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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