5

我正在开发一个简单的博客系统。这是我的models.py文件：

from django.contrib.auth.models import User
from django.db import models


class Comment(models.Model):
    user = models.ForeignKey(User)
    post = models.ForeignKey('Post')
    content = models.TextField()
    approved = models.NullBooleanField()

    class Meta:
        ordering = ('-id',)

    def __unicode__(self):
        return u'Comment by %s' % self.user


class Post(models.Model):
    user = models.ForeignKey(User)
    title = models.CharField(max_length=200)
    slug = models.CharField(max_length=50)
    content = models.TextField()

    class Meta:
        ordering = ('title',)

    def __unicode__(self):
        return self.title

这是我命名的夹具中的一些测试数据testdata.json（“some_author”用户是超级用户，密码是“Stack Overflow”）：

[
{
  "pk": 1, 
  "model": "auth.user", 
  "fields": {
    "username": "some_author", 
    "first_name": "Some", 
    "last_name": "Author", 
    "is_active": true, 
    "is_superuser": true, 
    "is_staff": true, 
    "last_login": "2014-07-02T20:18:49Z", 
    "groups": [], 
    "user_permissions": [], 
    "password": "pbkdf2_sha256$12000$PTl1hfgcIGZy$/0w1jNMBuKi9zk11JXhoS5WrbMBUgMDkZAhEvNEelbs=", 
    "email": "some_author@example.com", 
    "date_joined": "2014-07-02T20:18:29Z"
  }
},
{
  "pk": 2, 
  "model": "auth.user", 
  "fields": {
    "username": "some_reader", 
    "first_name": "Some", 
    "last_name": "Reader", 
    "is_active": true, 
    "is_superuser": false, 
    "is_staff": false, 
    "last_login": "2014-07-02T20:21:10Z", 
    "groups": [], 
    "user_permissions": [], 
    "password": "pbkdf2_sha256$12000$CtTGfFeOaRhd$oVR6zFSpK2qg1AZ4fgdBG/wt6Sr56dHsEIxFO99mHC8=", 
    "email": "some_reader@example.com", 
    "date_joined": "2014-07-02T20:21:10Z"
  }
},
{
  "pk": 3, 
  "model": "auth.user", 
  "fields": {
    "username": "another_reader", 
    "first_name": "Another", 
    "last_name": "Reader", 
    "is_active": true, 
    "is_superuser": false, 
    "is_staff": false, 
    "last_login": "2014-07-02T20:21:34Z", 
    "groups": [], 
    "user_permissions": [], 
    "password": "pbkdf2_sha256$12000$ZPnmV7fVeie3$08H2vv3A8Py4E92+uVAIiEaeg8CAL5deTyNAZj1YJMs=", 
    "email": "another_reader@example.com", 
    "date_joined": "2014-07-02T20:21:34Z"
  }
},
{
  "pk": 1, 
  "model": "blog.comment", 
  "fields": {
    "content": "Comment 1 of 1 on post 1: approved", 
    "post": 1, 
    "user": 2, 
    "approved": true
  }
},
{
  "pk": 2, 
  "model": "blog.comment", 
  "fields": {
    "content": "Comment 1 of 1 on post 2: not approved", 
    "post": 2, 
    "user": 2, 
    "approved": false
  }
},
{
  "pk": 3, 
  "model": "blog.comment", 
  "fields": {
    "content": "Comment 1 of 2 on post 3: approved", 
    "post": 3, 
    "user": 2, 
    "approved": true
  }
},
{
  "pk": 4, 
  "model": "blog.comment", 
  "fields": {
    "content": "Comment 2 of 2 on post 3: not approved", 
    "post": 3, 
    "user": 2, 
    "approved": false
  }
},
{
  "pk": 5, 
  "model": "blog.comment", 
  "fields": {
    "content": "Comment 1 of 2 on post 4: not approved", 
    "post": 4, 
    "user": 2, 
    "approved": false
  }
},
{
  "pk": 6, 
  "model": "blog.comment", 
  "fields": {
    "content": "Comment 2 of 2 on post 4: approved", 
    "post": 4, 
    "user": 2, 
    "approved": true
  }
},
{
  "pk": 7, 
  "model": "blog.comment", 
  "fields": {
    "content": "Comment 1 of 2 on post 5: approved", 
    "post": 5, 
    "user": 2, 
    "approved": true
  }
},
{
  "pk": 8, 
  "model": "blog.comment", 
  "fields": {
    "content": "Comment 2 of 2 on post 5: approved", 
    "post": 5, 
    "user": 2, 
    "approved": true
  }
},
{
  "pk": 9, 
  "model": "blog.comment", 
  "fields": {
    "content": "Comment 1 of 2 on post 6: not approved", 
    "post": 6, 
    "user": 2, 
    "approved": false
  }
},
{
  "pk": 10, 
  "model": "blog.comment", 
  "fields": {
    "content": "Comment 2 of 2 on post 6: not approved", 
    "post": 6, 
    "user": 2, 
    "approved": false
  }
},
{
  "pk": 11, 
  "model": "blog.comment", 
  "fields": {
    "content": "Comment 1 of 1 on post 7: approved", 
    "post": 7, 
    "user": 3, 
    "approved": true
  }
},
{
  "pk": 1, 
  "model": "blog.post", 
  "fields": {
    "content": "First post", 
    "slug": "post-1", 
    "user": 1, 
    "title": "Post 1"
  }
},
{
  "pk": 2, 
  "model": "blog.post", 
  "fields": {
    "content": "Second post", 
    "slug": "post-2", 
    "user": 1, 
    "title": "Post 2"
  }
},
{
  "pk": 3, 
  "model": "blog.post", 
  "fields": {
    "content": "Third post", 
    "slug": "post-3", 
    "user": 1, 
    "title": "Post 3"
  }
},
{
  "pk": 4, 
  "model": "blog.post", 
  "fields": {
    "content": "Fourth post", 
    "slug": "post-4", 
    "user": 1, 
    "title": "Post 4"
  }
},
{
  "pk": 5, 
  "model": "blog.post", 
  "fields": {
    "content": "Fifth post", 
    "slug": "post-5", 
    "user": 1, 
    "title": "Post 5"
  }
},
{
  "pk": 6, 
  "model": "blog.post", 
  "fields": {
    "content": "Sixth post", 
    "slug": "post-6", 
    "user": 1, 
    "title": "Post 6"
  }
},
{
  "pk": 7, 
  "model": "blog.post", 
  "fields": {
    "content": "Seventh post", 
    "slug": "post-7", 
    "user": 1, 
    "title": "Post 7"
  }
},
{
  "pk": 8, 
  "model": "blog.post", 
  "fields": {
    "content": "Eighth post", 
    "slug": "post-8", 
    "user": 1, 
    "title": "Post 8"
  }
}
]

