        def __init__(self, name):
            """
            Arguments:
             * name: the name of the field this expression references
            """
            self.name = name

        def __repr__(self):
            return "{}({})".format(self.__class__.__name__, self.name)

        def resolve_expression(self, query=None, allow_joins=True, reuse=None,
                               summarize=False, for_save=False, simple_col=False):
            return query.resolve_ref(self.name, allow_joins, reuse, summarize, simple_col)

        def asc(self, **kwargs):
            return OrderBy(self, **kwargs)

        def desc(self, **kwargs):
            return OrderBy(self, descending=True, **kwargs)

        def __eq__(self, other):
            return self.__class__ == other.__class__ and self.name == other.name

        def __hash__(self):
            return hash(self.name)

    class ResolvedOuterRef(F):
        """
        An object that contains a reference to an outer query.

        In this case, the reference to the outer query has been resolved because
        the inner query has been used as a subquery.
        """
        def as_sql(self, *args, **kwargs):
            raise ValueError(
                'This queryset contains a reference to an outer query and may '
                'only be used in a subquery.'
            )

        def _prepare(self, output_field=None):
            return self

        def relabeled_clone(self, relabels):
            return self

    class OuterRef(F):
        def resolve_expression(self, query=None, allow_joins=True, reuse=None,
                               summarize=False, for_save=False, simple_col=False):
            if isinstance(self.name, self.__class__):
                return self.name
            return ResolvedOuterRef(self.name)

        def _prepare(self, output_field=None):
            return self

    class Func(SQLiteNumericMixin, Expression):
        """An SQL function call."""
        function = None
        template = '%(function)s(%(expressions)s)'
        arg_joiner = ', '
        arity = None  # The number of arguments the function accepts.

        def __init__(self, *expressions, output_field=None, **extra):
            if self.arity is not None and len(expressions) != self.arity:
                raise TypeError(
                    "'%s' takes exactly %s %s (%s given)" % (
                        self.__class__.__name__,
                        self.arity,
                        "argument" if self.arity == 1 else "arguments",
                        len(expressions),
                    )
                )
            super().__init__(output_field=output_field)
            self.source_expressions = self._parse_expressions(*expressions)
            self.extra = extra

        def __repr__(self):
            args = self.arg_joiner.join(str(arg) for arg in self.source_expressions)
            extra = {**self.extra, **self._get_repr_options()}
            if extra:
                extra = ', '.join(str(key) + '=' + str(val) for key, val in sorted(extra.items()))
                return "{}({}, {})".format(self.__class__.__name__, args, extra)
            return "{}({})".format(self.__class__.__name__, args)

        def _get_repr_options(self):
            """Return a dict of extra __init__() options to include in the repr."""
            return {}

        def get_source_expressions(self):
            return self.source_expressions

        def set_source_expressions(self, exprs):
            self.source_expressions = exprs

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            c = self.copy()
            c.is_summary = summarize
            for pos, arg in enumerate(c.source_expressions):
                c.source_expressions[pos] = arg.resolve_expression(query, allow_joins, reuse, summarize, for_save)
            return c

        def as_sql(self, compiler, connection, function=None, template=None, arg_joiner=None, **extra_context):
            connection.ops.check_expression_support(self)
            sql_parts = []
            params = []
            for arg in self.source_expressions:
                arg_sql, arg_params = compiler.compile(arg)
                sql_parts.append(arg_sql)
                params.extend(arg_params)
            data = {**self.extra, **extra_context}
            # Use the first supplied value in this order: the parameter to this
            # method, a value supplied in __init__()'s **extra (the value in
            # `data`), or the value defined on the class.
            if function is not None:
                data['function'] = function
            else:
                data.setdefault('function', self.function)
            template = template or data.get('template', self.template)
            arg_joiner = arg_joiner or data.get('arg_joiner', self.arg_joiner)
            data['expressions'] = data['field'] = arg_joiner.join(sql_parts)
            return template % data, params

        def copy(self):
            copy = super().copy()
            copy.source_expressions = self.source_expressions[:]
            copy.extra = self.extra.copy()
            return copy

    class Value(Expression):
        """Represent a wrapped value as a node within an expression."""
        def __init__(self, value, output_field=None):
            """
            Arguments:
             * value: the value this expression represents. The value will be
               added into the sql parameter list and properly quoted.

