django-ormex 0.2.1

GroupConcat

类似于Concat，但对于连接相关多对多模型的字段值。例如，如果您有一个Author模型作为Book模型中的多对多关系（Book.authors = ManyToManyField(Author)）并且希望有一个与给定书籍关联的所有作者的连接列表。

给定以下模型

class Publisher(models.Model):
    """Publisher."""

    name = models.CharField(max_length=255)
    address = models.CharField(max_length=255)
    city = models.CharField(max_length=255)
    state_province = models.CharField(max_length=255)
    country = models.CharField(max_length=255)
    website = models.URLField(max_length=255)


class Author(models.Model):
    """Author."""

    salutation = models.CharField(max_length=255)
    name = models.CharField(max_length=255)
    email = models.EmailField(max_length=255)
    headshot = models.ImageField(upload_to='authors', null=True, blank=True)


class Book(models.Model):
    """Book."""

    title = models.CharField(max_length=255)
    authors = models.ManyToManyField('books.Author', related_name='books')
    publisher = models.ForeignKey(Publisher, related_name='books')
    publication_date = models.DateField()
    isbn = models.CharField(max_length=255, unique=True)
    price = models.DecimalField(max_digits=10, decimal_places=2)
    pages = models.PositiveIntegerField(default=200)
    stock_count = models.PositiveIntegerField(default=30)

我们可以这样使用GroupConcat

from ormex.aggregations import GroupConcat

book = Book.objects.all() \
        .values('id',
                'title',
                'pages',
                'price',
                'publisher__id',
                'publisher__name') \
        .annotate(
            authors__name=GroupConcat('authors__name', separator=', ')
        ) \
        .first()

输出将如下所示

{
    'authors__name': 'Finn Janssen, Dan Dijkman, Merel Wolf, Evy de Jong',
    'id': 14,
    'pages': 83,
    'price': Decimal('62.13'),
    'publisher__id': 19,
    'publisher__name': 'Rijn, de Bruyn and Verharen',
    'title': 'Laboriosam officia temporibus facere omnis odit.'
}

GroupConcat接受一个可选参数order_by，可用于调整字符串结果的排序顺序。如果给定为self，则按相同字段排序。为了按示例中上述作者名称排序，执行以下操作：

book = Book.objects.all() \
        .values('id',
                'title',
                'pages',
                'price',
                'publisher__id',
                'publisher__name') \
        .annotate(
            authors__name=GroupConcat('authors__name',
                                      separator=', ',
                                      order_by='self')
        ) \
        .first()

输出将如下所示

{
    'authors__name': 'Dan Dijkman, Evy de Jong, Finn Janssen, Merel Wolf',
    'id': 14,
    'pages': 83,
    'price': Decimal('62.13'),
    'publisher__id': 19,
    'publisher__name': 'Rijn, de Bruyn and Verharen',
    'title': 'Laboriosam officia temporibus facere omnis odit.'
}