Fix: write failure on nullable data relation fields

Closes #1159.

CHANGES.rst

@@ -10,6 +10,7 @@ Unreleased
 
 Fixed
 ~~~~~
+- Updating a field with a nullable data relation fails when value is null (`#1159`_)
 - ``cerberus.schema.SchemaError`` when ``VALIDATE_FILTERS = True``. (`#1154`_)
 - Serializers fails when array of types is in schema. (`#1112`_)
 - Replace the broken ``make audit`` shortcut with ``make check``, add the
@@ -52,6 +53,7 @@ Docs
 .. _`#1156`: https://github.com/pyeve/eve/issues/1156
 .. _`#1157`: https://github.com/pyeve/eve/issues/1157
 .. _`#1158`: https://github.com/pyeve/eve/issues/1158
+.. _`#1159`: https://github.com/pyeve/eve/issues/1159
 
 Version 0.8
 -----------

eve/io/mongo/validation.py

@@ -128,7 +128,7 @@ def _validate_data_relation(self, data_relation, field, value):
                 'embeddable': {'type': 'boolean', 'default': False},
                 'version': {'type': 'boolean', 'default': False}
              }} """
-        if not value and config.DOMAIN[self.resource]["schema"][field].get("nullable"):
+        if not value and self.schema[field].get("nullable"):
             return
 
         if "version" in data_relation and data_relation["version"] is True:

eve/tests/methods/post.py

@@ -892,21 +892,25 @@ def test_post_updating_a_document_with_nullable_data_relation_does_not_fail(self
         # See #1159.
         del (self.domain["contacts"]["schema"]["ref"]["required"])
 
-        user_schema = {
+        employee = {
             "employer": {
                 "type": "objectid",
                 "nullable": True,
-                "data_relation": {"resource": self.known_resource, "field": "_id"},
+                "data_relation": {"resource": self.known_resource},
             }
         }
-        self.app.register_resource("user_schema", {"schema": user_schema})
+        self.app.register_resource("employee", {"schema": employee})
 
         data = {"employer": None}
-        r, s = self.post("user_schema", data=data)
+        r, s = self.post("employee", data=data)
         self.assert201(s)
 
-        user_schema["employer"]["nullable"] = False
-        r, s = self.post("user_schema", data=data)
+        employee["employer"]["nullable"] = False
+        r, s = self.post("employee", data=data)
+        self.assert422(s)
+
+        del (employee["employer"]["nullable"])
+        r, s = self.post("employee", data=data)
         self.assert422(s)
 
     def perform_post(self, data, valid_items=[0]):