Get only certain fields of related object in Django

Question

Say I have such models in Django:

class User(models.Model):
    name=models.CharField(...)
    email=models.EmailField(...)
    photo=...
    <other fields>

class Comment(models.Model):
    user=models.ForeignKey(User, ...)

I have a function that receives Comment object and needs just name and email fields to send email (it is just example, obvious)

def sendEmail(comment):
    name, email=comment.user.name, comment.user.email

In implementation presented above, Django will fetch all fields of related object User (something roughly equal to select * from users where id=comment.user_id) Using values_list I can fetch just needed fields:

Users.objects.filter(id=comment.user_id).values_list('name', 'email')

but values_list only applicable to QuerySet object, not to model instance. My question is: is there any way to do the same thing using only "comment" object? It must be equal to select name, email from users where id=comment.user_id by complexity (I don't want to transfer a lot of data stored in some fields over network when I don't need it)

Answer 1

If you want to have comments with some additional user data, without having to retrieve whole User objects I believe it's better to fetch additional data when you fetch the comments:

Comment.objects.filter(user=user).values_list('user__name', 'user__email')

Obviously you can fetch other useful Comment fields.

OR:

Comment.objects.filter(user=user).annotate(author_name=F('user__name'),\ 
                                           author_email=F('user__email'))

This still uses the QuerySet API, but both approaches allow you to span relationships to get additional data without extra queries.