---
layout:    post
title: "SQLAlchemy-i18n with application factory, dynamic locales, and Celery"
date:      2017-03-14 00:25:52+01:00
tags:      [development, python, flask]
published: true
author:
    name: Gergely Polonkai
    email: gergely@polonkai.eu
---

[SQLAlchemy-i18n](https://sqlalchemy-i18n.readthedocs.io/) is an awesome
project if you need to make some of your model fields translatable.
However, it is only usable if your application class can be direectly
instantiated; if you use
[application factories](http://flask.pocoo.org/docs/0.12/patterns/appfactories/) and
you have the available locales in your config, you are on your own.

#### SQLAlchemy-i18n basics

According to the QuickStart section of the SQLAlchemy-i18n documentation,
you have to initialize the module like this:

```python
from flask_babel import get_locale
from sqlalchemy_i18n import make_translatable, translation_base, Translatable
import sqlalchemy_utils


make_translatable(options={'locales': ['fi', 'en']})


class Article(Translatable, Base):
    __tablename__ = 'articles'
    __translatable__ = {'locales': ['fi', 'en']}  # Available locales
    locale = 'en'  # Default locale

    id = sa.Column(sa.Integer, primary_key=True)
    author = sa.Column(sa.Unicode(255))


class ArticleTranslation(translation_base(Article)):
    __tablename__ = 'article_translations'

    name = sa.Column(sa.Unicode(255))
    content = sa.Column(sa.UnicodeText)
```

#### Add application factories to the mixture

In one of my current projects, I have the following config (excerpt):

```python
from flask_babel import lazy_gettext as _

class Config:
    AVAILABLE_LOCALES = {
        'en': _('English')
        'hu': _('Hungarian')
    }
    DEFAULT_LOCALE = 'en'
```

And the following application factory (again, excerpt):

```python
from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()

from app import models

def create_app(config_name):
    app = Flask(__name__)
    db.init_app(app)
```

Unfortunately, adding the `make_translatable` call to `create_app` and
leaving the `__translatable__` definition (assuming the options of the
`make_translatable` are the default) is not working (its value is not a
default for models), so here is what I came up with:

```python
from sqlalchemy_i18n import Translatable, translation_base

from app import db


class Article(Translatable, db.Model):
    __tablename__ = 'articles'
    __translatable__ = {
        'locales': current_app.config['AVAILABLE_LOCALES'],
    }
    locale = current_app.config['DEFAULT_LOCALE']

    id = db.Column(sa.Integer, primary_key=True)
    author = sa.Column(sa.Unicode(255))
```

This brings up a problem: my models have to be initialized with an active
app context.  Here is a solution for that:

    from config import config
    # I have removed the models import from here
    
    def create_app(config_name):
        app = Flask(__name__)
        app.config.from_object(config[config_name])
    
        make_translatable(options={
            'locales': list(app.config['AVAILABLE_LOCALES'].keys()),
        })
    
        with app.app_context():
            # Import models so they are registered with SQLAlchemy
            from app import models

The only drawback I found is that I cannot import my models without creating
the app first; however, given I need to instantiate the app to have a
working database connection, it’s actually not a problem.  Well, not until
Celery comes into play.

#### --Chemical X-- Celery

Up to this point I haven’t used Celery for this app.  When I set that up,
due to some import order madness (which I could overcome, but doesn’t worth
it), my models got imported before my Celery tasks and the Celery app, so my
well-crafted solution failed miserably.

Fortunately, SQLAlchemy-i18n doesn’t tamper with metaclasses neither does
monkey patching on my model classes.  I only have to extend `Translatable`,
and the magic happens on its own.  There are two properties `Translatable`
needs: `__translatable__`, a dictionary which can, among other things,
define the available locales (languages) for the model, and `locale`, which
sets the default locale.  As there are no tricky metaclasses are involved, I
could easily set these properties during app initialization time:

    from app import models
    
    
    def create_app(config):
        app = Flask(__name__)
    
        # Iterate over everything in the models module
        for _, member in models.__dict__.items():
            # We look for subclasses of Translatable, but not Translatable
            # itself
            if isinstance(member, type) and \
               issubclass(member, Translatable) and \
               member != Translatable:
                member.__translatable__['locales'] = list(app.config['AVAILABLE_LOCALES'].keys())
                member.locale = app.config['DEFAULT_LOCALE']

Honestly, I’m not particularly happy with this solution.  It works nice
with the current version of SQLAlchemy-i18n, but as soon as they switch
their implementation, I’m screwed (but that’s where testing comes in,
right?)