🐛 ManyRelatedField get_value clearing field on partial update

A PATCH to a serializer's non-related CharField was clearing an ancillary StringRelatedField(many=True) field.
The issue appears to be in the ManyRelatedField's get_value method, which was returning a [] instead of empty
when the request data was a MultiDict.

This fix mirrors code in fields.py, class Field, get_value, Ln. 272, which explicitly returns empty on a partial update.

Tests added to demonstrate the issue.

rest_framework/relations.py

@@ -338,7 +338,12 @@ class ManyRelatedField(Field):
         # We override the default field access in order to support
         # lists in HTML forms.
         if html.is_html_input(dictionary):
+            # Don't return [] if the update is partial
+            if self.field_name not in dictionary:
+                if getattr(self.root, 'partial', False):
+                    return empty
             return dictionary.getlist(self.field_name)
+
         return dictionary.get(self.field_name, empty)
 
     def to_internal_value(self, data):

tests/test_relations.py

-from .utils import mock_reverse, fail_reverse, BadType, MockObject, MockQueryset
+import pytest
+
 from django.core.exceptions import ImproperlyConfigured
+from django.utils.datastructures import MultiValueDict
+
 from rest_framework import serializers
+from rest_framework.fields import empty
 from rest_framework.test import APISimpleTestCase
-import pytest
+
+from .utils import mock_reverse, fail_reverse, BadType, MockObject, MockQueryset
 
 
 class TestStringRelatedField(APISimpleTestCase):
@@ -134,3 +139,34 @@ class TestSlugRelatedField(APISimpleTestCase):
     def test_representation(self):
         representation = self.field.to_representation(self.instance)
         assert representation == self.instance.name
+
+
+class TestManyRelatedField(APISimpleTestCase):
+    def setUp(self):
+        self.instance = MockObject(pk=1, name='foo')
+        self.field = serializers.StringRelatedField(many=True)
+        self.field.field_name = 'foo'
+
+    def test_get_value_regular_dictionary_full(self):
+        assert 'bar' == self.field.get_value({'foo': 'bar'})
+        assert empty == self.field.get_value({'baz': 'bar'})
+
+    def test_get_value_regular_dictionary_partial(self):
+        setattr(self.field.root, 'partial', True)
+        assert 'bar' == self.field.get_value({'foo': 'bar'})
+        assert empty == self.field.get_value({'baz': 'bar'})
+
+    def test_get_value_multi_dictionary_full(self):
+        mvd = MultiValueDict({'foo': ['bar1', 'bar2']})
+        assert ['bar1', 'bar2'] == self.field.get_value(mvd)
+
+        mvd = MultiValueDict({'baz': ['bar1', 'bar2']})
+        assert [] == self.field.get_value(mvd)
+
+    def test_get_value_multi_dictionary_partial(self):
+        setattr(self.field.root, 'partial', True)
+        mvd = MultiValueDict({'foo': ['bar1', 'bar2']})
+        assert ['bar1', 'bar2'] == self.field.get_value(mvd)
+
+        mvd = MultiValueDict({'baz': ['bar1', 'bar2']})
+        assert empty == self.field.get_value(mvd)