On Apr 7, 2011, at 2:30 PM, Lars wrote:
> Hello,
>
> I am pretty determined to find a way to get (a simplified version of)
> multiple inheritance working with SA. The simplification lies in that
> no overriding of attributes will be possible (but I don't know whether
> that is significant). I was thinking of a schema as follows:
>
> --------------------------------------------------------------------------------------
>
> metadata = MetaData()
>
> base1_table = Table("base1_table", metadata,
> Column('id', Integer, primary_key=True),
> Column('str', String)
> )
>
> base2_table = Table("base2_table", metadata,
> Column('id', Integer, primary_key=True),
> Column('int', Integer)
> )
>
> claz1_table = Table("claz1_table", metadata,
> Column('id', Integer, primary_key=True),
> Column('base1_id', None, ForeignKey('base1_table.id')),
> Column('base2_id', None, ForeignKey('base2_table.id')),
> Column('assoc_id', None, ForeignKey('assoc_table.id'))
> )
>
> assoc_table = Table("assoc_table", metadata,
> Column('id', Integer, primary_key=True),
> Column('name', String(50), nullable=False),
> Column('type', String(50), nullable=False)
> )
>
> base3_table = Table("base3_table", metadata,
> Column('id', Integer, primary_key=True),
> Column('assoc_id', None, ForeignKey('assoc_table.id')),
> Column('bool', Boolean)
> )
>
> claz2_table = Table("claz2_table", metadata,
> Column('id', Integer, primary_key=True),
> Column('base3_id', None, ForeignKey('base3_table.id')),
> Column('date', Date)
> )
>
> class base1(object):
> pass
> class base2(object):
> pass
> class base3(object):
> pass
> class claz1(base1, base2):
> pass
> class claz2(base3):
> pass
>
> # do mappings, relationships and e.g. be able to
>
> c1 = claz1(str = "hello", int = 17)
> setattr(c1, name, claz2(bool = True, date = Date(2010,9,10)))
You can just forego the "inherits" flag and map each class to the appropriate
join or base table. claz1 would be mapped to a join of the three tables
involved. The difficult part of course is the query side, if you're looking
to query "base1" or "base2" and get back "claz1" objects.
Alternatively, each class can be mapped to one table only, and relationship()
used to link to other tables. Again if you don't use the "inherits" flag,
you can maintain the class hierarchy on the Python side and use association
proxies to provide local access to attributes that are normally on the related
class. This would still not give you polymorphic loading but would grant a
little more flexibility in which tables are queried to start.
>
> -----------------------------------------------------------------------------------------------------------------
>
> I am still pretty new to SA. Can anyone give me any hints, tips,
> issues with this scheme (e.g. about how to do the mappings,
> descriptors, etc)?
>
> The step after will be to write factory functions/metaclasses to
> generate these dynamically.
>
> Multiple inheritance is very important for my use case.
>
> Cheers, Lars
>
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