Make ActiveRecord and Sequel actual plugins

[shioyama]

Currently on master, Mobility::Plugins::ActiveRecord, Mobility::Plugins::ActiveModel and Mobility::Plugins::Sequel are simply namespaces which hold modules called by Mobility::Plugins::Dirty depending on what gems are loaded.

This is really ugly, because it means that code that should be generic has to know every possible ORM variant.

With this PR, we use inversion of control to flip this situation around so that active_record and sequel are real plugins which themselves call active_record_dirty or sequel_dirty, active_record_cache or sequel_cache, etc. These latter plugins depend are triggered by inclusion of the generic ones (dirty, cache).

So basically, if you have:

class TranslatedAttributes < Mobility::Attributes
  plugin :active_record
  plugin :dirty
end

then the active_record plugin includes active_record_dirty, which itself depends on dirty (with include: false). This way, the AR specific dirty code is only triggered if you actually enable dirty. Same for Sequel.

The other thing that happens here is that we assume that if you have plugin :active_record, the model you're translating must be an ActiveRecord::Base model. Previously, the Dirty plugin would trigger on the included hook and check which class it was included into, but this meant that the model class would get more modules than necessary.

Now, everything is explicit and you must declare plugin :active_record, and when you do, Mobility expects translated classes to be ActiveRecord classes, etc. If (in that 0.001% of cases) you are using Mobility with both ActiveRecord and Sequel, you'd just have to create two module builders, one for each:

class ActiveRecordTranslatedAttributes < Mobility::Attributes
  plugin :active_record
  # ...
end

class SequelTranslatedAttributes < Mobility::Attributes
  plugin :sequel
  # ...
end

class ActiveRecordModel < ActiveRecord::Base
  include ActiveRecordTranslatedAttributes.new(:title)
end

class SequelModel < Sequel::Model
  include SequelTranslatedAttributes.new(:title)
end

So since everything is now more modular, this is still entirely possible.

The nice thing here is that all the ugly code which was implicitly trying to figure out what gems are loaded (to automatically switch between ORM classes) is gone. I think explicitness here is much clearer and safer.

Same goes for backends: the active_record plugin loads an active_record_backend plugin which overrides load_backend to first search for a backend under Mobility::Backends::ActiveRecord (same for sequel_backend). This again means a lot of magic becomes less magical, which I think is good.

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Update: I've also extracted UniquenessValidation into its own plugin, which is automatically included when you call plugin :active_record.

[shioyama]

I will be so happy when this is all gone.