Add a test for persisting a new doc containing nothing but a custom ID

This closes #1246.

tests/document/instance.py

@@ -364,6 +364,22 @@ class User(Document):
         exc = e.exception
         self.assertTrue("Field is required: ['username']" in exc.message)
 
+    def test_custom_id_field_save_doc_with_nothing_but_pk(self):
+        """Ensure we can still save a doc even if it only has a PK."""
+        class Sample(Document):
+            key = StringField(primary_key=True)
+            count = IntField()
+
+        Sample.drop_collection()
+
+        sample = Sample()
+        sample.key = '123'
+        sample.save()
+
+        sample = Sample.objects.first()
+        self.assertEqual(sample.key, '123')
+        self.assertEqual(sample.pk, '123')
+
     def test_document_not_registered(self):
         class Place(Document):
             name = StringField()