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Flask SQLAlchemy Integration - Documentation Suggestion #143

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williamjulianvicary opened this issue Jul 29, 2019 · 2 comments

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@williamjulianvicary
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commented Jul 29, 2019

Firstly, thank you for the great extension!!

I've ran into an error that I'm sure others will have ran into, it may be worth updating the docs with a warning about it.

Our structure was as follows:

  • Each model has it's own module
  • Each model module also contains a Schema and Manager for example UserModel, UserSchema, UserManager all defined within /models/user.py

Some background - with SQLAlchemy, with separate models, you need to import them all at runtime, before the DB is initialised to avoid circular dependancies within relationships.

When the UserSchema(ma.ModelSchema) is hit during import from app.models import * (in bootstrap) this initialises the models and attempts to execute the relationships. At this stage, we may not have a relationship requirement (which SQLAlchemy avoids using string based relationships) however as the ma.ModelSchema initialises the models it creates errors such as this:

sqlalchemy.exc.InvalidRequestError: When initializing mapper mapped class User->users, expression ‘Team’ failed to locate a name (“name ‘Team’ is not defined”). If this is a class name, consider adding this relationship() to the <class ‘app.models.user.User’> class after both dependent classes have been defined.

and, on subsequent loads:

sqlalchemy.exc.InvalidRequestError: Table ‘users_teams’ is already defined for this MetaData instance. Specify ‘extend_existing=True’ to redefine options and columns on an existing Table object.

The solution to this is to simply build the UserSchemas in a different import namespace, we've now got:

/schemas/user_schema.py
/models/user.py

And no more circular issues - hopefully this helps someone else, went around in circles (pun intended) for a few hours before I realised it was the ModelSchema causing it.

Could the docs be updated to make a point of explaining that the ModelSchema initialises the model, and therefore it's a good idea for them to be in separate import destinations?

@sloria

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commented Jul 30, 2019

Thank you for the suggestion and the thorough report. I agree that this needs to be documented better. It might be appropriate to document this in marshmallow-sqlalchemy then link to those docs in flask-marshmallow.

Would you be up for sending a PR?

@antgel

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commented Jul 31, 2019

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