Django：优化多对多查询

Question

我有两个模型：Post（文章）和 Tag（标签）：

class Tag(models.Model):
    """ Tag for blog entry """
    title           = models.CharField(max_length=255, unique=True)

class Post(models.Model):
    """ Blog entry """
    tags            = models.ManyToManyField(Tag)
    title           = models.CharField(max_length=255)
    text            = models.TextField()

我想输出博客文章的列表，以及每篇文章对应的一组标签。我希望能通过两个查询来实现这个流程：

1. 获取文章列表
2. 获取这些文章中使用的标签列表
3. 在Python中将标签和文章关联起来

我在最后一步遇到了问题，这里是我写的代码，但它给了我一个错误：'Tag' object has no attribute 'post__id'

#getting posts
posts = Post.objects.filter(published=True).order_by('-added')[:20]
#making a disc, like {5:<post>}
post_list = dict([(obj.id, obj) for obj in posts])
#gathering ids to list
id_list = [obj.id for obj in posts]

#tags used in given posts
objects = Tag.objects.select_related('post').filter(post__id__in=id_list)
relation_dict = {}
for obj in objects:
    #Here I get: 'Tag' object has no attribute 'post__id'
    relation_dict.setdefault(obj.post__id, []).append(obj)

for id, related_items in relation_dict.items():
    post_list[id].tags = related_items

你能看到这里有什么错误吗？我该如何使用django ORM来完成这个任务，还是说我必须写自定义的SQL？

编辑：

我通过原始查询解决了这个问题：

objects = Tag.objects.raw("""
    SELECT
        bpt.post_id,
        t.*
    FROM
        blogs_post_tags AS bpt,
        blogs_tag AS t
    WHERE
        bpt.post_id IN (""" + ','.join(id_list) + """)
        AND t.id = bpt.tag_id
""")
relation_dict = {}
for obj in objects:
    relation_dict.setdefault(obj.post_id, []).append(obj)

如果有人能告诉我如何避免这个问题，我将非常感激。

Answer 1

如果你一定要用两个查询来实现这个功能，我觉得你需要写一些自定义的SQL语句：

def custom_query(posts):
  from django.db import connection
  query = """
  SELECT "blogs_post_tags"."post_id", "blogs_tag"."title"
  FROM "blogs_post_tags"
  INNER JOIN "blogs_tags" ON ("blogs_post_tags"."tag_id"="blogs_tags"."id")
  WHERE "blogs_post_tags"."post_id" in %s
  """
  cursor=connection.cursor()
  cursor.execute(query,[posts,])
  results = {}
  for id,title in cursor.fetchall():
    results.setdefault(id,[]).append(title)
  return results

recent_posts = Post.objects.filter(published=True).order_by('-added')[:20]
post_ids = recent_posts.values_list('id',flat=True)
post_tags = custom_query(post_ids)

recent_posts 是你的帖子查询集，应该是通过一次查询来缓存的。
post_tags 是一个将帖子ID和标签标题对应起来的映射，也是通过一次查询得到的。

Answer 2

在这种情况下，我通常会这样做：

posts = Post.objects.filter(...)[:20]

post_id_map = {}
for post in posts:
    post_id_map[post.id] = post
    # Iteration causes the queryset to be evaluated and cached.
    # We can therefore annotate instances, e.g. with a custom `tag_list`.
    # Note: Don't assign to `tags`, because that would result in an update.
    post.tag_list = []

# We'll now need all relations between Post and Tag. 
# The auto-generated model that contains this data is `Post.tags.through`.
for t in Post.tags.through.select_related('tag').filter(post_id__in=post):
    post_id_map[t.post_id].tag_list.append(t.tag)

# Now you can iterate over `posts` again and use `tag_list` instead of `tags`.

如果能把这个模式封装起来就更好了，所以你可以考虑添加一个查询集的方法，比如 select_tags()，这样就可以自动帮你处理了。