- poc fix for #3149

3 files changed, 65 insertions, 7 deletions

diff --git a/lib/sqlalchemy/ext/declarative/api.py b/lib/sqlalchemy/ext/declarative/api.py
index daf8bffb5..251b06fed 100644
--- a/lib/sqlalchemy/ext/declarative/api.py
+++ b/lib/sqlalchemy/ext/declarative/api.py
@@ -13,7 +13,7 @@ from ...orm import synonym as _orm_synonym, mapper,\
     interfaces, properties
 from ...orm.util import polymorphic_union
 from ...orm.base import _mapper_or_none
-from ...util import OrderedDict
+from ...util import OrderedDict, classproperty
 from ... import exc
 import weakref
 
@@ -164,6 +164,45 @@ class declared_attr(interfaces._MappedAttribute, property):
     def __get__(desc, self, cls):
         return desc.fget(cls)
 
+    @classproperty
+    def column(cls):
+        return _declared_column
+
+    @classproperty
+    def property(cls):
+        return _declared_property
+
+    defer_defer_defer = False
+
+
+class _memoized_declared_attr(declared_attr):
+    def __init__(self, fget, cascading=False):
+        super(_memoized_declared_attr, self).__init__(fget)
+        self.reg = weakref.WeakKeyDictionary()
+        self._cascading = cascading
+
+    def __get__(desc, self, cls):
+        if desc.defer_defer_defer:
+            return desc
+        elif cls in desc.reg:
+            return desc.reg[cls]
+        else:
+            desc.reg[cls] = obj = desc.fget(cls)
+            return obj
+
+    @classproperty
+    def cascading(cls):
+        return lambda decorated: cls(decorated, cascading=True)
+
+
+class _declared_column(_memoized_declared_attr):
+    pass
+
+
+class _declared_property(_memoized_declared_attr):
+    defer_defer_defer = True
+
+
 
 def declarative_base(bind=None, metadata=None, mapper=None, cls=object,
                      name='Base', constructor=_declarative_constructor,
diff --git a/lib/sqlalchemy/ext/declarative/base.py b/lib/sqlalchemy/ext/declarative/base.py
index 94baeeb51..cee22637d 100644
--- a/lib/sqlalchemy/ext/declarative/base.py
+++ b/lib/sqlalchemy/ext/declarative/base.py
@@ -33,7 +33,7 @@ def _declared_mapping_info(cls):
 
 
 def _as_declarative(cls, classname, dict_):
-    from .api import declared_attr
+    from .api import declared_attr, _memoized_declared_attr
 
     # dict_ will be a dictproxy, which we can't write to, and we need to!
     dict_ = dict(dict_)
@@ -132,6 +132,14 @@ def _as_declarative(cls, classname, dict_):
                         "column_property(), relationship(), etc.) must "
                         "be declared as @declared_attr callables "
                         "on declarative mixin classes.")
+                elif isinstance(obj, _memoized_declared_attr): # and \
+                    if obj._cascading:
+                        dict_[name] = ret = obj.__get__(obj, cls)
+                    else:
+                        dict_[name] = ret = getattr(cls, name)
+                    if isinstance(ret, (Column, MapperProperty)) and \
+                            ret.doc is None:
+                        ret.doc = obj.__doc__
                 elif isinstance(obj, declarative_props):
                     dict_[name] = ret = \
                         column_copies[obj] = getattr(cls, name)
@@ -148,6 +156,7 @@ def _as_declarative(cls, classname, dict_):
 
     clsregistry.add_class(classname, cls)
     our_stuff = util.OrderedDict()
+    add_later = util.OrderedDict()
 
     for k in list(dict_):
 
@@ -157,7 +166,10 @@ def _as_declarative(cls, classname, dict_):
 
         value = dict_[k]
         if isinstance(value, declarative_props):
-            value = getattr(cls, k)
+            if value.defer_defer_defer:
+                add_later[k] = value
+            else:
+                value = getattr(cls, k)
 
         elif isinstance(value, QueryableAttribute) and \
                 value.class_ is not cls and \
@@ -324,7 +336,8 @@ def _as_declarative(cls, classname, dict_):
                  declared_columns,
                  column_copies,
                  our_stuff,
-                 mapper_args_fn)
+                 mapper_args_fn,
+                 add_later)
     if not defer_map:
         mt.map()
 
@@ -339,7 +352,8 @@ class _MapperConfig(object):
                  inherits,
                  declared_columns,
                  column_copies,
-                 properties, mapper_args_fn):
+                 properties, mapper_args_fn,
+                 add_later):
         self.mapper_cls = mapper_cls
         self.cls = cls
         self.local_table = table
@@ -348,6 +362,7 @@ class _MapperConfig(object):
         self.mapper_args_fn = mapper_args_fn
         self.declared_columns = declared_columns
         self.column_copies = column_copies
+        self.add_later = add_later
 
     def _prepare_mapper_arguments(self):
         properties = self.properties
@@ -410,6 +425,8 @@ class _MapperConfig(object):
             self.local_table,
             **mapper_args
         )
+        for k, v in self.add_later.items():
+            setattr(self.cls, k, v.fget(self.cls))
 
 
 class _DeferredMapperConfig(_MapperConfig):
diff --git a/lib/sqlalchemy/sql/schema.py b/lib/sqlalchemy/sql/schema.py
index f3af46c40..ea8c32fd4 100644
--- a/lib/sqlalchemy/sql/schema.py
+++ b/lib/sqlalchemy/sql/schema.py
@@ -1161,8 +1161,10 @@ class Column(SchemaItem, ColumnClause):
         existing = getattr(self, 'table', None)
         if existing is not None and existing is not table:
             raise exc.ArgumentError(
-                "Column object already assigned to Table '%s'" %
-                existing.description)
+                "Column object '%s' already assigned to Table '%s'" % (
+                    self.key,
+                    existing.description
+                ))
 
         if self.key in table._columns:
             col = table._columns.get(self.key)