Validation on many to many field when default=None

docs/community/contributing.md

@@ -81,6 +81,23 @@ To run the tests, clone the repository, and then:
     # Run the tests
     ./runtests.py
 
+---
+
+**Note:**  
+If your tests require access to the database, do not forget to inherit from `django.test.TestCase`.  
+For example:
+
+    from django.test import TestCase
+
+    class MyDatabaseTest(TestCase):
+        def test_something(self):
+            # Your test code here
+            pass
+
+You can reuse existing models defined in `tests/models.py` for your tests.
+
+---
+
 ### Test options
 
 Run using a more concise output style.
@@ -99,6 +116,10 @@ Shorter form to run the tests for a given test method.
 
     ./runtests.py test_this_method
 
+**Note:** If you do not want the output to be captured (for example, to see print statements directly), you can use the `-s` flag:
+
+    ./runtests.py -s
+
 Note: The test case and test method matching is fuzzy and will sometimes run other tests that contain a partial string match to the given  command line input.
 
 ### Running against multiple environments

rest_framework/serializers.py

@@ -1090,6 +1090,14 @@ def get_fields(self):
         # Determine the fields that should be included on the serializer.
         fields = {}
 
+        # If it's a ManyToMany field, and the default is None, then raises an exception to prevent exceptions on .set()
+        for field_name in declared_fields.keys():
+            if field_name in info.relations and info.relations[field_name].to_many and declared_fields[field_name].default is None:
+                raise ValueError(
+                    f"The field '{field_name}' on serializer '{self.__class__.__name__}' is a ManyToMany field and cannot have a default value of None. "
+                    "Please set an appropriate default value, such as an empty list, or remove the default."
+                )
+
         for field_name in field_names:
             # If the field is explicitly declared on the class then use that.
             if field_name in declared_fields:

tests/test_serializer.py

@@ -6,12 +6,14 @@
 
 import pytest
 from django.db import models
+from django.test import TestCase
 
 from rest_framework import exceptions, fields, relations, serializers
 from rest_framework.fields import Field
 
 from .models import (
-    ForeignKeyTarget, NestedForeignKeySource, NullableForeignKeySource
+    ForeignKeyTarget, ManyToManySource, ManyToManyTarget,
+    NestedForeignKeySource, NullableForeignKeySource
 )
 from .utils import MockObject
 
@@ -64,6 +66,7 @@ def setup_method(self):
         class ExampleSerializer(serializers.Serializer):
             char = serializers.CharField()
             integer = serializers.IntegerField()
+
         self.Serializer = ExampleSerializer
 
     def test_valid_serializer(self):
@@ -774,3 +777,35 @@ def test_nested_key(self):
         ret = {'a': 1}
         self.s.set_value(ret, ['x', 'y'], 2)
         assert ret == {'a': 1, 'x': {'y': 2}}
+
+
+class TestWarningManyToMany(TestCase):
+    def test_warning_many_to_many(self):
+        """Tests that using a PrimaryKeyRelatedField for a ManyToMany field breaks with default=None."""
+        class ManyToManySourceSerializer(serializers.ModelSerializer):
+            targets = serializers.PrimaryKeyRelatedField(
+                many=True,
+                queryset=ManyToManyTarget.objects.all(),
+                default=None
+            )
+
+            class Meta:
+                model = ManyToManySource
+                fields = '__all__'
+
+        # Instantiates with invalid data (not value for a ManyToMany field to force using the default)
+        serializer = ManyToManySourceSerializer(data={
+            "name": "Invalid Example",
+        })
+
+        error_msg = "The field 'targets' on serializer 'ManyToManySourceSerializer' is a ManyToMany field and cannot have a default value of None. Please set an appropriate default value, such as an empty list, or remove the default."
+
+        # Calls to get_fields() should raise a ValueError
+        with pytest.raises(ValueError) as exc_info:
+            serializer.get_fields()
+        assert str(exc_info.value) == error_msg
+
+        # Calls to is_valid() should behave the same
+        with pytest.raises(ValueError) as exc_info:
+            serializer.is_valid(raise_exception=True)
+        assert str(exc_info.value) == error_msg