             * output_field: an instance of the model field type that this
               expression will return, such as IntegerField() or CharField().
            """
            super().__init__(output_field=output_field)
            self.value = value

        def __repr__(self):
            return "{}({})".format(self.__class__.__name__, self.value)

        def as_sql(self, compiler, connection):
            connection.ops.check_expression_support(self)
            val = self.value
            output_field = self._output_field_or_none
            if output_field is not None:
                if self.for_save:
                    val = output_field.get_db_prep_save(val, connection=connection)
                else:
                    val = output_field.get_db_prep_value(val, connection=connection)
                if hasattr(output_field, 'get_placeholder'):
                    return output_field.get_placeholder(val, compiler, connection), [val]
            if val is None:
                # cx_Oracle does not always convert None to the appropriate
                # NULL type (like in case expressions using numbers), so we
                # use a literal SQL NULL
                return 'NULL', []
            return '%s', [val]

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            c = super().resolve_expression(query, allow_joins, reuse, summarize, for_save)
            c.for_save = for_save
            return c

        def get_group_by_cols(self):
            return []

    class DurationValue(Value):
        def as_sql(self, compiler, connection):
            connection.ops.check_expression_support(self)
            if connection.features.has_native_duration_field:
                return super().as_sql(compiler, connection)
            return connection.ops.date_interval_sql(self.value), []

    class RawSQL(Expression):
        def __init__(self, sql, params, output_field=None):
            if output_field is None:
                output_field = fields.Field()
            self.sql, self.params = sql, params
            super().__init__(output_field=output_field)

        def __repr__(self):
            return "{}({}, {})".format(self.__class__.__name__, self.sql, self.params)

        def as_sql(self, compiler, connection):
            return '(%s)' % self.sql, self.params

        def get_group_by_cols(self):
            return [self]

    class Star(Expression):
        def __repr__(self):
            return "'*'"

        def as_sql(self, compiler, connection):
            return '*', []

    class Random(Expression):
        output_field = fields.FloatField()

        def __repr__(self):
            return "Random()"

        def as_sql(self, compiler, connection):
            return connection.ops.random_function_sql(), []

    class Col(Expression):

        contains_column_references = True

        def __init__(self, alias, target, output_field=None):
            if output_field is None:
                output_field = target
            super().__init__(output_field=output_field)
            self.alias, self.target = alias, target

        def __repr__(self):
            return "{}({}, {})".format(
                self.__class__.__name__, self.alias, self.target)

        def as_sql(self, compiler, connection):
            qn = compiler.quote_name_unless_alias
            return "%s.%s" % (qn(self.alias), qn(self.target.column)), []

        def relabeled_clone(self, relabels):
            return self.__class__(relabels.get(self.alias, self.alias), self.target, self.output_field)

        def get_group_by_cols(self):
            return [self]

        def get_db_converters(self, connection):
            if self.target == self.output_field:
                return self.output_field.get_db_converters(connection)
            return (self.output_field.get_db_converters(connection) +
                    self.target.get_db_converters(connection))

    class SimpleCol(Expression):
        """
        Represents the SQL of a column name without the table name.

        This variant of Col doesn't include the table name (or an alias) to
        avoid a syntax error in check constraints.
        """
        contains_column_references = True

        def __init__(self, target, output_field=None):
            if output_field is None:
                output_field = target
            super().__init__(output_field=output_field)
            self.target = target

        def __repr__(self):
            return '{}({})'.format(self.__class__.__name__, self.target)

        def as_sql(self, compiler, connection):
            qn = compiler.quote_name_unless_alias
            return qn(self.target.column), []

        def get_group_by_cols(self):
            return [self]

        def get_db_converters(self, connection):
            if self.target == self.output_field:
                return self.output_field.get_db_converters(connection)
            return (
                self.output_field.get_db_converters(connection) +
                self.target.get_db_converters(connection)
            )

    class Ref(Expression):
        """
        Reference to column alias of the query. For example, Ref('sum_cost') in
        qs.annotate(sum_cost=Sum('cost')) query.
        """
        def __init__(self, refs, source):
            super().__init__()
            self.refs, self.source = refs, source

        def __repr__(self):
            return "{}({}, {})".format(self.__class__.__name__, self.refs, self.source)

        def get_source_expressions(self):
            return [self.source]

        def set_source_expressions(self, exprs):
            self.source, = exprs

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            # The sub-expression `source` has already been resolved, as this is
            # just a reference to the name of `source`.
            return self

