Adding a foreign key dynamically

[RexE]

I'm changing my framework to use the SQLAlchemy declarative system internally. However, I want to keep the API the same. Here is how users of my framework declare a foreign key:

from myframework import Model, Link

class Bar(Model)

class Foo(Model):

  bar = Link(Bar)

At first I thought I could simply change the implementation of Link to this:

def Link(cls):
  return relationship(cls.__name__)

But actually I see that in SQLAlchemy I also need an Integer column that contains the id. So, I think I would need to patch the class at runtime to ensure these 2 attributes are defined. First, I would make a simple definition of Link to store the class it points to:

class Link:

  def __init__(self, cls):

    self.cls = cls

Then after I import the user's module, I patch all subclasses of Model:

for cls in Model.__subclasses__():

    for k, v in list(cls.__dict__.items()):

        if isinstance(v, Link):

            ClsName = v.cls.__name__

            setattr(cls, k, relationship(f'{cls.__module__}.{ClsName}'))

            setattr(

                cls,

                f'{k}_id',

                Column(

                    Integer,

                    ForeignKey(f'{cls.__tablename__}_{ClsName.lower()}.id'),

                ),

            )

However, I get this:

sqlalchemy.exc.NoReferencedTableError: Foreign key associated with column 'live_bid.player_id' could not find table 'live_bid_player' with which to generate a foreign key to target column 'id'

I guess I am patching it too late. My next idea is to put this patching code inside the metaclass of Model, but before I go too far down the wrong path I wanted to ask here if there is a better way :)

Thank you!

[Mike Bayer]

Likely patching too early.   For this kind of thing you want to use an event like "mapper_configured" , which will give you a space where a particular mapper is all set up, all the Table objects needed should have been defined, and you can add additional attributes.   there's a ten-year-old example of what you are trying to do at https://techspot.zzzeek.org/2011/05/17/magic-a-new-orm/.   the code there seems to be mostly still how things work.

Thank you!

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[RexE]

OK thank you! I will give it a try.