Question about PrimaryKeyRelatedField with `many=True`

[cheehong1030]

I have some questions regarding the following code snippet:

model = PrimaryKeyRelatedField(
    pk_field=IntegerField(),
    queryset=MachineModel.objects.filter(active=True),
    many=True,
    required=True,
)

When PrimaryKeyRelatedField is set with many=True, it generates these queries. It seems like they are not using join.

SELECT "api_application_machinemodel"."id", "api_application_machinemodel"."brand_id", "api_application_machinemodel"."name", "api_application_machinemodel"."description", "api_application_machinemodel"."active" FROM "api_application_machinemodel" WHERE ("api_application_machinemodel"."active" AND "api_application_machinemodel"."id" = 1) LIMIT 21; args=(1,); alias=default
SELECT "api_application_machinemodel"."id", "api_application_machinemodel"."brand_id", "api_application_machinemodel"."name", "api_application_machinemodel"."description", "api_application_machinemodel"."active" FROM "api_application_machinemodel" WHERE ("api_application_machinemodel"."active" AND "api_application_machinemodel"."id" = 2) LIMIT 21; args=(2,); alias=default
SELECT "api_application_machinemodel"."id", "api_application_machinemodel"."brand_id", "api_application_machinemodel"."name", "api_application_machinemodel"."description", "api_application_machinemodel"."active" FROM "api_application_machinemodel" WHERE ("api_application_machinemodel"."active" AND "api_application_machinemodel"."id" = 3) LIMIT 21; args=(3,); alias=default
SELECT "api_application_machinemodel"."id", "api_application_machinemodel"."brand_id", "api_application_machinemodel"."name", "api_application_machinemodel"."description", "api_application_machinemodel"."active" FROM "api_application_machinemodel" WHERE ("api_application_machinemodel"."active" AND "api_application_machinemodel"."id" = 4) LIMIT 21; args=(4,); alias=default
SELECT "api_application_machinemodel"."id", "api_application_machinemodel"."brand_id", "api_application_machinemodel"."name", "api_application_machinemodel"."description", "api_application_machinemodel"."active" FROM "api_application_machinemodel" WHERE ("api_application_machinemodel"."active" AND "api_application_machinemodel"."id" = 5) LIMIT 21; args=(5,); alias=default

Is this behavior a bug, or is it expected not to use join?

[browniebroke]

Are you prefeching the relationship in your view? You'll need to use Django's prefetch_related().

[cheehong1030]

Are you prefeching the relationship in your view? You'll need to use Django's prefetch_related().

This is for validation purposes, so I cannot use prefetch_related.

def put(self, request):
    serializer = MachinePartUpdateSerializer(data=request.data)
    serializer.is_valid()

Here is the serializer:

class MachinePartSerializer(ModelSerializer):
    brand = PrimaryKeyRelatedField(
        pk_field=IntegerField(),
        queryset=MachineBrand.objects.filter(active=True),
        required=True,
    )
    # TODO: performance issue n+1
    model = PrimaryKeyRelatedField(
        pk_field=IntegerField(),
        queryset=MachineModel.objects.filter(active=True),
        many=True,
        required=True,
    )

    class Meta:
        model = MachinePart
        fields = ["id", "brand", "model", "name", "description"]

Here is my model using a many-to-many relationship:

class MachinePart(Model):
    brand = ForeignKey(MachineBrand, on_delete=models.CASCADE)
    model = ManyToManyField(MachineModel, related_name="parts")
    name = CharField(max_length=255)
    part_number = CharField(max_length=255)
    description = TextField(blank=True, null=True)
    active = BooleanField(default=True)

So, how should I handle prefetch the relations

[browniebroke]

Ah ok, I thought you were doing a read-only request, I didn't realise it was a PUT request. I don't know the answer in this case.

[cheehong1030]

This might be a bug. It seems that the case of PrimaryKeyRelatedField with many=True wasn't considered.

[sevdog]

The problem lies in the implementation of ManyrelatedField which is extremely simple and does not implement any kind of DB-level optimization. The same problem is in ListSerializer.create method. To keep those elements "simple" they just delegate DB operations to children, but this is not-optimal when there are many elements.

This is not technically a bug, because it does what it is expected, yet it is a bad usage of resources and should be addressed.

[cheehong1030]

Maybe this could open a PR to optimize it.

[sreedhar742]

The behavior you're observing is due to how Django's PrimaryKeyRelatedField interacts with the database when many=True is used. Each item in the list of primary keys results in an individual query to fetch the corresponding MachineModel instance. This can lead to N+1 query problems, where N is the number of related objects being fetched.
Validation Per Instance:

PrimaryKeyRelatedField :
fetches each related object separately to ensure it exists and matches the given queryset.
The queries you see are part of this validation process.
No Bulk Query:
By default, PrimaryKeyRelatedField does not perform a single bulk query to fetch all related objects at once.

[cheehong1030]

So, is there no solution for this issue?

[sevdog]

This may be handled like it is done in Django core MultipleChoiceField: build a "bulk" select and than check if every provided key is in the returned queryset:

https://github.com/django/django/blob/40d5516385448a73426aad396778f369a363eda9/django/forms/models.py#L1622-L1658

This is not an hard task, since an implementation is ready for use for the same use-case.

[tomviner]

We hit this today. Searching the repo I see an attempt was made a while back:

• Optimize out O(n) queries when updating ManyRelatedField #4917
• #4917 - Remove O(n) queries in m2m updates #5093
• Proof of Concept for single lookup on HyperlinkRelatedField #5150

And there's also a approach suggested here: #8919 (comment) which is to try a single self.child_relation.bulk_to_internal_value(data) call before falling back to the current

[
    self.child_relation.to_internal_value(item)
    for item in data
]