the typical hook for this kind of thing is __declare_last__, works after
everything has been compiled.
you might have an easier time also (or might not matter) if you build against
the Table (i.e. Foo.__table__.alias(), etc.) instead of firing up the class
with aliased(Foo), but if you're in __declare_last__ might not matter anyway.
On Mar 18, 2013, at 12:20 PM, millerdev <miller...@gmail.com> wrote:
> Hi,
>
> Using declarative here, and I'm trying to create a column_property with a
> correlated subquery that returns a count of records with a matching value in
> some other column. Here's what I've tried. Option 1 is the best, option 2 is
> ugly but second best, option 3 is not a good option since there are many
> other classes involved and the place where I'd need to put that code is far
> away from where it logically belongs.
>
> from sqlalchemy import *
> from sqlalchemy.orm import *
> from sqlalchemy.ext.declarative import *
> from sqlalchemy.ext.declarative import declared_attr
>
> Base = declarative_base()
> option = 1
>
> class Foo(Base):
> __tablename__ = 'foo'
> id = Column(Integer, primary_key=True)
> bar_id = Column(Integer, ForeignKey("bar.id"))
> name = Column(String)
>
> if option == 1:
> # does not work (see first traceback below)
> @declared_attr
> def name_count(cls):
> clx = aliased(cls)
> return column_property(
> select(func.count([clx.id]))
> .where(clx.name == cls.name)
> .correlate(cls.__table__))
>
> if option == 2:
> # does not work (see second traceback below)
> _foo = aliased(Foo)
> Foo.name_count = column_property(
> select([func.count(_foo.id)])
> .where(_foo.name == Foo.name)
> .correlate(Foo.__table__))
>
>
> class Bar(Base):
> __tablename__ = 'bar'
> id = Column(Integer, primary_key=True)
> name = Column(String)
>
>
> if option == 3:
> # works, but really not where I want to put this code
> _foo = aliased(Foo)
> Foo.name_count = column_property(
> select([func.count(_foo.id)])
> .where(_foo.name == Foo.name)
> .correlate(Foo.__table__))
>
>
> Option 1 traceback:
>
> Traceback (most recent call last):
> File "temp/example.py", line 8, in <module>
> class Foo(Base):
> File ".../python2.7/site-packages/sqlalchemy/ext/declarative.py", line
> 1348, in __init__
> _as_declarative(cls, classname, cls.__dict__)
> File ".../python2.7/site-packages/sqlalchemy/ext/declarative.py", line
> 1181, in _as_declarative
> value = getattr(cls, k)
> File ".../python2.7/site-packages/sqlalchemy/ext/declarative.py", line
> 1554, in __get__
> return desc.fget(cls)
> File "temp/example.py", line 15, in name_count
> clx = aliased(cls)
> File ".../python2.7/site-packages/sqlalchemy/orm/util.py", line 385, in
> aliased
> return AliasedClass(element, alias=alias, name=name,
> adapt_on_names=adapt_on_names)
> File ".../python2.7/site-packages/sqlalchemy/orm/util.py", line 298, in
> __init__
> self.__mapper = _class_to_mapper(cls)
> File ".../python2.7/site-packages/sqlalchemy/orm/util.py", line 673, in
> _class_to_mapper
> raise exc.UnmappedClassError(class_or_mapper)
> sqlalchemy.orm.exc.UnmappedClassError: Class '__main__.Foo' is not mapped
>
>
> Option 2 traceback:
>
> Traceback (most recent call last):
> File "temp/example.py", line 16, in <module>
> select([func.count(_foo.id)])
> File ".../python2.7/site-packages/sqlalchemy/sql/expression.py", line 1229,
> in __call__
> return func(*c, **o)
> File ".../python2.7/site-packages/sqlalchemy/sql/functions.py", line 16, in
> __call__
> args = [_literal_as_binds(c) for c in args]
> File ".../python2.7/site-packages/sqlalchemy/sql/expression.py", line 1440,
> in _literal_as_binds
> return element.__clause_element__()
> File ".../python2.7/site-packages/sqlalchemy/orm/attributes.py", line 117,
> in __clause_element__
> return self.comparator.__clause_element__()
> File ".../python2.7/site-packages/sqlalchemy/util/langhelpers.py", line
> 506, in oneshot
> result = self.fget(obj, *args, **kw)
> File ".../python2.7/site-packages/sqlalchemy/orm/properties.py", line 156,
> in __clause_element__
> return self.adapter(self.prop.columns[0])
> File ".../python2.7/site-packages/sqlalchemy/orm/util.py", line 334, in
> __adapt_element
> return self.__adapter.traverse(elem).\
> File ".../python2.7/site-packages/sqlalchemy/sql/visitors.py", line 185, in
> traverse
> return replacement_traverse(obj, self.__traverse_options__, replace)
> File ".../python2.7/site-packages/sqlalchemy/sql/visitors.py", line 281, in
> replacement_traverse
> obj = clone(obj, **opts)
> File ".../python2.7/site-packages/sqlalchemy/sql/visitors.py", line 270, in
> clone
> newelem = replace(elem)
> File ".../python2.7/site-packages/sqlalchemy/sql/visitors.py", line 182, in
> replace
> e = v.replace(elem)
> File ".../python2.7/site-packages/sqlalchemy/sql/util.py", line 720, in
> replace
> return self._corresponding_column(col, True)
> File ".../python2.7/site-packages/sqlalchemy/sql/util.py", line 695, in
> _corresponding_column
> require_embedded=require_embedded)
> File ".../python2.7/site-packages/sqlalchemy/sql/expression.py", line 2492,
> in corresponding_column
> if self.c.contains_column(column):
> File ".../python2.7/site-packages/sqlalchemy/util/langhelpers.py", line
> 485, in __get__
> obj.__dict__[self.__name__] = result = self.fget(obj)
> File ".../python2.7/site-packages/sqlalchemy/sql/expression.py", line 2558,
> in columns
> self._populate_column_collection()
> File ".../python2.7/site-packages/sqlalchemy/sql/expression.py", line 3704,
> in _populate_column_collection
> col._make_proxy(self)
> File ".../python2.7/site-packages/sqlalchemy/schema.py", line 1103, in
> _make_proxy
> fk = [ForeignKey(f.column) for f in self.foreign_keys]
> File ".../python2.7/site-packages/sqlalchemy/util/langhelpers.py", line
> 485, in __get__
> obj.__dict__[self.__name__] = result = self.fget(obj)
> File ".../python2.7/site-packages/sqlalchemy/schema.py", line 1392, in
> column
> tname)
> sqlalchemy.exc.NoReferencedTableError: Foreign key associated with column
> 'foo.bar_id' could not find table 'bar' with which to generate a foreign key
> to target column 'id'
>
>
> It looks like the problem is that mapper state is not being initialized
> properly prior to setting up the column_property in the first two options. Is
> there another type of property similar to declared_attr that will allow me to
> add the column_property some place in the initialization sequence where
> mappers have been sufficiently initialized to allow constructing a select
> statement?
>
> ~ Daniel
>
>
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