Question

Starting from an existing (SQLite) database with foreign keys, can SQLAlchemy automatically build relationships?

SQLAlchemy classes are automatically created via __table_args__ = {'autoload': True}.

The goal would be to easily access data from related tables without having to add all the relationships one by one by hand (i.e. without using sqlalchemy.orm.relationship() and sqlalchemy.orm.backref).

Answer 1

[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, orm
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship



metadata = MetaData()
Base = declarative_base()
Base.metadata = metadata

db = 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']}]