Let's say I have the following two models:
class Parent(models.Model):
name = models.CharField(max_length=48)
class Child(models.Model):
name = models.CharField(max_length=48)
movement = models.ForeignKey(Parent, related_name='children')
And I have the following DRF generics.ListAPIView where I want to be able to search/filter on Child objects but actually return the related Parent objects:
class ParentSearchByChildNameView(generics.ListAPIView):
"""
Return a list of parents who have a child with the given name
"""
serializer_class = ParentSerializer
def get_queryset(self):
child_name = self.request.query_params.get('name')
queryset = Child.objects.all()
if child_name is not None:
queryset = queryset.filter(name__contains=child_name)
matched_parents = Parent.objects.filter(children__in=queryset).distinct()
return matched_parents
Now, this works well for me. If a Parent object has 3 Child objects which all match the given "name" query_param, then I only get one Parent object back, which is what I want:
{
"next": null,
"previous": null,
"results": [
{
"url": "URL to parent",
"name": "Parent #1",
}
]
}
However, what I also want is to indicate the matched Child object IDs within the result. If I may illustrate in JSON what I want:
{
"next": null,
"previous": null,
"results": [
{
"url": "URL to parent",
"name": "Parent #1",
"matched_child": [1, 3, 7]
}
]
}
Is this something I can do with built-in tools, without expensively and repeatedly hitting the database?
Without making use of a database-specific feature, you can work with a Prefetch object [Django-doc]:
from django.db.models import Prefetch
class ParentSearchByChildNameView(generics.ListAPIView):
"""
Return a list of parents who have a child with the given name
"""
serializer_class = ParentSerializer
def get_queryset(self):
child_name = self.request.query_params.get('name')
return = Parent.objects.filter(
children__name__contains=child_name
).prefetch_related(
Prefetch('children', Child.objects.filter(name=child_name), to_attr='matched_children')
).distinct()For the serializer we then can work with a PrimaryKeyRelatedField [drf-doc]:
class ParentSerializer(serializers.ModelSerializer)
matched_children = serializers.PrimaryKeyRelatedField(
many=True,
read_only=True
)
class Meta:
model = Parent
fields = ['url', 'name', 'matched_children']If you work with postgresql, you can work with an ArrayAgg expression [Django-doc]:
from django.contrib.postgres.aggregates import ArrayAgg
class ParentSearchByChildNameView(generics.ListAPIView):
"""
Return a list of parents who have a child with the given name
"""
serializer_class = ParentSerializer
def get_queryset(self):
child_name = self.request.query_params.get('name')
return = Parent.objects.filter(
children__name__contains=child_name
).annotate(matched_children=ArrayAgg('children__pk'))In the serializer you thus add a ListField that will list the children with:
class ParentSerializer(serializers.ModelSerializer)
matched_children = serializers.ListField(
child=serializers.IntegerField(),
read_only=True
)
class Meta:
model = Parent
fields = ['url', 'name', 'matched_children']