Django annotate several same objects in QuerySet by different related object - django

Question

I got:
# models
class Building(models.Model):
...
class Flat(models.Model):
building = models.ForeignKey(Building)
class Profile(models.Model):
flats = models.ManyToManyField(Flat)
# logic
building = Building.objects.create()
flat_1 = Flat.objects.create(building=building)
flat_2 = Flat.objects.create(building=building)
profile = Profile.objects.create()
profile.flats.add(flat_1)
profile.flats.add(flat_2)
profiles = Profile.objects.filter(flats__building=building)
I got in profiles 2 same profiles. How i can annotate each of them by different flat like this: profiles.first().flat == flat_1 and profiles.last().flat == flat_2?
Maybe Subquery() but how?
UPD I need this in some DRF list view. Output in JSON must be something like:
[
{
"profile_id": 1,
"flat_id": 2
},
{
"profile_id": 1,
"flat_id": 3
}
]

Answer 1

To obtain that output, you could do:
data = Profile.objects.all().values('flats', 'id')
return Response(data=data)
in your DRF view.

Answer 2

You don't have to profile instances ...
I wrote the code for your exact needs at the end, but first wrote a couple of things that might be of interest.
In your code sample, you've created only one profile, I'm sure you are not getting 2 instances of Profile that are equals but only one.
The thing is if you have a QuerySet with only one entry, then:
profiles.first() == profiles.last() # True
since profile.first() and profiles.last() are the same instance.
You should try creating 2 Profile instances:
building = Building.objects.create()
flat_1 = Flat.objects.create(building=building)
flat_2 = Flat.objects.create(building=building)
profile_1 = Profile.objects.create() # You coud/should use bulk_create here.
profile_2 = Profile.objects.create()
profile_1.flats.add(flat_1)
profile_2.flats.add(flat_2)
Then
profiles = Profile.objects.filter(flats__building=building)
will return two different profile objects.
On the other hand, obtaining the JSON like you want ...
Following the example, you posted, filter flats by profile and get the values (this also works if you have more that one profile).
Flat.objects.filter(profile=profile_1).values('profile__id', 'id')
This will return something like ("id" stands for flats ids):
[
{
"profile__id": 1,
"id": 1
},
{
"profile__id": 1,
"id": 3
}
]
If you do not filter by profile (and you have more than one) you could get something like:
[
{
"profile__id": 1,
"id": 1
},
{
"profile__id": 2,
"id": 3
},
{
"profile__id": 2,
"id": 4
},
...
]
Annotating to get the EXACT json you want:
Filter as shown previously annotate, and get desired values:
Flat.objects.filter(profile=profile_1).annotate(
flat_id=F('id')
).annotate(
profile_id=F('profile__id')
).values(
'profile_id', 'flat_id'
)
will give exactly what you want:
[
{
"profile_id": 1,
"flat_id": 2
},
{
"profile_id": 1,
"flat_id": 3
}
]

Answer 3

You can do that with the right serializer and the right annotation:
The serializer:
class FlatSerializer(serializers.ModelSerializer):
class Meta:
model = Flat
fields = ('flat_id', 'building_id')
flat_id = serializers.CharField(read_only=True)
Then I would simply query Flats rather than profiles and serialize:
flats = Flat.objects \
.annotate(flat_id=F('id')) \
.filter(building=building)
serialized = FlatSerializer(flats, many=True)
print(serialized.data) # [ { flat_id: 1, building_id: 1 }, { flat_id: 2, building_id: 1 } ]
Let me know if that works for you