Django 查询相关对象

Question

我有两个模型 Post 和 UserBookmarks。

models.py 共 Project/App1

from App2 import UserBookmarks

class Post(models.Model):
        id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
        author = models.ForeignKey(User, on_delete=models.CASCADE)
        title = models.CharField(verbose_name="Title", max_length=20)
        content = models.TextField(verbose_name="Post Content", max_length=2000)
        ...
        bookmarks = GenericRelation(UserBookmarks, related_query_name='post')

models.py 共 Project/App2

bookmarkable_models = models.Q(app_label='App', model='post') | models.Q(app_label='App', model='model-b') | models.Q(app_label='App', model='model-c')

class UserBookmarks(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    user = models.ForeignKey(User, on_delete=models.CASCADE, null=False, blank=False)
    content_type = models.ForeignKey(ContentType, limit_choices_to=bookmarkable_models, on_delete=models.CASCADE, null=True, blank=False)
    object_id = models.CharField(max_length=50, blank=False)
    content_object = GenericForeignKey('content_type', 'object_id')
    date_added = models.DateTimeField(auto_now_add=True, blank=False)

现在我想检查用户是否已经将 post 作为我的 views.py 添加到他的书签中，但我不明白我对此的查询应该是什么样子。目前我正在做这样的事情：

def post_add_bookmark(request, pk):
    post = get_object_or_404(Post, pk=pk)
    if request.method == 'GET':
        if post.bookmarks.filter(user=request.user).exists():
            messages.error(request,
                           'You already added this Post to your Bookmarks')
            return redirect('post_detail', pk=post.pk)
        else:
            post.bookmarks.create(user=request.user, object_id=post.pk)
            messages.success(request, 'Post has been added to your Bookmarks')
            return redirect('post_detail', pk=post.pk)
    else:
        messages.error(request, 'Something went wrong')
        return redirect('post_detail', pk=post.pk)

可以smb。向我解释这个查询必须是什么样子才能检查它是否已经存在。目前我收到以下错误：

Cannot query "peter123": Must be "UserBookmarks" instance.

Answer 1

您可以检查 post 是否已被用户添加为书签：

post<b>.bookmarks</b>.filter(<b>user=request.user</b>).exists()

然而，在数据库级别强制执行此操作可能是个好主意，这样就不可能为同一用户存储相同的书签：

class UserBookmarks(models.Model):
    # …

    class Meta:
        <b>constraints</b> = [
            models.UniqueConstraint(fields=['user, content_type', 'object_id'], name='unique_bookmark')
        ]

或在django-2.2之前：

class UserBookmarks(models.Model):
    # …

    class Meta:
        <b>unique_together</b> = [['user, content_type', 'object_id']]

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