Models need a url_for()

[jace]

Flask's url_for is view-centric. Given a view name and parameters, it generates a URL. We often have to work the reverse: given a model instance, create a URL to its view. All BaseMixin models should have a url_for(view="view") method that implementations can override to return a URL.

To be decided: If a view (default value view, implying the primary public view) is not known by the model, should the method:

1. Return None, implying no known view? or
2. Raise an exception, making it easier to catch gaps in implementation?

[jace]

Given the experience with SQLAlchemy's .first() vs .one(), where the former is almost always used because it fits the mental model better, I think None is the correct response.

[jace]

Should the parameter to url_for be called view or action? The latter implies a URL to a view that performs a certain action. The dual use of view to mean both the URL endpoint and a default action of viewing (ie, read-only access) is confusing. Naming the parameter action helps alleviate the confusion.

[jace]

One downside to having url_for in the model: it can't be used to retrieve the URL for a new action because there is no instance on which to call url_for. It can't be a classmethod either because the endpoint may need a container, which is not known unless explicitly passed.

This suggests that the new action must be in the scope of the container.

[jace]

This was supplemented with a declarative approach in #77 (implemented in #79).

[jace]

We've since switched over from .first() to .one() (more often .one_or_none()) because it helped catch programmer errors with the lack of appropriate unique constraints.

Similarly, url_for() should raise BuildError instead of returning None when an unknown view is requested for.

[jace]

@iambibhas Could you work on a PR that replaces return with raise BuildError here? It'll break some templates in various apps, but we don't know which, so once we deploy this we'll have to be alert and fix those apps as error reports come in.

coaster/coaster/sqlalchemy/mixins.py

Lines 229 to 231 in 02a76cd

	if action not in self.url_for_endpoints:
	# FIXME: Legacy behaviour, fails silently, but shouldn't. url_for itself raises a BuildError
	return