Insert new model adapters at the start of the list

[ghiculescu]

This makes implementing new model adapters easier. The example in the docs adds an adapter for non-activerecord. That works fine - none of the builtin adapters will work with it. But consider the use case where you want to add a more specific adapter for activerecord, and don't want to fork cancancan.

module CanCan
  module ModelAdapters
    class IDProxyAdapter < ActiveRecord5Adapter
      AbstractAdapter.inherited(self)

      def self.for_class?(model_class)
        super
      end

Currently this will not work, because IDProxyAdapter will come after ActiveRecord5Adapter in CanCan::ModelAdapters::AbstractAdapters#subclasses. So ActiveRecord5Adapter will always get used.

By adding new adapters to the start of the subclasses list, we inverse this. All the builtin adapters will end up last in the list; all adapters added in userland will be considered first.

This is safe with all built in adapters because they are written in an exclusive way (AR4, AR5). As a result, no tests should break here. However, this could cause problems for users who have added AR adapters in the wild if they haven't added to them the start of the subclasses list. For that reason I think this should be considered a breaking change.

If this is accepted I'm happy to also tidy up the docs to make it more clear how easy adapters are to add.

[coorasse]

I like it! Give me some time when I am back from holidays to properly schedule it in a release.

[ghiculescu]

No rush. Enjoy the summer!

[coorasse]

Good! This can be merged, although forking cancancan is not the way to go anyway. a separate adapter can be written without forking. The docs are outdated. I'll take care of updating them