Expire memoizations on setattr/delattr, check in delattr

Fixed bug where declarative would not update the state of the
:class:`.Mapper` as far as what attributes were present, when additional
attributes were added or removed after the mapper attribute collections had
already been called and memoized.  Addtionally, a ``NotImplementedError``
is now raised if a fully mapped attribute (e.g. column, relationship, etc.)
is deleted from a class that is currently mapped, since the mapper will not
function correctly if the attribute has been removed.

Change-Id: Idaca8e0237b31aa1d6564d94c3a179d7dc6b5df9
Fixes: #4133

3 files changed, 24 insertions, 1 deletions

diff --git a/lib/sqlalchemy/ext/declarative/api.py b/lib/sqlalchemy/ext/declarative/api.py
index b08d3ce30..865cd16f0 100644
--- a/lib/sqlalchemy/ext/declarative/api.py
+++ b/lib/sqlalchemy/ext/declarative/api.py
@@ -21,7 +21,7 @@ import re
 
 from .base import _as_declarative, \
     _declarative_constructor,\
-    _DeferredMapperConfig, _add_attribute
+    _DeferredMapperConfig, _add_attribute, _del_attribute
 from .clsregistry import _class_resolver
 
 
@@ -68,6 +68,8 @@ class DeclarativeMeta(type):
     def __setattr__(cls, key, value):
         _add_attribute(cls, key, value)
 
+    def __delattr__(cls, key):
+        _del_attribute(cls, key)
 
 def synonym_for(name, map_column=False):
     """Decorator that produces an :func:`.orm.synonym` attribute in conjunction
diff --git a/lib/sqlalchemy/ext/declarative/base.py b/lib/sqlalchemy/ext/declarative/base.py
index 5d0eab34e..544bb2497 100644
--- a/lib/sqlalchemy/ext/declarative/base.py
+++ b/lib/sqlalchemy/ext/declarative/base.py
@@ -677,10 +677,29 @@ def _add_attribute(cls, key, value):
             )
         else:
             type.__setattr__(cls, key, value)
+            cls.__mapper__._expire_memoizations()
     else:
         type.__setattr__(cls, key, value)
 
 
+def _del_attribute(cls, key):
+
+    if '__mapper__' in cls.__dict__ and \
+            key in cls.__dict__ and not cls.__mapper__._dispose_called:
+        value = cls.__dict__[key]
+        if isinstance(
+                value,
+                (Column, ColumnProperty, MapperProperty, QueryableAttribute)
+        ):
+            raise NotImplementedError(
+                "Can't un-map individual mapped attributes on a mapped class.")
+        else:
+            type.__delattr__(cls, key)
+            cls.__mapper__._expire_memoizations()
+    else:
+        type.__delattr__(cls, key)
+
+
 def _declarative_constructor(self, **kwargs):
     """A simple constructor that allows initialization from kwargs.
 
diff --git a/lib/sqlalchemy/orm/mapper.py b/lib/sqlalchemy/orm/mapper.py
index a30a8c243..e856c6c79 100644
--- a/lib/sqlalchemy/orm/mapper.py
+++ b/lib/sqlalchemy/orm/mapper.py
@@ -86,6 +86,7 @@ class Mapper(InspectionAttr):
     """
 
     _new_mappers = False
+    _dispose_called = False
 
     def __init__(self,
                  class_,
@@ -1274,6 +1275,7 @@ class Mapper(InspectionAttr):
     def dispose(self):
         # Disable any attribute-based compilation.
         self.configured = True
+        self._dispose_called = True
 
         if hasattr(self, '_configure_failed'):
             del self._configure_failed