Two reproducible issues in DRF nested serializers with many to many through model

tests/conftest.py

@@ -63,6 +63,7 @@ def pytest_configure(config):
             'rest_framework.authtoken',
             'tests.authentication',
             'tests.generic_relations',
+            'tests.issue',
             'tests.importable',
             'tests',
         ),

tests/issue/migrations/0001_initial.py

@@ -0,0 +1,42 @@
+# Generated by Django 3.1.13 on 2022-01-23 09:00
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = [
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Item',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('name', models.CharField(max_length=10)),
+            ],
+        ),
+        migrations.CreateModel(
+            name='ItemAmount',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('amount', models.IntegerField()),
+                ('item', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, to='issue.item')),
+            ],
+        ),
+        migrations.CreateModel(
+            name='Summary',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('items', models.ManyToManyField(through='issue.ItemAmount', to='issue.Item')),
+            ],
+        ),
+        migrations.AddField(
+            model_name='itemamount',
+            name='summary',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, to='issue.summary'),
+        ),
+    ]

tests/issue/models.py

@@ -0,0 +1,15 @@
+from django.db import models
+
+
+class Item(models.Model):
+    name = models.CharField(max_length=10)
+
+
+class ItemAmount(models.Model):
+    summary = models.ForeignKey('Summary', on_delete=models.CASCADE)
+    item = models.ForeignKey(Item, on_delete=models.CASCADE)
+    amount = models.IntegerField()
+
+
+class Summary(models.Model):
+    items = models.ManyToManyField(Item, through=ItemAmount)

tests/issue/serializers.py

@@ -0,0 +1,30 @@
+from rest_framework import serializers
+
+from . import models
+
+
+class ItemAmountSerializer(serializers.ModelSerializer):
+    item = serializers.PrimaryKeyRelatedField(queryset=models.Item.objects.all())
+
+    class Meta:
+        model = models.ItemAmount
+        fields = ('item', 'amount')
+
+
+class SummarySerializer(serializers.ModelSerializer):
+    items = ItemAmountSerializer(many=True)
+
+    def create(self, validated_data):
+        items = validated_data.pop('items')
+        instance = super().create(validated_data)
+        for item in items:
+            instance.items.add(
+                item['item'], through_defaults=dict(
+                    amount=item['amount']
+                )
+            )
+        return instance
+
+    class Meta:
+        model = models.Summary
+        fields = ('items', )

tests/issue/test_issue.py

@@ -0,0 +1,66 @@
+from django.test import TestCase, override_settings
+from django.urls import reverse
+
+from rest_framework.test import APIClient
+from rest_framework.utils import json
+
+from . import models, serializers
+
+
+class TestSerializer(TestCase):
+    def test_create(self):
+        item = models.Item.objects.create(name='test')
+        data = {
+            "items": [
+                {
+                    "item": item.id,
+                    "amount": 100,
+                }
+            ],
+        }
+        serializer = serializers.SummarySerializer(data=data)
+        serializer.is_valid(raise_exception=True)
+        expected_data = {
+            "items": [
+                {
+                    "item": item,
+                    "amount": 100,
+                }
+            ],
+        }
+        assert serializer.validated_data == expected_data
+        serializer.save()
+        assert models.Summary.objects.count() == 1
+
+
+@override_settings(ROOT_URLCONF='tests.issue.urls')
+class TestIssueViewSet(TestCase):
+    def test_create(self):
+        api_client = APIClient()
+        item = models.Item.objects.create(name='test')
+        data = {
+            "items": [
+                {
+                    "item": item.id,
+                    "amount": 100,
+                }
+            ],
+        }
+        response = api_client.post(reverse('summary-list'), data)
+        print(response.content)
+        assert response.status_code == 201
+
+    def test_create_with_json(self):
+        api_client = APIClient()
+        item = models.Item.objects.create(name='test')
+        data = {
+            "items": [
+                {
+                    "item": item.id,
+                    "amount": 100,
+                }
+            ],
+        }
+        response = api_client.post(reverse('summary-list'), json.dumps(data), content_type='application/json')
+        print(response.content)
+        assert response.status_code == 201

tests/issue/urls.py

@@ -0,0 +1,10 @@
+from rest_framework.routers import DefaultRouter
+
+from . import views
+
+app_name = "issue"
+router = DefaultRouter()
+
+router.register(r'summary', views.SummaryViewSet, basename='summary')
+
+urlpatterns = router.urls

tests/issue/views.py

@@ -0,0 +1,13 @@
+from rest_framework import viewsets
+from rest_framework.permissions import AllowAny
+
+from . import models, serializers
+
+
+class SummaryViewSet(viewsets.ModelViewSet):
+    """
+    A simple ViewSet for viewing and editing accounts.
+    """
+    queryset = models.Summary.objects.all()
+    serializer_class = serializers.SummarySerializer
+    permission_classes = [AllowAny]