[Sqlalchemy-tickets] Issue #3150: use-case specific declared_attr (mixin.*?) descriptors (zzzeek/sqlalchemy)

[Mike B. <iss...@bi...>]

New issue 3150: use-case specific declared_attr (mixin.*?) descriptors
https://bitbucket.org/zzzeek/sqlalchemy/issue/3150/use-case-specific-declared_attr-mixin

Mike Bayer:

poc should allow us to consolidate #2670, #2952, #3149, #3050.

the new descriptors include the ability to cache the result per class, or to do "cascade", guarantees that the callable fn is called only once per target class, as well as to name attributes that are set up after the mapping is complete, so that relationship and column_property declared_attrs have the whole mapping to work with when they are called.  and the whole thing doesn't modify any existing functionality, only adds new things we can take time to stabilize.  win win win win.


```
#!python

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base, declared_attr, has_inherited_table

Base = declarative_base()


class VehicleModel(Base):
    __tablename__ = "vehicle_model"
    id = Column(Integer, primary_key=True)
    name = Column(String(20))


class VehicleInfo(object):
    vehicle_plate_region = Column(String(5))
    vehicle_plate = Column(String(20))

    @declared_attr.column
    def vehicle_model_id(cls):
        return Column(Integer, ForeignKey("vehicle_model.id"))

    @declared_attr.property
    def vehicle_model(cls):
        # 1. called after the class is fully mapped
        # 2. called only once for each class

        assert cls.__table__ is not None and \
            cls.__table__.c.vehicle_model_id.shares_lineage(
                cls.vehicle_model_id.__clause_element__()
            )

        return relationship(VehicleModel, foreign_keys=[cls.vehicle_model_id])

    @declared_attr.column.cascading
    def id(cls):
        if has_inherited_table(cls):
            return Column(Integer, ForeignKey("vehicle.id"), primary_key=True)
        else:
            return Column(Integer, primary_key=True)


class Vehicle(VehicleInfo, Base):
    __tablename__ = 'vehicle'


class SubVehicle(Vehicle):
    __tablename__ = 'subveh'

    @declared_attr.property
    def some_other_thing(cls):
        # called way at the end
        assert cls.id.__clause_element__().references(Vehicle.__table__.c.id)
        return column_property(cls.id)

configure_mappers()


```

poc patch, however still needs integration for #2670:


```
#!diff

diff --git a/lib/sqlalchemy/ext/declarative/api.py b/lib/sqlalchemy/ext/declarative/api.py
index daf8bff..8adec1a 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 +164,49 @@ 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 desc._cascading:
+            search = [cls]
+        else:
+            search = cls.__mro__
+        for super_ in search:
+            if super_ in desc.reg:
+                return desc.reg[super_]
+        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 94baeeb..cee2263 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 f3af46c..ea8c32f 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)

```