Annotate on related field

[Ramashish Baranwal]

Hi,

I have the following model definitions:

class Tag(models.Model):
    name = models.CharField(max_length=16)

class Blog(models.Model):
    name = models.CharField(max_length=16)
    tags = models.ManyToManyField(Tag, blank=True)

I want to select blogs with more than one tags. Here is what I am doing (for two tags)-

# select blogs for python and java
blogs = Blog.objects.filter(tags__name='python').filter(tags__name='java')

This INNER joins Tag with Blog twice, each time filtering on the given name. This works well. Now I want to retrieve the matching tags. If it were a single join, I would have done-

blogs = blogs.annotate(tag=F('tags__name'))

Doing this still works, but only retrieves the last tag. How do I retrieve the tag name for each join, giving them different names? Something like-

# Below should give tag1 = 'python', tag2 = 'java'
blogs = blogs.annotate(tag1=?, tag2=?)

Thanks,
Ramashish

[Todor Velichkov]

By "retrieving the matching tags" you mean fetching `Tag` objects?

Well I guess what you can do is to use `prefetch_related` with custom `Prefetch` object.

blogs = Blog.objects.prefetch_related(
    models.Prefetch('tags',
        queryset=Tag.objects.filter(name__in=['python', 'java'])
        to_attr='my_tags'
    )
)

#usage
for blog in blogs:
    print blog.my_tags