[newforms-admin] Custom FilterSpecs

I really like the idea behind this, but I'd need to review the code closer and won't take the time to do it right now since it's a new feature. I'd rather focus on bugs preventing newforms-admin from being used.

Also see #3400 for another ideas about filters which could benefit from this scheme...

This ticket isn't critical to merge newforms-admin into trunk. Tagging with nfa-someday.

How about FilterSpec?.insert method to do prepend and FilterSpec?.register to do append? discussed somewhere, no?

I'm attaching my crack at this issue; in particular, I wanted to allow custom FilterSpecs that aren't associated with a field. This way, you can throw together a filter for fairly arbitrary queries.

Much of the field-specific code from FilterSpec has been moved out to a subclass, FieldFilterSpec; that should be used as the superclass for custom field-based filterspecs, while FilterSpec can be used for non-field-based ones.

The list_filter syntax gains two further options besides field names: a FilterSpec subclass, or a tuple of ('field_name', FieldFilterSpecSubclass).

An example combining all three:

list_filter = (
    'field1',
    ('field2', SomeFieldFilterSpec),
    SomeFilterSpec,
)

Custom FilterSpecs?, also allowing fieldless FilterSpecs?

Tweaked the summary and description.

^{that was me; sorry.}

updated version of my simple patch,

Honza_Kral: I modified the filterspec definition to allow for users to register their own filters in admin. The mechanism is simple - I just reverted the order of the registry so that newly registered specs will come first. That way if you register your own filter via FilterSpec.register, it will be used before the default one.

I'm not sure having a reversed registry is intuitive. It certainly makes overriding FilterSpecs? simple. Except if I were to register two FilterSpecs? I would expect them to be tested the order I add them, the same way it is done in the rest of Django. Maybe the FilterSpec?'s should be in a Priority Queue instead of a Queue?

I had created a solution which doesn't reverse the registry ticket:8330. But I'm realizing now that this wouldn't allow for overriding other filters like Boolean. I'll keep playing.

Suggestion: make FieldFilterSpec?'s similar to how widgets work in newforms admin.