I'm hoping someone might be able to help me. I am working with Django Rest Framework, and attempting to create an API that allows users to search for Providers that provide specific Procedures in particular Regions, and only return the relevant details.
Set up
I have these models (heavily simplified):
# models.py
from django.db import models
from django.core.validators import MinValueValidator, MaxValueValidator
class Region(models.Model):
name = models.CharField(max_length=100)
class Meta:
ordering = ["name"]
class Procedure(models.Model):
description = models.CharField(max_length=100)
class Meta:
ordering = ["description"]
class Provider(models.Model):
provider_name = models.CharField(max_length=200)
class Meta:
ordering = ["provider_name"]
class Provision(models.Model):
fk_procedure = models.ForeignKey(
Procedure,
related_name="providers",
on_delete=models.RESTRICT,
)
fk_provider = models.ForeignKey(
Provider,
related_name="services",
on_delete=models.CASCADE,
)
discount = models.FloatField(
validators=[MaxValueValidator(100), MinValueValidator(0)],
default=0,
)
class Meta:
ordering = ["-discount"]
unique_together = ["fk_procedure", "fk_provider"]
class ProvisionLinkRegion(models.Model):
fk_provision = models.ForeignKey(
Provision,
related_name="regions",
on_delete=models.CASCADE,
)
fk_region = models.ForeignKey(
Region,
related_name="services",
on_delete=models.RESTRICT,
)
location = models.BooleanField(default=False)
As you can see, there is a ManyToMany link between Provision and Region via ProvisionLinkRegion. I haven't defined this as a ManyToMany field though, as I need to store additional details (location) about the pairing.
I have defined the following serializers on these models:
# serializers.py
from rest_framework import serializers
from models import (
Provider,
Region,
Procedure,
ProvisionLinkRegion,
Provision,
)
class ProvisionLinkRegionSerializer(serializers.ModelSerializer):
class Meta:
model = ProvisionLinkRegion
fields = ["fk_region", "location"]
class ProvisionDetailsSerializer(serializers.ModelSerializer):
regions = ProvisionLinkRegionSerializer(many=True)
class Meta:
model = Provision
fields = ["fk_procedure", "discount", "mff_opt_out", "regions"]
class ProviderProvisionSerializer(serializers.ModelSerializer):
services = ProvisionDetailsSerializer(many=True)
number_services = serializers.IntegerField()
class Meta:
model = Provider
fields = [
"provider_name",
"services",
"number_services",
]
And have defined my API like this:
# api.py
from django.db.models import Prefetch, Count
from rest_framework import generics, pagination, permissions, status
from rest_framework.response import Response
from serializers import ProviderProvisionSerializer
from models import (
Provider,
ProvisionLinkRegion,
Provision,
)
class CustomPagination(pagination.PageNumberPagination):
page_size_query_param = "limit"
def get_paginated_response(self, data):
return Response(
{
"pagination": {
"previous": self.get_previous_link(),
"next": self.get_next_link(),
"count": self.page.paginator.count,
"current_page": self.page.number,
"total_pages": self.page.paginator.num_pages,
"items_on_page": len(data),
},
"results": data,
}
)
class ProvisionListAPI(generics.ListAPIView):
permission_classes = [permissions.IsAuthenticated]
serializer_class = ProviderProvisionSerializer
pagination_class = CustomPagination
def get_queryset(self):
queryset = Provider.objects.distinct()
# Extract the query parameters
params = self.request.query_params
region_list = params["region"].split(",")
procedure_list = param["procedure"].split(",")
# Build up the prefetch Provision table filtering on the regions
# and services
services_prefetch_qs = (
Provision.objects.distinct()
.filter(regions__fk_region__in=region_list)
.filter(fk_procedure__in=procedure_list)
.prefetch_related(
Prefetch(
"regions",
queryset=ProvisionLinkRegion.objects.filter(
fk_region__in=region_list
),
)
)
)
# Apply the filters and prefetch required tables
queryset = queryset.filter(
services__regions__fk_region__in=region_list
).prefetch_related(
Prefetch("services", queryset=services_prefetch_qs),
)
# Add the ordering parameters
queryset = (
queryset.annotate(
number_services=Count("services", distinct=True) # FIXME
)
.filter(number_services__gt=0)
.order_by("-number_services")
)
return queryset.all()
def list(self, response):
# Check it has the right headers
params = self.request.query_params
if "procedure" not in params:
return Response(
{"detail": "procedure not provided as a query parameter"},
status.HTTP_400_BAD_REQUEST,
)
if "region" not in params:
return Response(
{"detail": "region not provided as a query parameter"},
status.HTTP_400_BAD_REQUEST,
)
# Paginate and filter queryset
queryset = self.get_queryset()
page = self.paginate_queryset(queryset)
if page is not None:
serializer = self.get_serializer(page, many=True)
return self.get_paginated_response(serializer.data)
serializer = self.get_serializer(queryset, many=True)
return Response(serializer.data, status=status.HTTP_200_OK)
My problem is, the number_services value is not correct, as it's not doing the count on the full filtered results. It's only doing it on the pre-filtered ones (although the region filter does work). I also don't want any providers to appear when they don't have any services (hence the .filter(number_services__gt=0)).
