'Intercept' ActiveRecord::Relation's ARel without killing a kitten

Question

In the gem I'm making, I want to allow developer to add a class method I've written, let's call it interceptor , to a model, in classic Devise syntax:

class User < ActiveRecord::Base
  has_interceptor
end

This allows you to call User.interceptor , which returns an Interceptor object that does magic things with querying the database through the Squeel gem. All good.

However, I'd like to find a graceful way of allowing the developer to scope the queries the interceptor performs, first. This can be accomplished by allowing interceptor to take in an ActiveRecord::Relation and chain Squeel off of that, and otherwise fall back on the model. This implementation works as follows:

# Builds on blank ARel from User:
User.interceptor.perform_magic
#=> "SELECT `users`.* FROM `users` WHERE interceptor magic"

# Build on scoped ARel from Relation:
User.interceptor( User.where('name LIKE (?)', 'chris') ).perform_magic
#=> "SELECT `users`.* FROM `users`  WHERE `users`.`name` LIKE 'chris' AND  interceptor magic"

Which is effective, but ugly. What I really want is something like:

# Build on scoped ARel:
User.where('name LIKE (?)', 'chris').interceptor.perform_magic
#=> "SELECT `users`.* FROM `users`  WHERE `users`.`name` LIKE 'chris' AND  interceptor magic"

Essentially, I'd like to 'tap in' to the ActiveRecord::Relation chain and steal it's ARel, passing it into my Interceptor object to modify it before I evaluate it. But every way I can think of to do this involves code so horrifying, I know God would kill a kitten if I implemented it. I don't need that blood on my hands. Help me save a kitten?

ISSUES:

Adding to my complications,

class User < ActiveRecord::Base
  has_interceptor :other_interceptor_name
end

allows you to call User.other_interceptor_name , and models can have multiple interceptors. It works well, but makes using method_missing an even worse idea than normal.

Answer 1

I ended up hacking ActiveRecord::Relation 's method_missing after all, it didn't turn out too ugly. Here's the full process, from beginning to end.

My gem defines an Interceptor class, intended to be a DSL that developers may subclass. This object takes in some root ARel, from a Model or a Relation , and manipulates the query further before rendering.

# gem/app/interceptors/interceptor.rb
class Interceptor
  attr_accessor :name, :root, :model
  def initialize(name, root)
    self.name = name
    self.root = root
    self.model = root.respond_to?(:klass) ? root.klass : root
  end
  def render
    self.root.apply_dsl_methods.all.to_json
  end
  ...DSL methods...
end

Implemented:

# sample/app/interceptors/user_interceptor.rb
class UserInterceptor < Interceptor
  ...DSL...
end

Then I give models the has_interceptor method that defines new interceptors and builds an interceptors mapping:

# gem/lib/interceptors/model_additions.rb
module Interceptor::ModelAdditions

  def has_interceptor(name=:interceptor, klass=Interceptor)
    cattr_accessor :interceptors unless self.respond_to? :interceptors
    self.interceptors ||= {}
    if self.has_interceptor? name
      raise Interceptor::NameError,
        "#{self.name} already has a interceptor with the name '#{name}'. "\
        "Please supply a parameter to has_interceptor other than:"\
        "#{self.interceptors.join(', ')}"
    else
      self.interceptors[name] = klass
      cattr_accessor name
      # Creates new Interceptor that builds off the Model
      self.send("#{name}=", klass.new(name, self))
    end
  end

  def has_interceptor?(name=:interceptor)
    self.respond_to? :interceptors and self.interceptors.keys.include? name.to_s
  end

end

ActiveRecord::Base.extend Interceptor::ModelAdditions

Implemented:

# sample/app/models/user.rb
class User < ActiveRecord::Base
  # User.interceptor, uses default Interceptor Class
  has_interceptor
  # User.custom_interceptor, uses custom CustomInterceptor Class
  has_interceptor :custom_interceptor, CustomInterceptor

  # User.interceptors #show interceptor mappings
  #=> {
  #     interceptor: #<Class:Interceptor>,
  #     custom_interceptor: #<Class:CustomInterceptor>
  #   }
  # User.custom_interceptor #gets an instance
  #=> #<CustomInterceptor:0x005h3h234h33>
end

With that alone, you can call User.interceptor and build an Interceptor with a clean query as the root for all interceptor query manipulation. However, with a little more effort, we can extend ActiveRecord::Relation so that you can call interceptor methods as an endpoint in a chain of scopes:

# gem/lib/interceptor/relation_additions.rb
module Interceptor::RelationAdditions

  delegate :has_interceptor?, to: :klass

  def respond_to?(method, include_private = false)
    self.has_interceptor? method
  end

protected

  def method_missing(method, *args, &block)
    if self.has_interceptor? method
      # Creates new Interceptor that builds off of a Relation
      self.klass.interceptors[method.to_s].new(method.to_s, self)
    else
      super
    end
  end

end

ActiveRecord::Relation.send :include, Interceptor::RelationAdditions

Now, User.where('created_at > (?)', Time.current - 2.weeks).custom_interceptor will apply all the scoping set up in the Interceptor DSL on top of whatever query you build on the model.