A bit of history: this gem was inspired by digging deeper into Draper with an eye on refactoring.

A common task for an application is to match its parts against each other, be it inferring a default model class for a controller, or looking up a decorator for a model, or whatever else.

Modern frameworks tend to use naming conventions over configuration to achieve that. Unfortunately, every one has to implement them on its own struggling through numerous bugs. Moreover, inconsistencies across these implementations lead to misunderstanding and endless tickets when actual behavior fails to meet user expectations based on other frameworks.

So, meet

It is meant to be The One to Rule Them All — the library to provide a generic name-based lookup for a plenty of cases.

Install the gem and add to the application’s Gemfile by executing:

$ bundle add magic-lookup

If bundler is not being used to manage dependencies, install the gem by executing:

$ gem install magic-lookup

These are the steps to set up an automatic class inference:

1. Define a base class extending Magic::Lookup.
2. Define .name_for method for that class implementing your lookup logic.
3. From the base class, inherit classes to be looked up.

class Scope
  extend Magic::Lookup

  def self.name_for object_class
    object_class.name
        .delete_suffix('Model')
        .concat('Scope')
  end
end

class MyScope < Scope
end

Scope.for MyModel    # => MyScope
Scope.for OtherModel # => nil

When no class is found, nil is returned. If you need to raise an exception in this case, you can use Magic::Lookup::Error like this:

scope_class = Scope.for(object.class) or
    raise Magic::Lookup::Error.for(object, Scope)

These are additional steps to set up a reverse inference:

1. Define a base module including Magic::Lookup::Scope.
2. Include the base module in the classes to be looked up.

module Scopable
  include Magic::Lookup::Scope
end

class Model
  include Scopable
end

class MyModel < Model
end

Scopable.for MyScope    # => MyModel
Scopable.for OtherScope # => nil

You can mark a directory under app for eager loading. E. g., this will ensure that all the classes under app/models are available for reverse lookup in development environment:

module Scopable
  include Magic::Lookup::Scope[:models]
end

Lookup is provided not only for the class itself, but to any of its ancestors as well.

Both of matching classes — the target and its match — may be namespaced independently.

One can specify a namespace to look in:

Scope.for MyModel              # => MyScope
Scope.for MyModel, MyNamespace # => MyNamespace::MyScope

Multiple default lookup namespaces may be set for the base class:

Scope.namespaces << MyNamespace # => [nil, MyNamespace]
Scope.for MyModel               # => MyNamespace::MyScope

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install.

Bug reports and pull requests are welcome on GitHub at https://github.com/Alexander-Senko/magic-lookup. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the code of conduct.

The gem is available as open source under the terms of the MIT License.

Everyone interacting in the Magic Lookup project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.