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

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
@@ -531,7 +531,30 @@ 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):