Tricky Django GenericRelation查询

问题描述

假设我有几个代表现实生活中的对象的模型：人，主席， Room

我还有一个集合模型，代表这些模型的一些记录集。

每个模型都可以是多个集合的成员 - 因此，我还创建了一个成员资格模型，它表示一个对象是一个集合的成员。定义如下：

  class Membership（models.Model）：
 content_type = models.ForeignKey（ContentType） 
 object_id = models.PositiveIntegerField（）
 content_object = generic.GenericForeignKey（'content_type'，'object_id'）
 
 collection = models.ForeignKey（'Collection'，related_name =成员）
  

我想要创建一个 QuerySet 给定一个集合，表示给定模型的所有成员 。我知道我可以以编程方式执行，但是我需要一个QuerySet，它可以被过滤，订购等。

编辑： p>

显然，这可以使用原始SQL：

  SELECT * FROM 
（modelx INNER JOIN membership ON modelx.id = membership.object_id）
 WHERE 
（membership.collection_id =< my-collection-id> AND 
 membership.content_type_id = modelx-type-id>）
  

但是可以使用Django查询语言来表示？ >

解决方案

似乎我已经找到了解决方案，通过使用 QuerySet 的 extra 方法：

  def members_of_model（collection，cls）：
 cls_type = ContentType.objects.get_for_model（cls）
 cm_tablename = CollectionMembership._meta.db_table 
 cls_tablename = cls._meta.db_table 
 return cls.objects.all（）。extra（tables = [cm_tablename]，
其中= ['％s.content_type_id = %% s'％cm_tablename，
'％s.collection_id = %% s'％cm_tablename，
'％s.object_id =％s.id'％（cm_tablename，cls_tablename）]，
 params = [cls_type.id，collection.id]）
  

这将返回一个特定模型的有效QuerySet，它包含作为特定集合成员的所有记录。

Suppose I have a few models representing real life objects: "Person", "Chair", "Room"

I also have a "Collection" model, which represents some collection of records of these models.

Each model can be a member of more than on collection - therefore, I have also created a "Membership" model, which represents an object is a member of a collection. It is defined as follows:

class Membership(models.Model):
   content_type   = models.ForeignKey(ContentType)
   object_id      = models.PositiveIntegerField()
   content_object = generic.GenericForeignKey('content_type', 'object_id')

   collection     = models.ForeignKey('Collection', related_name="members")

I want to be able to create a QuerySet, which given a collection, represents all its members of a given model. I know I can do it programmatically, but I need it in a QuerySet, which can be filtered, ordered etc.

EDIT:

Obviously this can be done using raw SQL:

   SELECT * FROM 
       ( modelx INNER JOIN membership ON modelx.id = membership.object_id) 
   WHERE 
       ( membership.collection_id=<my-collection-id> AND    
         membership.content_type_id=<modelx-type-id> )

But can it be represented using the Django query language?

解决方案

It seems I have found the solution, by using QuerySet's extra method:

def members_of_model(collection,cls):
    cls_type = ContentType.objects.get_for_model(cls)
    cm_tablename = CollectionMembership._meta.db_table
    cls_tablename = cls._meta.db_table
    return cls.objects.all().extra(tables=[cm_tablename],
                                   where=[ '%s.content_type_id=%%s' % cm_tablename,
                                           '%s.collection_id=%%s' % cm_tablename,
                                           '%s.object_id=%s.id' % (cm_tablename, cls_tablename) ],
                                   params=[cls_type.id,collection.id] )

This returns a valid QuerySet of a specific model, which holds all records which are members of a specific collection.

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