我正在尝试在数据库中查询所有博客文章以及满足这两个条件的每个博客文章的最新评论：

  1. 评论是由“一些读者”（user_id = 2）发表的
  2. 该评论已被批准

我希望查询返回所有博客文章，即使他们没有满足上述两个条件的评论。对于没有符合上述两个条件的评论的博文，返回的评论栏应该是NULL. 我有这个使用原始 SQL：

for p in Post.objects.raw(
    '''
    SELECT blog_post.id,
           blog_post.title,
           blog_comment.content
    FROM   blog_post
           LEFT OUTER JOIN (SELECT post_id,
                                   MAX(id) AS latest
                            FROM   blog_comment
                            WHERE  user_id = 2
                                   AND approved = 1
                            GROUP  BY post_id) AS x
                        ON x.post_id = blog_post.id
           LEFT OUTER JOIN blog_comment
                        ON blog_comment.post_id = x.post_id
                           AND blog_comment.id = x.latest
    ORDER  BY blog_post.id;
    '''
):
    print '%s: %s' % (
        p.title,
        p.content,
    )

上面的代码输出这个（这是我想要的）：

Post 1: Comment 1 of 1 on post 1: approved
Post 2: None
Post 3: Comment 1 of 2 on post 3: approved
Post 4: Comment 2 of 2 on post 4: approved
Post 5: Comment 2 of 2 on post 5: approved
Post 6: None
Post 7: None
Post 8: None

我的问题是：是否有可能（有效地）做同样的事情，但不诉诸原始 SQL？我喜欢尽可能避免原始查询。

4

2 回答 2

3

如果没有 django orm 范式中的原始 sql，您将无法做到这一点。但是您可以通过对 db 的两个查询来做到这一点：

from django.db.models import Max
posts = Post.objects.annotate(Max('comment_set__id'))
comments_cache = Comment.objects.filter(id__in= posts.values('id', flat=True))
comments_dict = dict([(item.id, item) for item in comments_cache])
for item in posts:
     print post, comments_dict[item.id]

我经常进行复杂的查询，但仍然找不到比在缓存对象中使用少量查询获取所需的所有数据以及根据需要对其进行分组更好的方法。

请不要使用如下代码：

#get_comment: return self.comment_set.filter(user=user, approved=True).latest('id')
for post in Post.objects.all():
    print post.get_comment(request.user)

它将产生 len(posts) 对数据库的 sql 查询。这是不好的做法。

于 2014-07-07T08:21:51.017 回答
2

您想要所有帖子，但评论应该是符合指定条件的评论，否则None. For that you can add a method/property inPost` 模型可以按照您想要的方式获取最近的评论。

不要认为使用 ORM 有很好的方法。

在您的模型中添加以下方法，

class Post(models.Model):
    ...
    #your code

    def get_comment(self, user):
        comment = None
        try:
            comment = self.comment_set.filter(user=user, approved=True).latest('id')
        except Comment.DoesNotExist:
            comment = None #no comment matching criteria

        return comment

从您的视图或其他代码中，您可以执行

for post in Post.objects.all():
    print post.get_comment(request.user)
于 2014-07-03T04:40:46.887 回答