python - SqlAlchemy: Convert inherited type from one to another -

let's have 2 different types both on same database table (single table inheritance):

class employee(db.model):     id = db.column(db.integer, primary_key = true)     name = db.column(db.string, nullable = false)     discriminator = db.column('type', string)     __mapper_args__ = {'polymorphic_on': discriminator}  class manager(employee):     __mapper_args__ = {'polymorphic_identity': 'manager'}     division = db.column(db.string, nullable = false)     role = db.column(db.string, nullable = false)  class worker(employee):     __mapper_args__ = {'polymorphic_identity': 'worker'}     title = db.column(db.string, nullable = false) 

(yes, i'm using flask-sqlalchemy , not plain vanilla) how might go converting 1 declarative model another. is, if "worker" promoted "manager?" how do that? have write raw sql that?

sorry if has been asked before couldn't find googles. please note, contrived example.

it's kludgy, , causes warning, can brute-force modify discriminator column setting property:

john_smith = session.query(employee).filter_by(name='john smith').one() john_smith.discriminator = 'manager' session.commit() 

this cause warning like,

sawarning: flushing object <worker @ 0xdeadbeef> incompatible polymorphic identity 'manager'; object may not refresh and/or load correctly     mapper._validate_polymorphic_identity(mapper, state, dict_) 

you can ignore that, long fix issues cause. safest thing close session (session.close()) or expunge (session.expunge_all()) after commit.

if must, can fix issues john's object alone expunging john session (session.expunge(john_smith)). have careful that; remaining references john_smith keep object, although thankfully detached session , won't allowed them.

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i tried other obvious options well. neither worked, both illustrative of sqlalchemy's session object stores , how:

1. session.refresh(john_smith) fails with

   invalidrequesterror: not refresh instance '<worker @ 0xdeadbeef>' 

   that's because sqlalchemy queries database worker (not employee) , can't find 1 name of john smith, because database knows john got promoted due fancy new value in type column.

2. session.expire(john_smith) succeeds fails update john new class, , subsequent access him result in

   objectdeletederror: instance '<worker @ 0xdeadbeef>' has been deleted, or row otherwise not present. 

   sqlalchemy still thinks john worker, , tries query him worker. that's because he's still persisted in session.identity_map, looks this:

   {(saexample2.employee, (1,)): <saexample2.worker @ 0xdeadbeef>} 

   so there's john, listed explicitly worker object. when expunge() john session, entry in dictionary gets purged. when expire() him, of mapped properties marked stale, still exists in dictionary.