        def relabeled_clone(self, relabels):
            return self

        def as_sql(self, compiler, connection):
            return connection.ops.quote_name(self.refs), []

        def get_group_by_cols(self):
            return [self]

    class ExpressionList(Func):
        """
        An expression containing multiple expressions. Can be used to provide a
        list of expressions as an argument to another expression, like an
        ordering clause.
        """
        template = '%(expressions)s'

        def __init__(self, *expressions, **extra):
            if not expressions:
                raise ValueError('%s requires at least one expression.' % self.__class__.__name__)
            super().__init__(*expressions, **extra)

        def __str__(self):
            return self.arg_joiner.join(str(arg) for arg in self.source_expressions)

    class ExpressionWrapper(Expression):
        """
        An expression that can wrap another expression so that it can provide
        extra context to the inner expression, such as the output_field.
        """

        def __init__(self, expression, output_field):
            super().__init__(output_field=output_field)
            self.expression = expression

        def set_source_expressions(self, exprs):
            self.expression = exprs[0]

        def get_source_expressions(self):
            return [self.expression]

        def as_sql(self, compiler, connection):
            return self.expression.as_sql(compiler, connection)

        def __repr__(self):
            return "{}({})".format(self.__class__.__name__, self.expression)

    class When(Expression):
        template = 'WHEN %(condition)s THEN %(result)s'

        def __init__(self, condition=None, then=None, **lookups):
            if lookups and condition is None:
                condition, lookups = Q(**lookups), None
            if condition is None or not getattr(condition, 'conditional', False) or lookups:
                raise TypeError("__init__() takes either a Q object or lookups as keyword arguments")
            if isinstance(condition, Q) and not condition:
                raise ValueError("An empty Q() can't be used as a When() condition.")
            super().__init__(output_field=None)
            self.condition = condition
            self.result = self._parse_expressions(then)[0]

        def __str__(self):
            return "WHEN %r THEN %r" % (self.condition, self.result)

        def __repr__(self):
            return "<%s: %s>" % (self.__class__.__name__, self)

        def get_source_expressions(self):
            return [self.condition, self.result]

        def set_source_expressions(self, exprs):
            self.condition, self.result = exprs

        def get_source_fields(self):
            # We're only interested in the fields of the result expressions.
            return [self.result._output_field_or_none]

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            c = self.copy()
            c.is_summary = summarize
            if hasattr(c.condition, 'resolve_expression'):
                c.condition = c.condition.resolve_expression(query, allow_joins, reuse, summarize, False)
            c.result = c.result.resolve_expression(query, allow_joins, reuse, summarize, for_save)
            return c

        def as_sql(self, compiler, connection, template=None, **extra_context):
            connection.ops.check_expression_support(self)
            template_params = extra_context
            sql_params = []
            condition_sql, condition_params = compiler.compile(self.condition)
            template_params['condition'] = condition_sql
            sql_params.extend(condition_params)
            result_sql, result_params = compiler.compile(self.result)
            template_params['result'] = result_sql
            sql_params.extend(result_params)
            template = template or self.template
            return template % template_params, sql_params

        def get_group_by_cols(self):
            # This is not a complete expression and cannot be used in GROUP BY.
            cols = []
            for source in self.get_source_expressions():
                cols.extend(source.get_group_by_cols())
            return cols

    class Case(Expression):
        """
        An SQL searched CASE expression:

            CASE
                WHEN n > 0
                    THEN 'positive'
                WHEN n < 0
                    THEN 'negative'
                ELSE 'zero'
            END
        """
        template = 'CASE %(cases)s ELSE %(default)s END'
        case_joiner = ' '

        def __init__(self, *cases, default=None, output_field=None, **extra):
            if not all(isinstance(case, When) for case in cases):
                raise TypeError("Positional arguments must all be When objects.")
            super().__init__(output_field)
            self.cases = list(cases)
            self.default = self._parse_expressions(default)[0]
            self.extra = extra

        def __str__(self):
            return "CASE %s, ELSE %r" % (', '.join(str(c) for c in self.cases), self.default)

        def __repr__(self):
            return "<%s: %s>" % (self.__class__.__name__, self)

        def get_source_expressions(self):
            return self.cases + [self.default]

        def set_source_expressions(self, exprs):
            *self.cases, self.default = exprs

