Test using model objects for dotted source default (#5880)

… when path components may be null.

Ref #5375, #5727

tests/models.py

@@ -69,6 +69,17 @@ class NullableUUIDForeignKeySource(RESTFrameworkModel):
                                on_delete=models.CASCADE)
 
 
+class NestedForeignKeySource(RESTFrameworkModel):
+    """
+    Used for testing FK chain. A -> B -> C.
+    """
+    name = models.CharField(max_length=100)
+    target = models.ForeignKey(NullableForeignKeySource, null=True, blank=True,
+                               related_name='nested_sources',
+                               verbose_name='Intermediate target object',
+                               on_delete=models.CASCADE)
+
+
 # OneToOne
 class OneToOneTarget(RESTFrameworkModel):
     name = models.CharField(max_length=100)

tests/test_serializer.py

@@ -14,6 +14,9 @@ from rest_framework import fields, relations, serializers
 from rest_framework.compat import unicode_repr
 from rest_framework.fields import Field
 
+from .models import (
+    ForeignKeyTarget, NestedForeignKeySource, NullableForeignKeySource
+)
 from .utils import MockObject
 
 try:
@@ -453,6 +456,22 @@ class TestDefaultOutput:
 
         assert Serializer({'a': {'b': {'c': 'abc'}}}).data == {'c': 'abc'}
 
+        # Same test using model objects to exercise both paths in
+        # rest_framework.fields.get_attribute() (#5880)
+        class ModelSerializer(serializers.Serializer):
+            target = serializers.CharField(default='x', source='target.target.name')
+
+        a = NestedForeignKeySource(name="Root Object", target=None)
+        assert ModelSerializer(a).data == {'target': 'x'}
+
+        b = NullableForeignKeySource(name="Intermediary Object", target=None)
+        a.target = b
+        assert ModelSerializer(a).data == {'target': 'x'}
+
+        c = ForeignKeyTarget(name="Target Object")
+        b.target = c
+        assert ModelSerializer(a).data == {'target': 'Target Object'}
+
     def test_default_for_nested_serializer(self):
         class NestedSerializer(serializers.Serializer):
             a = serializers.CharField(default='1')