Ian Fiddes Ian Fiddes - 2 months ago 34
Python Question

Using multiple levels of inheritance with sqlalchemy declarative base

I have many tables with identical columns. The difference is the table names themselves. I want to set up a inheritance chain to minimize code duplication. The following single layer inheritance works the way I want it to:

from sqlalchemy import Column, Integer, Text
from sqlalchemy.ext.declarative import declarative_base, declared_attr
from sqlalchemy.orm import sessionmaker

engine = sqlalchemy.create_engine('sqlite:///monDom5.db')


class Base(object):
"""base for all table classes"""
__abstract__ = True
__table_args__ = {'autoload': True, 'autoload_with': engine}
@declared_attr
def __tablename__(cls):
return cls.__name__.lower()


Base = declarative_base(cls=Base)


class TransMap_HgmIntronVector(Base):
AlignmentId = Column(Text, primary_key=True)


But requires me to specify the
AlignmentId
column for every instance of the
Hgm
base. I would instead like to do this, but get a
sqlalchemy.exc.InvalidRequestError
when I try to actually use it:

from sqlalchemy import Column, Integer, Text
from sqlalchemy.ext.declarative import declarative_base, declared_attr
from sqlalchemy.orm import sessionmaker

engine = sqlalchemy.create_engine('sqlite:///monDom5.db')

class Base(object):
"""base for all table classes"""
__abstract__ = True
__table_args__ = {'autoload': True, 'autoload_with': engine}
@declared_attr
def __tablename__(cls):
return cls.__name__.lower()


# model for all Hgm tables

class Hgm(Base):
__abstract__ = True
AlignmentId = Column(Text, primary_key=True)


Base = declarative_base(cls=Hgm)

class TransMap_HgmIntronVector(Hgm):
pass



metadata = Base.metadata
Session = sessionmaker(bind=engine)
session = Session()


Leads to the error

>>> metadata = Base.metadata
>>> Session = sessionmaker(bind=engine)
>>> session = Session()
>>> session.query(TransMap_HgmIntronVector).all()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/cluster/home/ifiddes/anaconda2/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 1260, in query
return self._query_cls(entities, self, **kwargs)
File "/cluster/home/ifiddes/anaconda2/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 110, in __init__
self._set_entities(entities)
File "/cluster/home/ifiddes/anaconda2/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 118, in _set_entities
entity_wrapper(self, ent)
File "/cluster/home/ifiddes/anaconda2/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 3829, in __init__
"expected - got '%r'" % (column, )
sqlalchemy.exc.InvalidRequestError: SQL expression, column, or mapped entity expected - got '<class '__main__.TransMap_HgmIntronVector'>'

Answer

An example is in the docs. In particular, __abstract__ = True is not necessary. This works fine:

class Base(object):
    @declared_attr
    def __tablename__(cls):
        return cls.__name__.lower()

class Hgm(Base):
    AlignmentId = Column(Text, primary_key=True)

Base = declarative_base(cls=Hgm)

class TransMap_HgmIntronVector(Base):
    pass

Note that it may be simpler to just use a mixin for the identical columns instead.

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