Why isn't SQLAlchemy default column value available before object is committed?

Question

Recently I figured out that SQLAlchemy\'s Column default doesn\'t work as I expect it to:

>>> Base = declarative_base()
>>> class TestModel         


        

Answer 1

You can use force_instant_defaults listener from sqlalchemy_utils to change this behavior:

from sqlalchemy_utils import force_instant_defaults

force_instant_defaults()

class TestModel(Base):
    __tablename__ = 'tmodel'
    id = sa.Column(sa.Integer, primary_key=True)
    foo = sa.Column(sa.Integer, default=0)

model = TestModel()
assert model.foo == 0

Answer 2

one would prefer default over server default for one of four reasons:

1. you'd like to run a Python function, not a SQL function, for the default (or a SQL expression that needs some per-INSERT Python state also).

2. the default is part of a primary key column. the ORM can't load a row back without the primary key, so server_default is generally not useful for a PK column when using the ORM.

3. the SQL expression you want to run isn't supported by the database as a "server default".

4. You're dealing with a schema you can't/don't want to change.

In this case, when you'd like "foo" to be "0" in your application independent of database operations, the choices are:

1. use __init__(). It's python!

2. use events.

Here's __init__():

class TestModel(Base):
   # ...

   def __init__(self):
       self.foo = 0

Here's events (specifically the init event):

from sqlalchemy import event

@event.listens_for(Foo, "init")
def init(target, args, kwargs):
    target.foo = 0

Answer 3

You can use the init event to fill defaults. This event listener will do it:

from sqlalchemy import event
from sqlalchemy.orm import mapper
from sqlalchemy.inspection import inspect


def instant_defaults_listener(target, args, kwargs):
    for key, column in inspect(target.__class__).columns.items():
        if column.default is not None:
            if callable(column.default.arg):
                setattr(target, key, column.default.arg(target))
            else:
                setattr(target, key, column.default.arg)


event.listen(mapper, 'init', instant_defaults_listener)