flask-sqlalchemy cross database with "dynamic" schema

Question

I'm attempting to use the "application factory" pattern from flask, but I seem to have a chicken-and-egg problem with my models. http://flask.pocoo.org/docs/patterns/appfactories/

I am importing my views in the create_app function, which imports my models. So, I don't have a config in the app when my models are being defined. This is usually fine, using the bind keys, I can set up models to connect to different dbs.

However, in this case, I have two sets of models, one from the default database, and another set on another db connection - and I'd like to cross-db join. I know that the usual method is to add

__table_args__ = { 'schema' : 'other_db_name' }

to my "other db" models.

But...depending on the config, the 'other_db_name' may be different.

So, now I have models being defined that require the schema name from the config, but with no schema from the config to put in the class definition. I also may simply be missing something in flask I wasn't aware of.

(Side note - an easy fix for this is to configure Sqlalchemy to always output the schema name in the query, no matter what - but I can't seem to find a setting for this.)

If anyone has any input on this, I'd be very much obliged. Thanks!

Miguel Grinberg · Accepted Answer · 2013-08-27 04:29:36Z

5

Never tried this myself, but you can probably force __table_args__ to have lazy evaluation by making it a declared_attr.

from sqlalchemy.ext.declarative import declared_attr

class MyModel(MySQLSettings, MyOtherMixin, Base):
    __tablename__='my_model'

    @declared_attr
    def __table_args__(cls):
        return { 'schema': current_app.config['OTHER_DB_NAME'] }

edited Aug 27, 2013 at 4:29

answered Aug 27, 2013 at 1:20

Miguel Grinberg

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4 Comments

Hoopes Over a year ago

This is a great idea, but doesn't seem to work. It looks like the declarative base still looks for table_args on definition, and I'm still not in an application context when I get there. Thanks for the reply!

Miguel Grinberg Over a year ago

You should probably investigate why this does not work. The SQLAlchemy documentation mentions the use of declared_attr for __table_args__ a few times. See here, for example.

Hoopes Over a year ago

Yes, I agree with you that I should be able to have that function called for table_args - but I still don't yet have the config set (by the application context) by the time that declared_attr is called to set the table args. So - I don't think it's lazily evaluated to the point beyond the flask app starting up...

Miguel Grinberg Over a year ago

I would think you need to initialize app.config before you initialize app.db. Is that the order you are using?

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