Django:优化多对多查询

Question

我有Post和Tag模型:

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()

我需要输出博客条目列表和每篇文章的一组标记。我希望能够做到这一点，只有两个查询，使用这个工作流:

1. 获取帖子列表
2. 获取这些帖子中使用的标签列表
3. 链接标签到python的文章

我在最后一步遇到了麻烦，这是我想出的代码，但是在给我'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

你能看到一个错误吗?我如何使用django ORM解决这个任务，或者我将不得不写一个自定义SQL?

编辑:

我可以用原始查询解决这个问题:

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)

Answer 1

在这种情况下，我通常是这样做的:

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`.

如果以某种方式封装此模式会更好，因此您可能希望添加QuerySet方法(例如select_tags())来为您完成此操作。

Answer 2

如果你必须在两个查询中拥有它，我认为你需要自定义SQL:

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Post QuerySet，应该缓存从一个查询。
post_tags是post id到标签标题的映射，来自一个查询。