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            c = self.copy()
            c.is_summary = summarize
            for pos, case in enumerate(c.cases):
                c.cases[pos] = case.resolve_expression(query, allow_joins, reuse, summarize, for_save)
            c.default = c.default.resolve_expression(query, allow_joins, reuse, summarize, for_save)
            return c

        def copy(self):
            c = super().copy()
            c.cases = c.cases[:]
            return c

        def as_sql(self, compiler, connection, template=None, case_joiner=None, **extra_context):
            connection.ops.check_expression_support(self)
            if not self.cases:
                return compiler.compile(self.default)
            template_params = {**self.extra, **extra_context}
            case_parts = []
            sql_params = []
            for case in self.cases:
                try:
                    case_sql, case_params = compiler.compile(case)
                except EmptyResultSet:
                    continue
                case_parts.append(case_sql)
                sql_params.extend(case_params)
            default_sql, default_params = compiler.compile(self.default)
            if not case_parts:
                return default_sql, default_params
            case_joiner = case_joiner or self.case_joiner
            template_params['cases'] = case_joiner.join(case_parts)
            template_params['default'] = default_sql
            sql_params.extend(default_params)
            template = template or template_params.get('template', self.template)
            sql = template % template_params
            if self._output_field_or_none is not None:
                sql = connection.ops.unification_cast_sql(self.output_field) % sql
            return sql, sql_params

    class Subquery(Expression):
        """
        An explicit subquery. It may contain OuterRef() references to the outer
        query which will be resolved when it is applied to that query.
        """
        template = '(%(subquery)s)'
        contains_aggregate = False

        def __init__(self, queryset, output_field=None, **extra):
            self.queryset = queryset
            self.extra = extra
            super().__init__(output_field)

        def _resolve_output_field(self):
            if len(self.queryset.query.select) == 1:
                return self.queryset.query.select[0].field
            return super()._resolve_output_field()

        def copy(self):
            clone = super().copy()
            clone.queryset = clone.queryset.all()
            return clone

        def resolve_expression(self, query=None, allow_joins=True, reuse=None, summarize=False, for_save=False):
            clone = self.copy()
            clone.is_summary = summarize
            clone.queryset.query.bump_prefix(query)

            # Need to recursively resolve these.
            def resolve_all(child):
                if hasattr(child, 'children'):
                    [resolve_all(_child) for _child in child.children]
                if hasattr(child, 'rhs'):
                    child.rhs = resolve(child.rhs)

            def resolve(child):
                if hasattr(child, 'resolve_expression'):
                    resolved = child.resolve_expression(
                        query=query, allow_joins=allow_joins, reuse=reuse,
                        summarize=summarize, for_save=for_save,
                    )
                    # Add table alias to the parent query's aliases to prevent
                    # quoting.
                    if hasattr(resolved, 'alias') and resolved.alias != resolved.target.model._meta.db_table:
                        clone.queryset.query.external_aliases.add(resolved.alias)
                    return resolved
                return child

            resolve_all(clone.queryset.query.where)

            for key, value in clone.queryset.query.annotations.items():
                if isinstance(value, Subquery):
                    clone.queryset.query.annotations[key] = resolve(value)

            return clone

        def get_source_expressions(self):
            return [
                x for x in [
                    getattr(expr, 'lhs', None)
                    for expr in self.queryset.query.where.children
                ] if x
            ]

        def relabeled_clone(self, change_map):
            clone = self.copy()
            clone.queryset.query = clone.queryset.query.relabeled_clone(change_map)
            clone.queryset.query.external_aliases.update(
                alias for alias in change_map.values()
                if alias not in clone.queryset.query.alias_map
            )
            return clone

        def as_sql(self, compiler, connection, template=None, **extra_context):
            connection.ops.check_expression_support(self)
            template_params = {**self.extra, **extra_context}
            template_params['subquery'], sql_params = self.queryset.query.get_compiler(connection=connection).as_sql()

            template = template or template_params.get('template', self.template)
            sql = template % template_params
            return sql, sql_params

        def _prepare(self, output_field):
            # This method will only be called if this instance is the "rhs" in an
            # expression: the wrapping () must be removed (as the expression that
            # contains this will provide them). SQLite evaluates ((subquery))
            # differently than the other databases.
            if self.template == '(%(subquery)s)':
                clone = self.copy()
                clone.template = '%(subquery)s'
                return clone
            return self

    class Exists(Subquery):
        template = 'EXISTS(%(subquery)s)'
        output_field = fields.BooleanField()

        def __init__(self, *args, negated=False, **kwargs):
            self.negated = negated
            super().__init__(*args, **kwargs)

