How to get only related objects in django serializer with manyToMany? - django

Question

class BookingSerializer(serializers.ModelSerializer):
class Meta:
model = Booking
fields = "__all__"
class EmployeeSerializer(serializers.ModelSerializer):
bookings_st = BookingSerializer(many=True, read_only=True)
class Meta:
model = Employee
fields = "__all__"
class ProjectSerializer(serializers.ModelSerializer):
employees = EmployeeSerializer(read_only=True, many=True)
class Meta:
model = Project
fields = "__all__"
class Employee(models.Model):
name = models.CharField(max_length=127)
lastname = models.CharField(max_length=127)
class Project(models.Model):
title = models.CharField(max_length=127)
employees = models.ManyToManyField(Employee,
related_name='employees')
class Booking(models.Model):
start = models.DateField()
end = models.DateField()
employee = models.ForeignKey(Employee,
on_delete=models.CASCADE,
related_name='bookings_st')
project = models.ForeignKey(Project,
on_delete=models.CASCADE,
related_name='bookings_st')
I get nested object, but how to get in Emploee only related to both (project and employee) bookings? Now I just get all bookings that this employee has.
I mean that structure:
project_1:
emploee_1:
[bookings_that_belong_to_THIS_PROJECT]

Answer 1

A possibility here is to leverage a SerializerMethodField together with the serializer's context object:
class BookingSerializer(serializers.ModelSerializer):
class Meta:
model = Booking
fields = "__all__"
class EmployeeSerializer(serializers.ModelSerializer):
bookings_st = serializers.SerializerMethodField()
def get_bookings_st(self, employee):
project = self.context.get("project")
return BookingSerializer(
employee.bookings_st.filter(project=project),
many=True,
).data
class Meta:
model = Employee
fields = "__all__"
class ProjectSerializer(serializers.ModelSerializer):
employees = serializers.SerializerMethodField()
def get_employees(self, project):
return EmployeeSerializer(
project.employees.all(),
many=True,
context={**self.context, "project": project}
).data
class Meta:
model = Project
fields = "__all__"
This wouldn't be super performant if you're using that when listing projects or when retrieving a specific project with many employees. Depending on your use case - if you're only using it to retrieve a specific project, for example, you could leverage prefetch_related and the Prefetch class to prefetch custom employees + bookings querysets by filtering out the specific project at hand (and you could then use the regular serializers).