I think that it's related to not filtering on the main Provider queryset like I do with the region i.e. to include:
queryset = queryset.filter(services__fk_procedure__in=procedure_list)
But when I include this, it doesn't actually remove the services, but just the providers that don't provide ANY of those services, so the count is still off.
Example
If my data with no filtering or prefetching looks like this:
"results": [
{
"provider_name": "Provider 2.0",
"services": [
{
"fk_procedure": 3,
"discount": 0.05,
"regions": [
{
"fk_region": 1,
"location": true
},
{
"fk_region": 2,
"location": false
}
{
"fk_region": 3,
"location": true
}
]
},
{
"fk_procedure": 5,
"discount": 0.05,
"regions": [
{
"fk_region": 1,
"location": true
}
]
}
]
},
{
"provider_name": "Test Provider",
"services": [
{
"fk_procedure": 2,
"discount": 0.00,
"regions": [
{
"fk_region": 1,
"location": true
}
]
}
]
}
]
If I then run this on it:
GET /api/v1/provision?page=1&limit=10®ion=1,3&services=3`
I want to show all the providers and the relevant details where they relate to either region 1 or 3, and procedure 3.
Actual Result
{
"pagination": {
"previous": null,
"next": null,
"count": 2,
"current_page": 1,
"total_pages": 1,
"items_on_page": 2
},
"results": [
{
"provider_name": "Provider 2.0",
"services": [
{
"fk_procedure": 3,
"discount": 0.05,
"regions": [
{
"fk_region": 1,
"location": true
},
{
"fk_region": 3,
"location": true
}
]
}
],
"number_services": 2
},
{
"provider_name": "Test Provider",
"services": [],
"number_services": 1
}
]
}
Desired Result
{
"pagination": {
"previous": null,
"next": null,
"count": 2,
"current_page": 1,
"total_pages": 1,
"items_on_page": 2
},
"results": [
{
"provider_name": "Provider 2.0",
"services": [
{
"fk_procedure": 3,
"discount": 0.05,
"regions": [
{
"fk_region": 1,
"location": true
},
{
"fk_region": 3,
"location": true
}
]
}
],
"number_services": 1,
}
]
}
Things I've tried
SerializerMethodField
I've been able to get number_services to work using a SerializerMethodField by including:
# serializers.py
class ProviderProvisionSerializer(serializers.ModelSerializer):
...
number_services = serializers.SerializerMethodField()
...
class Meta:
...
def get_number_services(self, obj):
return obj.services.count()
Unfortunately though, I can't use this for ordering, or filtering within the API, and I also can't use it with pagination either, so is pretty useless for what I need it for.
Subquery
In the get_queryset method for the API, I've also tried using what I currently have as a subquery, and using the main queryset where the ID is in the other one, but then I lose all the prefetch from the first subquery, and have regions and services that don't relate to my filter.
## TLDR
How do I filter a queryset in a get_queryset method for a ListAPIView on properties of the children of the main model, and be able to return a count of the remaining children after the filter has taken place?