Can SQLAlchemy automatically create relationships from a database schema?

[Update] As of SQLAlchemy 0.9.1 there is Automap extension for doing that.

For SQLAlchemy < 0.9.0 it is possible to use sqlalchemy reflection.

SQLAlchemy reflection loads foreign/primary keys relations between tables. But doesn't create relations between mapped classes. Actually reflection doesn't create mapped classes for you - you have to specify mapped class name.

Actually I think that reflection support for loading foreign keys is a great helper and time saving tool. Using it you can build a query using joins without need to specify which columns to use for a join.

from sqlalchemy import *from sqlalchemy import create_engine, ormfrom sqlalchemy.ext.declarative import declarative_basefrom sqlalchemy.orm import relationshipmetadata = MetaData()Base = declarative_base()Base.metadata = metadatadb = create_engine('<db connection URL>',echo=False)metadata.reflect(bind=db)cause_code_table = metadata.tables['cause_code']ndticket_table = metadata.tables['ndticket']sm = orm.sessionmaker(bind=db, autoflush=True, autocommit=True, expire_on_commit=True)session = orm.scoped_session(sm)q = session.query(ndticket_table,cause_code_table).join(cause_code_table)for r in q.limit(10):    print r

Also when I was using reflection to run queries to existing database - I had to define only mapped classes names, table bindings, relations, BUT there were no need to define table columns for these relations.

class CauseCode(Base):    __tablename__ = "cause_code"class NDTicket(Base):    __tablename__ = "ndticket"    cause_code = relationship("CauseCode", backref = "ndticket")q = session.query(NDTicket)for r in q.limit(10):    print r.ticket_id, r.cause_code.cause_code

Overall SQLAlchemy reflection is already powerful tool and save me time, so adding relations manually is a small overhead for me.

If I would have to develop functionality that will add relations between mapped objects using existing foreign keys, I would start from using reflection with inspector. Using get_foreign_keys() method gives all information required to build relations - referred table name, referred column name and column name in target table. And would use this information for adding property with relationship into mapped class.

insp = reflection.Inspector.from_engine(db)print insp.get_table_names()print insp.get_foreign_keys(NDTicket.__tablename__)>>>[{'referred_table': u'cause_code', 'referred_columns': [u'cause_code'], 'referred_schema': None, 'name': u'SYS_C00135367', 'constrained_columns': [u'cause_code_id']}]

As of SQLAlchemy 0.9.1 the (for now experimental) Automap extension would seem to do just that: http://docs.sqlalchemy.org/en/rel_0_9/orm/extensions/automap.html