fix: Convert NDB keys to Datastore keys for serialization.

See comments in code for design considerations. This is a compromise
solution.

Fixes googleapis#284.

google/cloud/ndb/model.py

@@ -1687,7 +1687,23 @@ def _validate(self, value):
         """
         methods = self._find_methods("_validate", "_to_base_type")
         call = self._apply_list(methods)
-        return call(value)
+        value = call(value)
+
+        # Legacy NDB, because it didn't delegate to Datastore for serializing
+        # entities, would directly write a Key protocol buffer for a key. We,
+        # however, need to transform NDB keys to Datastore keys before
+        # delegating to Datastore to generate protocol buffers. You might be
+        # tempted to do this in KeyProperty._to_base_type, and that works great
+        # for properties of KeyProperty type. If, however, you're computing a
+        # key in a ComputedProperty, ComputedProperty doesn't know to call
+        # KeyProperty's base type. (Probably ComputedProperty should take
+        # another property type as a constructor argument for this purpose,
+        # but that wasn't part of the original design and adding it introduces
+        # backwards compatibility issues.) See: Issue #184
+        if isinstance(value, key_module.Key):
+            value = value._key  # Datastore key
+
+        return value
 
     def _call_shallow_validation(self, value):
         """Call the "initial" set of ``_validate()`` methods.

tests/system/test_crud.py

@@ -1004,3 +1004,36 @@ class SomeKind(ndb.Model):
 
         key.delete()
         assert key.get() is None
+
+
+@pytest.mark.usefixtures("client_context")
+def test_computed_key_property(dispose_of):
+    """Regression test for #284.
+
+    https://github.com/googleapis/python-ndb/issues/284
+    """
+
+    class AModel(ndb.Model):
+        s_foo = ndb.StringProperty()
+
+    class BModel(ndb.Model):
+        s_bar = ndb.StringProperty()
+        key_a = ndb.KeyProperty(kind="AModel", indexed=True)
+
+    class CModel(ndb.Model):
+        s_foobar = ndb.StringProperty()
+        key_b = ndb.KeyProperty(kind="BModel", indexed=True)
+        key_a = ndb.ComputedProperty(  # Issue here
+            lambda self: self.key_b.get().key_a if self.key_b else None,
+        )
+
+    key_a = AModel(s_foo="test").put()
+    dispose_of(key_a._key)
+    key_b = BModel(s_bar="test", key_a=key_a).put()
+    dispose_of(key_b._key)
+    key_c = CModel(s_foobar="test", key_b=key_b).put()
+    dispose_of(key_c._key)
+
+    entity = key_c.get()
+    assert entity.key_a == key_a
+    assert entity.key_b == key_b