# Django 2.2.9.dev20191202104323 文档

# 多对一关联

要定义一个多对一关联，使用 `ForeignKey`:

    from django.db import models

    class Reporter(models.Model):
        first_name = models.CharField(max_length=30)
        last_name = models.CharField(max_length=30)
        email = models.EmailField()

        def __str__(self):
            return "%s %s" % (self.first_name, self.last_name)

    class Article(models.Model):
        headline = models.CharField(max_length=100)
        pub_date = models.DateField()
        reporter = models.ForeignKey(Reporter, on_delete=models.CASCADE)

        def __str__(self):
            return self.headline

        class Meta:
            ordering = ['headline']

下面是可以使用PythonAPI工具执行的操作示例。

创建一些 Reporters：

    >>> r = Reporter(first_name='John', last_name='Smith', email='john@example.com')
    >>> r.save()

    >>> r2 = Reporter(first_name='Paul', last_name='Jones', email='paul@example.com')
    >>> r2.save()

创建一个 Article：

    >>> from datetime import date
    >>> a = Article(id=None, headline="This is a test", pub_date=date(2005, 7, 27), reporter=r)
    >>> a.save()

    >>> a.reporter.id
    1

    >>> a.reporter
    <Reporter: John Smith>

注意你必须先保存对象，然后再给它指定外键关系。比如，使用未保存的 `Reporter` 创建 `Article` ，会引发 `ValueError`：

    >>> r3 = Reporter(first_name='John', last_name='Smith', email='john@example.com')
    >>> Article.objects.create(headline="This is a test", pub_date=date(2005, 7, 27), reporter=r3)
    Traceback (most recent call last):
    ...
    ValueError: save() prohibited to prevent data loss due to unsaved related object 'reporter'.

Article 对象可以访问与它们相关联的 Reporter 对象：

    >>> r = a.reporter

通过 Reporter 对象来创建一个 Article

    >>> new_article = r.article_set.create(headline="John's second story", pub_date=date(2005, 7, 29))
    >>> new_article
    <Article: John's second story>
    >>> new_article.reporter
    <Reporter: John Smith>
    >>> new_article.reporter.id
    1

创建一个新的article：

    >>> new_article2 = Article.objects.create(headline="Paul's story", pub_date=date(2006, 1, 17), reporter=r)
    >>> new_article2.reporter
    <Reporter: John Smith>
    >>> new_article2.reporter.id
    1
    >>> r.article_set.all()
    <QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>

添加相同的article到一个不同的article集合，请观察它怎么移动：

    >>> r2.article_set.add(new_article2)
    >>> new_article2.reporter.id
    2
    >>> new_article2.reporter
    <Reporter: Paul Jones>

添加错误类型的对象，会引发TypeError：

    >>> r.article_set.add(r2)
    Traceback (most recent call last):
    ...
    TypeError: 'Article' instance expected, got <Reporter: Paul Jones>

    >>> r.article_set.all()
    <QuerySet [<Article: John's second story>, <Article: This is a test>]>
    >>> r2.article_set.all()
    <QuerySet [<Article: Paul's story>]>

    >>> r.article_set.count()
    2

    >>> r2.article_set.count()
    1

注意在最后的例子里，那篇 article 的 reporter 已经从 John 变为 Paul。

相关管理器也提供字段查询。只要你需要，API会自动跟踪关系，使用双下划线来分隔，你可以根据需要获取深层关系。这没有限制。比如：

    >>> r.article_set.filter(headline__startswith='This')
    <QuerySet [<Article: This is a test>]>

    # Find all Articles for any Reporter whose first name is "John".
    >>> Article.objects.filter(reporter__first_name='John')
    <QuerySet [<Article: John's second story>, <Article: This is a test>]>

这里的查询是完全匹配：

    >>> Article.objects.filter(reporter__first_name='John')
    <QuerySet [<Article: John's second story>, <Article: This is a test>]>

在相关字段上查询两次。这里转化成WHERE从句里的AND条件。

    >>> Article.objects.filter(reporter__first_name='John', reporter__last_name='Smith')
    <QuerySet [<Article: John's second story>, <Article: This is a test>]>

对于相关查询，你可以提供主键值或显式传递相关对象：

    >>> Article.objects.filter(reporter__pk=1)
    <QuerySet [<Article: John's second story>, <Article: This is a test>]>
    >>> Article.objects.filter(reporter=1)
    <QuerySet [<Article: John's second story>, <Article: This is a test>]>
    >>> Article.objects.filter(reporter=r)
    <QuerySet [<Article: John's second story>, <Article: This is a test>]>

    >>> Article.objects.filter(reporter__in=[1,2]).distinct()
    <QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>
    >>> Article.objects.filter(reporter__in=[r,r2]).distinct()
    <QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>

你也可以使用查询集而不是实例的文字列表：

    >>> Article.objects.filter(reporter__in=Reporter.objects.filter(first_name='John')).distinct()
    <QuerySet [<Article: John's second story>, <Article: This is a test>]>

反向查询：

    >>> Reporter.objects.filter(article__pk=1)
    <QuerySet [<Reporter: John Smith>]>
    >>> Reporter.objects.filter(article=1)
    <QuerySet [<Reporter: John Smith>]>
    >>> Reporter.objects.filter(article=a)
    <QuerySet [<Reporter: John Smith>]>

    >>> Reporter.objects.filter(article__headline__startswith='This')
    <QuerySet [<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]>
    >>> Reporter.objects.filter(article__headline__startswith='This').distinct()
    <QuerySet [<Reporter: John Smith>]>

反向查询的计数与 distinct() ：

    >>> Reporter.objects.filter(article__headline__startswith='This').count()
    3
    >>> Reporter.objects.filter(article__headline__startswith='This').distinct().count()
    1

可以循环查询：

    >>> Reporter.objects.filter(article__reporter__first_name__startswith='John')
    <QuerySet [<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]>
    >>> Reporter.objects.filter(article__reporter__first_name__startswith='John').distinct()
    <QuerySet [<Reporter: John Smith>]>
    >>> Reporter.objects.filter(article__reporter=r).distinct()
    <QuerySet [<Reporter: John Smith>]>

如果删除了一个reporter，他的articles将被删除（假设使用设置了 `CASCADE` 的 `django.db.models.ForeignKey.on_delete` 来定义主键，这是默认设置）：

    >>> Article.objects.all()
    <QuerySet [<Article: John's second story>, <Article: Paul's story>, <Article: This is a test>]>
    >>> Reporter.objects.order_by('first_name')
    <QuerySet [<Reporter: John Smith>, <Reporter: Paul Jones>]>
    >>> r2.delete()
    >>> Article.objects.all()
    <QuerySet [<Article: John's second story>, <Article: This is a test>]>
    >>> Reporter.objects.order_by('first_name')
    <QuerySet [<Reporter: John Smith>]>

可以在查询中使用JOIN进行删除：

    >>> Reporter.objects.filter(article__headline__startswith='This').delete()
    >>> Reporter.objects.all()
    <QuerySet []>
    >>> Article.objects.all()
    <QuerySet []>