Django - queryset for multiple models that related with foreignkey

Question

I have models.py like this:

class Post(models.Model):
    name = models.CharField(max_length=100,blank=True)
    author = models.ForeignKey(User, null=True, blank=True)
    ...

class Categories(models.Model):
   category = models.CharField(max_length=200)
   post_id = models.ForeignKey(Post)

class Tags(models.Model):
  tag = = models.CharField(max_length=200)
  tag_id = models.ForeignKey(Post)

How do I get the all the related objects if given the author id. eg. In the view.py

def AllPost(request, userid):
    posts = Post.objects.filter(author=userid)
    for post in Posts:
         category = Categories.filter(post_id = post.id)
         tag = Tags.filter(post_id = post.id)

The problem of the function above will causes extra query for each of the post. Is there any way to call all the objects in single query??

possible duplicate of JOIN two tables containing same foreign key in Django — user764357
– user764357, Commented Jul 22, 2014 at 3:26

laffuste · Accepted Answer · 2014-07-22 03:59:34Z

1

For django >= 1.4, use prefecth_related.

class Post(models.Model):
    name = models.CharField(max_length=100,blank=True)
    author = models.ForeignKey(User, null=True, blank=True)
    ...

class Categories()models.Model:
   category = models.CharField(max_length=200)
   post = models.ForeignKey(Post, related_name='categories')

def get_all_posts(request, user_id):
    posts = Post.objects.filter(author=user_id).prefetch_related('categories')
    for post in posts:
         categories = post.categories

(Note: some code errors fixed.)

For django < 1.4, you can use django_prefetch plugin.

answered Jul 22, 2014 at 3:59

laffuste

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1 Comment

dev-jim Over a year ago

But how about the Tags? Can i prefetch_related more than one objects? like prefetch_related('categories', 'tags')?

ruddra · Accepted Answer · 2014-07-22 08:03:22Z

1

How about like this:

Categories= Category.objects.filter(tag_id__author=userid) #assuming userid is an User object like for example: request.User
Tags= Tags.objects.filter(post_id__auther=userid)

if you want to get both category and tags from Post in single query, then you will get ValuesQuerySet from following query:

data= Post.objects.filter(auther=userid).values_list('category', 'tags') #or .values_list('category__category', 'tags__tag')

answered Jul 22, 2014 at 8:03

ruddra

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