Handle unset fields with 'many=True' (#7574)

* Handle unset fields with 'many=True'

The docs note:

  When serializing fields with dotted notation, it may be necessary to
  provide a `default` value if any object is not present or is empty
  during attribute traversal.

However, this doesn't work for fields with 'many=True'. When using
these, the default is simply ignored.

The solution is simple: do in 'ManyRelatedField' what we were already
doing for 'Field', namely, catch possible 'AttributeError' and
'KeyError' exceptions and return the default if there is one set.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes: #7550

* Add test cases for #7550

Signed-off-by: Stephen Finucane <stephen@that.guru>

rest_framework/relations.py

@@ -10,7 +10,7 @@ from django.utils.encoding import smart_str, uri_to_iri
 from django.utils.translation import gettext_lazy as _
 
 from rest_framework.fields import (
-    Field, empty, get_attribute, is_simple_callable, iter_options
+    Field, SkipField, empty, get_attribute, is_simple_callable, iter_options
 )
 from rest_framework.reverse import reverse
 from rest_framework.settings import api_settings
@@ -535,7 +535,30 @@ class ManyRelatedField(Field):
         if hasattr(instance, 'pk') and instance.pk is None:
             return []
 
-        relationship = get_attribute(instance, self.source_attrs)
+        try:
+            relationship = get_attribute(instance, self.source_attrs)
+        except (KeyError, AttributeError) as exc:
+            if self.default is not empty:
+                return self.get_default()
+            if self.allow_null:
+                return None
+            if not self.required:
+                raise SkipField()
+            msg = (
+                'Got {exc_type} when attempting to get a value for field '
+                '`{field}` on serializer `{serializer}`.\nThe serializer '
+                'field might be named incorrectly and not match '
+                'any attribute or key on the `{instance}` instance.\n'
+                'Original exception text was: {exc}.'.format(
+                    exc_type=type(exc).__name__,
+                    field=self.field_name,
+                    serializer=self.parent.__class__.__name__,
+                    instance=instance.__class__.__name__,
+                    exc=exc
+                )
+            )
+            raise type(exc)(msg)
+
         return relationship.all() if hasattr(relationship, 'all') else relationship
 
     def to_representation(self, iterable):

tests/test_model_serializer.py

@@ -1025,6 +1025,73 @@ class Issue2704TestCase(TestCase):
         assert serializer.data == expected
 
 
+class Issue7550FooModel(models.Model):
+    text = models.CharField(max_length=100)
+    bar = models.ForeignKey(
+        'Issue7550BarModel', null=True, blank=True, on_delete=models.SET_NULL,
+        related_name='foos', related_query_name='foo')
+
+
+class Issue7550BarModel(models.Model):
+    pass
+
+
+class Issue7550TestCase(TestCase):
+
+    def test_dotted_source(self):
+
+        class _FooSerializer(serializers.ModelSerializer):
+            class Meta:
+                model = Issue7550FooModel
+                fields = ('id', 'text')
+
+        class FooSerializer(serializers.ModelSerializer):
+            other_foos = _FooSerializer(source='bar.foos', many=True)
+
+            class Meta:
+                model = Issue7550BarModel
+                fields = ('id', 'other_foos')
+
+        bar = Issue7550BarModel.objects.create()
+        foo_a = Issue7550FooModel.objects.create(bar=bar, text='abc')
+        foo_b = Issue7550FooModel.objects.create(bar=bar, text='123')
+
+        assert FooSerializer(foo_a).data == {
+            'id': foo_a.id,
+            'other_foos': [
+                {
+                    'id': foo_a.id,
+                    'text': foo_a.text,
+                },
+                {
+                    'id': foo_b.id,
+                    'text': foo_b.text,
+                },
+            ],
+        }
+
+    def test_dotted_source_with_default(self):
+
+        class _FooSerializer(serializers.ModelSerializer):
+            class Meta:
+                model = Issue7550FooModel
+                fields = ('id', 'text')
+
+        class FooSerializer(serializers.ModelSerializer):
+            other_foos = _FooSerializer(source='bar.foos', default=[], many=True)
+
+            class Meta:
+                model = Issue7550FooModel
+                fields = ('id', 'other_foos')
+
+        foo = Issue7550FooModel.objects.create(bar=None, text='abc')
+
+        assert FooSerializer(foo).data == {
+            'id': foo.id,
+            'other_foos': [],
+        }
+
+
 class DecimalFieldModel(models.Model):
     decimal_field = models.DecimalField(
         max_digits=3,