[Sqlalchemy-tickets] [sqlalchemy] #2952: "cascading" declared_attr ?

[sqlalchemy <mi...@zz...>]

#2952: "cascading" declared_attr ?
-------------------------+------------------------------------
 Reporter:  zzzeek       |           Owner:  zzzeek
     Type:  enhancement  |          Status:  new
 Priority:  high         |       Milestone:  0.9.xx
Component:  declarative  |        Severity:  major - 1-3 hours
 Keywords:               |  Progress State:  not decided upon
-------------------------+------------------------------------
 use case:

 {{{
 #!python
 from sqlalchemy import *
 from sqlalchemy.orm import *
 from sqlalchemy.ext.declarative import declarative_base, declared_attr,
 cascading_declared_attr

 Base = declarative_base()

 class HasId(object):
     @cascading_declared_attr
     def id(cls):
         if cls.__name__=='Content':
             return Column('id', Integer, primary_key=True)
         else:
             return Column('id', Integer, ForeignKey('content.id'),
 primary_key=True)

 class Content(HasId, Base):

     content_type = Column(String(20))

     @cascading_declared_attr
     def __tablename__(cls):
         return cls.__name__.lower()

 class Project(Content):
     currency  = Column(String(3))
     target = Column(Integer)
     current = Column(Integer)
 }}}

 {{{
 #!diff
 diff --git a/lib/sqlalchemy/ext/declarative/__init__.py
 b/lib/sqlalchemy/ext/declarative/__init__.py
 index 0ee4e33..4566324 100644
 --- a/lib/sqlalchemy/ext/declarative/__init__.py
 +++ b/lib/sqlalchemy/ext/declarative/__init__.py
 @@ -1300,7 +1300,7 @@ Mapped instances then make usage of
  from .api import declarative_base, synonym_for, comparable_using, \
      instrument_declarative, ConcreteBase, AbstractConcreteBase, \
      DeclarativeMeta, DeferredReflection, has_inherited_table,\
 -    declared_attr, as_declarative
 +    declared_attr, cascading_declared_attr, as_declarative


  __all__ = ['declarative_base', 'synonym_for', 'has_inherited_table',
 diff --git a/lib/sqlalchemy/ext/declarative/api.py
 b/lib/sqlalchemy/ext/declarative/api.py
 index 2418c6e..dc6c42b 100644
 --- a/lib/sqlalchemy/ext/declarative/api.py
 +++ b/lib/sqlalchemy/ext/declarative/api.py
 @@ -162,6 +162,26 @@ class declared_attr(interfaces._MappedAttribute,
 property):
      def __get__(desc, self, cls):
          return desc.fget(cls)

 +class cascading_declared_attr(declared_attr):
 +    """A :class:`.declared_attr` that will be invoked for all subclasses.
 +
 +    Use :class:`.cascading_declared_attr` when a particular
 ``@declared_attr``
 +    needs to be invoked individually for each subclass in a hierarchy::
 +
 +        class HasId(object):
 +            @cascading_declared_attr
 +            def id(cls):
 +                if has_inherited_table(cls):
 +                    return Column(Integer, ForeignKey("content.id"),
 primary_key=True)
 +                else:
 +                    return Column(Integer, primary_key=True)
 +
 +        class Content(HasId, Base):
 +            pass
 +
 +        class SubContent(Content):
 +            pass
 +    """

  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 4fda9c7..a8702cc 100644
 --- a/lib/sqlalchemy/ext/declarative/base.py
 +++ b/lib/sqlalchemy/ext/declarative/base.py
 @@ -31,7 +31,7 @@ def _declared_mapping_info(cls):


  def _as_declarative(cls, classname, dict_):
 -    from .api import declared_attr
 +    from .api import declared_attr, cascading_declared_attr

      # dict_ will be a dictproxy, which we can't write to, and we need to!
      dict_ = dict(dict_)
 @@ -96,6 +96,12 @@ def _as_declarative(cls, classname, dict_):
                              "not applying to subclass %s."
                              % (base.__name__, name, base, cls))
                  continue
 +            elif isinstance(obj, cascading_declared_attr):
 +                ret = obj.__get__(obj, cls)
 +                dict_[name] = column_copies[obj] = ret
 +                if isinstance(ret, (Column, MapperProperty)) and \
 +                    ret.doc is None:
 +                    ret.doc = obj.__doc__
              elif base is not cls:
                  # we're a mixin.
                  if isinstance(obj, Column):
 @@ -125,8 +131,8 @@ def _as_declarative(cls, classname, dict_):
                          "be declared as @declared_attr callables "
                          "on declarative mixin classes.")
                  elif isinstance(obj, declarative_props):
 -                    dict_[name] = ret = \
 -                            column_copies[obj] = getattr(cls, name)
 +                    ret = getattr(cls, name)
 +                    dict_[name] = column_copies[obj] = ret
                      if isinstance(ret, (Column, MapperProperty)) and \
                          ret.doc is None:
                          ret.doc = obj.__doc__
 }}}

--
Ticket URL: <http://www.sqlalchemy.org/trac/ticket/2952>
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