        def __invert__(self):
            return type(self)(self.queryset, negated=(not self.negated), **self.extra)

        def resolve_expression(self, query=None, *args, **kwargs):
            # As a performance optimization, remove ordering since EXISTS doesn't
            # care about it, just whether or not a row matches.
            self.queryset = self.queryset.order_by()
            return super().resolve_expression(query, *args, **kwargs)

        def as_sql(self, compiler, connection, template=None, **extra_context):
            sql, params = super().as_sql(compiler, connection, template, **extra_context)
            if self.negated:
                sql = 'NOT {}'.format(sql)
            return sql, params

        def as_oracle(self, compiler, connection, template=None, **extra_context):
            # Oracle doesn't allow EXISTS() in the SELECT list, so wrap it with a
            # CASE WHEN expression. Change the template since the When expression
            # requires a left hand side (column) to compare against.
            sql, params = self.as_sql(compiler, connection, template, **extra_context)
            sql = 'CASE WHEN {} THEN 1 ELSE 0 END'.format(sql)
            return sql, params

    class OrderBy(BaseExpression):
        template = '%(expression)s %(ordering)s'

        def __init__(self, expression, descending=False, nulls_first=False, nulls_last=False):
            if nulls_first and nulls_last:
                raise ValueError('nulls_first and nulls_last are mutually exclusive')
            self.nulls_first = nulls_first
            self.nulls_last = nulls_last
            self.descending = descending
            if not hasattr(expression, 'resolve_expression'):
                raise ValueError('expression must be an expression type')
            self.expression = expression

        def __repr__(self):
            return "{}({}, descending={})".format(
                self.__class__.__name__, self.expression, self.descending)

        def set_source_expressions(self, exprs):
            self.expression = exprs[0]

        def get_source_expressions(self):
            return [self.expression]

        def as_sql(self, compiler, connection, template=None, **extra_context):
            if not template:
                if self.nulls_last:
                    template = '%s NULLS LAST' % self.template
                elif self.nulls_first:
                    template = '%s NULLS FIRST' % self.template
            connection.ops.check_expression_support(self)
            expression_sql, params = compiler.compile(self.expression)
            placeholders = {
                'expression': expression_sql,
                'ordering': 'DESC' if self.descending else 'ASC',
                **extra_context,
            }
            template = template or self.template
            params *= template.count('%(expression)s')
            return (template % placeholders).rstrip(), params

        def as_sqlite(self, compiler, connection):
            template = None
            if self.nulls_last:
                template = '%(expression)s IS NULL, %(expression)s %(ordering)s'
            elif self.nulls_first:
                template = '%(expression)s IS NOT NULL, %(expression)s %(ordering)s'
            return self.as_sql(compiler, connection, template=template)

        def as_mysql(self, compiler, connection):
            template = None
            if self.nulls_last:
                template = 'IF(ISNULL(%(expression)s),1,0), %(expression)s %(ordering)s '
            elif self.nulls_first:
                template = 'IF(ISNULL(%(expression)s),0,1), %(expression)s %(ordering)s '
            return self.as_sql(compiler, connection, template=template)

        def get_group_by_cols(self):
            cols = []
            for source in self.get_source_expressions():
                cols.extend(source.get_group_by_cols())
            return cols

        def reverse_ordering(self):
            self.descending = not self.descending
            if self.nulls_first or self.nulls_last:
                self.nulls_first = not self.nulls_first
                self.nulls_last = not self.nulls_last
            return self

        def asc(self):
            self.descending = False

        def desc(self):
            self.descending = True

    class Window(Expression):
        template = '%(expression)s OVER (%(window)s)'
        # Although the main expression may either be an aggregate or an
        # expression with an aggregate function, the GROUP BY that will
        # be introduced in the query as a result is not desired.
        contains_aggregate = False
        contains_over_clause = True
        filterable = False

        def __init__(self, expression, partition_by=None, order_by=None, frame=None, output_field=None):
            self.partition_by = partition_by
            self.order_by = order_by
            self.frame = frame

            if not getattr(expression, 'window_compatible', False):
                raise ValueError(
                    "Expression '%s' isn't compatible with OVER clauses." %
                    expression.__class__.__name__
                )

            if self.partition_by is not None:
                if not isinstance(self.partition_by, (tuple, list)):
                    self.partition_by = (self.partition_by,)
                self.partition_by = ExpressionList(*self.partition_by)

