URL Converters should have access to app context

Expected Behavior

The following code should create a new URL converter that converts a specific field to a model (or None).

class ModelConverter(BaseConverter):

    def to_python(self, value):
        return models.Example.query.filter(models.Example.name == value).first()

    def to_url(self, value):
        if isinstance(value, str):
            return value
        return value.name

Actual Behavior

The converters don’t have access to the app context so an error is thrown.

Bad Solution

class OrganizationConverter(BaseConverter):
    app = None
    def to_python(self, value):
        with self.app.app_context():
            return repositories.Organization.query.filter(repositories.Organization.name == value).first()

    def to_url(self, value):
        if isinstance(value, str):
            return value
        return value.name

This solution tries to get around it by creating a new app context. The problem is that the context disappears, killing the database session at the same time. This prevents SQLAlchemy features (such as lazy loading).

Environment

• Python version: 3.6 and 3.7
• Flask version: 1.0.2
• SQLAlchemy version: 1.2.17
• Flask-SQLAlchemy version: 2.3.2
• Werkzeug version: 0.14.1

Additional Resources

This issue has also come up on stack overflow

[davidism]

I’ve also ran into this issue with converters, but it’s might not be straightforward to change at this point.

URL matching is done as part of creating the request context, not pushing the context:

https://github.com/pallets/flask/blob/0b5b4a66ef99c8b91569dd9b9b34911834689d3f/flask/ctx.py#L313

So we’d need to move it into RequestContext.push, after it’s actually pushed so that request is populated:

https://github.com/pallets/flask/blob/0b5b4a66ef99c8b91569dd9b9b34911834689d3f/flask/ctx.py#L379

It’s a more internal part of Flask, and I haven’t really seen any discussion about overriding or using these parts, but I’d have to understand that more before making the change.

[michaeltoohig]

Is this well documented yet? I’m here since I read the 1.1.1 release notes and was looking for some examples of how this feature could be used. The test above by @eladm26 clears things up but it took awhile for me to find it here.

Here’s the part from the release notes:

URL routing is performed after the request context is pushed. This allows custom URL converters to access current_app and request. This makes it possible to implement converters such as one that queries the database for a model based on the ID in the URL.

Thanks.