class ModelA(models.Model):
    name = models.CharField(max_length=1)

class ModelB(models.Model):
    name = models.CharField(max_length=1)

class ModelAItems(models.Model):
    model_a = models.ForeignKey(ModelA, on_delete=models.CASCADE, related_name='items')
    model_b = models.ForeignKey(ModelB, on_delete=models.CASCADE)


class ModelAItemsSerializer(serializers.ModelSerializer):
    Model_a_items = ModelAItemsSerializer(many=True)

class ModelBSerializer(serializers.ModelSerializer):
    pass

class ModelB(serializers.ModelSerializer):
    model_a = serializer.SerializerMethodField()

    def get_model_b(self, obj):
        return ModelASerializer(ModelA.objects.filter(items_у_каждого_инстанса_A_должно_быть_только_ те items где items__model_b=obj))