Many-to-Many relationship in Django

Question

I have the following 3 models:

class Platform(models.Model):
    title = models.CharField(max_length=100, unique=True)

class Profile(models.Model):
    title = models.CharField(max_length=110, unique=True)
    platform = models.ManyToManyField(Platform)

class Register(models.Model):
    ...
    profile = models.ManyToManyField(Profile)
    ...

My views.py

def info(request):
    ...
    registers=Register.objects.all()
    ...
    for register in registers:
        profile= register.profile....???

I need to know the profile or profiles from a query of the Register model

is possible?

Willem Van Onsem · Accepted Answer · 2020-09-04T00:51:21.250

0

You can obtain all the Profiles related to one or more registers with:

Profile.objects.filter(register__in=registers)

or you can obtain the Profiles of a Register object:

def info(request):
    # …
    registers=Register.objects.all()
    # …
    for register in registers:
        profiles = register.profile.all()

but here you will hit the database per register, which might not scale very well.

You can, like @IainShelvington says, use .prefetch_related(…) [Django-doc] to fetch all these related Profiles in memory with one extra query and do the JOIN at the Django/Python level:

def info(request):
    # …
    registers=Register.objects.prefetch_related('profile')
    # …
    for register in registers:
        profiles = register.profile.all()

edited Sep 04 '20 at 00:51

answered Sep 03 '20 at 23:45

Willem Van Onsem

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`Register.objects.prefetch_related('profile_set')` would eliminate the repeated database calls – Iain Shelvington Sep 03 '20 at 23:46
if I use ```register.profile_set.all()``` I get the following error: ```AttributeError: 'Register' object has no attribute 'profile_set'``` – David Castro Sep 04 '20 at 00:26
@DavidCastro: did you specify a `related_name` in the `ManyToManyField`? – Willem Van Onsem Sep 04 '20 at 00:27
No, I have ```profile= models.ManyToManyField(Profile)``` – David Castro Sep 04 '20 at 00:49
@DavidCastro: never mind, I misread it. Then it is `register.profile.all()`. You might however want to rename `profile` to `profiles` (so plural), since it is a *collection* of `Profile` objects. – Willem Van Onsem Sep 04 '20 at 00:51
Yes now, thank you!!!. Why doesn't it accept ```..._ set``` as indicated by Django? – David Castro Sep 04 '20 at 13:13
@DavidCastro: no, the `..._set` is the relation in *reverse*. But here we use the `ManyToMany` in a *forward* manner. – Willem Van Onsem Sep 04 '20 at 18:13

Many-to-Many relationship in Django

1 Answers1