            if self.order_by is not None:
                if isinstance(self.order_by, (list, tuple)):
                    self.order_by = ExpressionList(*self.order_by)
                elif not isinstance(self.order_by, BaseExpression):
                    raise ValueError(
                        'order_by must be either an Expression or a sequence of '
                        'expressions.'
                    )
            super().__init__(output_field=output_field)
            self.source_expression = self._parse_expressions(expression)[0]

        def _resolve_output_field(self):
            return self.source_expression.output_field

        def get_source_expressions(self):
            return [self.source_expression, self.partition_by, self.order_by, self.frame]

        def set_source_expressions(self, exprs):
            self.source_expression, self.partition_by, self.order_by, self.frame = exprs

        def as_sql(self, compiler, connection, template=None):
            connection.ops.check_expression_support(self)
            expr_sql, params = compiler.compile(self.source_expression)
            window_sql, window_params = [], []

            if self.partition_by is not None:
                sql_expr, sql_params = self.partition_by.as_sql(
                    compiler=compiler, connection=connection,
                    template='PARTITION BY %(expressions)s',
                )
                window_sql.extend(sql_expr)
                window_params.extend(sql_params)

            if self.order_by is not None:
                window_sql.append(' ORDER BY ')
                order_sql, order_params = compiler.compile(self.order_by)
                window_sql.extend(''.join(order_sql))
                window_params.extend(order_params)

            if self.frame:
                frame_sql, frame_params = compiler.compile(self.frame)
                window_sql.extend(' ' + frame_sql)
                window_params.extend(frame_params)

            params.extend(window_params)
            template = template or self.template

            return template % {
                'expression': expr_sql,
                'window': ''.join(window_sql).strip()
            }, params

        def __str__(self):
            return '{} OVER ({}{}{})'.format(
                str(self.source_expression),
                'PARTITION BY ' + str(self.partition_by) if self.partition_by else '',
                'ORDER BY ' + str(self.order_by) if self.order_by else '',
                str(self.frame or ''),
            )

        def __repr__(self):
            return '<%s: %s>' % (self.__class__.__name__, self)

        def get_group_by_cols(self):
            return []

    class WindowFrame(Expression):
        """
        Model the frame clause in window expressions. There are two types of frame
        clauses which are subclasses, however, all processing and validation (by no
        means intended to be complete) is done here. Thus, providing an end for a
        frame is optional (the default is UNBOUNDED FOLLOWING, which is the last
        row in the frame).
        """
        template = '%(frame_type)s BETWEEN %(start)s AND %(end)s'

        def __init__(self, start=None, end=None):
            self.start = Value(start)
            self.end = Value(end)

        def set_source_expressions(self, exprs):
            self.start, self.end = exprs

        def get_source_expressions(self):
            return [self.start, self.end]

        def as_sql(self, compiler, connection):
            connection.ops.check_expression_support(self)
            start, end = self.window_frame_start_end(connection, self.start.value, self.end.value)
            return self.template % {
                'frame_type': self.frame_type,
                'start': start,
                'end': end,
            }, []

        def __repr__(self):
            return '<%s: %s>' % (self.__class__.__name__, self)

        def get_group_by_cols(self):
            return []

        def __str__(self):
            if self.start.value is not None and self.start.value < 0:
                start = '%d %s' % (abs(self.start.value), connection.ops.PRECEDING)
            elif self.start.value is not None and self.start.value == 0:
                start = connection.ops.CURRENT_ROW
            else:
                start = connection.ops.UNBOUNDED_PRECEDING

            if self.end.value is not None and self.end.value > 0:
                end = '%d %s' % (self.end.value, connection.ops.FOLLOWING)
            elif self.end.value is not None and self.end.value == 0:
                end = connection.ops.CURRENT_ROW
            else:
                end = connection.ops.UNBOUNDED_FOLLOWING
            return self.template % {
                'frame_type': self.frame_type,
                'start': start,
                'end': end,
            }

        def window_frame_start_end(self, connection, start, end):
            raise NotImplementedError('Subclasses must implement window_frame_start_end().')

    class RowRange(WindowFrame):
        frame_type = 'ROWS'

        def window_frame_start_end(self, connection, start, end):
            return connection.ops.window_frame_rows_start_end(start, end)

    class ValueRange(WindowFrame):
        frame_type = 'RANGE'

        def window_frame_start_end(self, connection, start, end):
            return connection.ops.window_frame_range_start_end(start, end)