feat: Include ORM associations in CollectionDecorator (#845)

## Description
Include all QueryMethods from the ORM in CollectionDecorator. The default adapter is :active_record

* Why was this change required?
It was necessary to delegate or define a method of the ORM which you are using in your decorator to make an instance of CollectionDecorator able to call it.
* Is there something you aren't happy with or that needs extra attention?
In order to support other ORM associations, we'll need to write a method `allowed?` for each strategy at `lib/draper/query_methods/load_strategy.rb`

## Testing
1. Create a decorator for the model and its association
```ruby
class OrderHistoryDecorator < Draper::Decorator
  delegate_all
end

class OrderDecorator < Draper::Decorator
  delegate_all

  decorates_association :order_histories, with: OrderHistoryDecorator
end
```

2. Call any query method in the decorated instance
```ruby
pry(main)> Order.last.decorate.order_histories.includes(:user)
```

## References
* Issue #702 
* Issue #812

Gemfile

@@ -3,7 +3,7 @@ source "https://rubygems.org"
 gemspec
 
 platforms :ruby do
-  gem "sqlite3"
+  gem 'sqlite3', '~> 1.3.6'
 end
 
 platforms :jruby do

README.md

@@ -326,6 +326,17 @@ your `ArticleDecorator` and they'll return decorated objects:
 @article = ArticleDecorator.find(params[:id])
 ```
 
+### Decorated Query Methods
+By default, Draper will decorate all [QueryMethods](https://api.rubyonrails.org/classes/ActiveRecord/QueryMethods.html)
+of ActiveRecord.
+If you're using another ORM, in order to support it, you can tell Draper to use a custom strategy:
+
+```ruby
+Draper.configure do |config|
+  config.default_query_methods_strategy = :mongoid
+end
+```
+
 ### When to Decorate Objects
 
 Decorators are supposed to behave very much like the models they decorate, and

lib/draper.rb

@@ -23,6 +23,7 @@
 require 'draper/decorated_association'
 require 'draper/helper_support'
 require 'draper/view_context'
+require 'draper/query_methods'
 require 'draper/collection_decorator'
 require 'draper/undecorate'
 require 'draper/decorates_assigned'

lib/draper/collection_decorator.rb

@@ -2,6 +2,7 @@ module Draper
   class CollectionDecorator
     include Enumerable
     include Draper::ViewHelpers
+    include Draper::QueryMethods
     extend Draper::Delegation
 
     # @return the collection being decorated.

lib/draper/configuration.rb

@@ -11,5 +11,13 @@ def default_controller
     def default_controller=(controller)
       @@default_controller = controller
     end
+
+    def default_query_methods_strategy
+      @@default_query_methods_strategy ||= :active_record
+    end
+
+    def default_query_methods_strategy=(strategy)
+      @@default_query_methods_strategy = strategy
+    end
   end
 end

lib/draper/query_methods.rb

@@ -0,0 +1,19 @@
+require_relative 'query_methods/load_strategy'
+
+module Draper
+  module QueryMethods
+    # Proxies missing query methods to the source class if the strategy allows.
+    def method_missing(method, *args, &block)
+      return super unless strategy.allowed? method
+
+      object.send(method, *args, &block).decorate
+    end
+
+    private
+
+    # Configures the strategy used to proxy the query methods, which defaults to `:active_record`.
+    def strategy
+      @strategy ||= LoadStrategy.new(Draper.default_query_methods_strategy)
+    end
+  end
+end

lib/draper/query_methods/load_strategy.rb

@@ -0,0 +1,21 @@
+module Draper
+  module QueryMethods
+    module LoadStrategy
+      def self.new(name)
+        const_get(name.to_s.camelize).new
+      end
+
+      class ActiveRecord
+        def allowed?(method)
+          ::ActiveRecord::Relation::VALUE_METHODS.include? method
+        end
+      end
+
+      class Mongoid
+        def allowed?(method)
+          raise NotImplementedError
+        end
+      end
+    end
+  end
+end

spec/draper/configuration_spec.rb

@@ -6,20 +6,44 @@ module Draper
       Draper.configure { |config| expect(config).to be Draper }
     end
 
-    it 'defaults default_controller to ApplicationController' do
-      expect(Draper.default_controller).to be ApplicationController
+    describe '#default_controller' do
+      it 'defaults default_controller to ApplicationController' do
+        expect(Draper.default_controller).to be ApplicationController
+      end
+
+      it 'allows customizing default_controller through configure' do
+        default = Draper.default_controller
+
+        Draper.configure do |config|
+          config.default_controller = CustomController
+        end
+
+        expect(Draper.default_controller).to be CustomController
+
+        Draper.default_controller = default
+      end
     end
 
-    it 'allows customizing default_controller through configure' do
-      default = Draper.default_controller
+    describe '#default_query_methods_strategy' do
+      let!(:default) { Draper.default_query_methods_strategy }
+
+      subject { Draper.default_query_methods_strategy }
 
-      Draper.configure do |config|
-        config.default_controller = CustomController
+      context 'when there is no custom strategy' do
+        it { is_expected.to eq(:active_record) }
       end
 
-      expect(Draper.default_controller).to be CustomController
+      context 'when using a custom strategy' do
+        before do
+          Draper.configure do |config|
+            config.default_query_methods_strategy = :mongoid
+          end
+        end
 
-      Draper.default_controller = default
+        after { Draper.default_query_methods_strategy = default }
+
+        it { is_expected.to eq(:mongoid) }
+      end
     end
   end
 end

spec/draper/query_methods/load_strategy_spec.rb

@@ -0,0 +1,26 @@
+require 'spec_helper'
+require 'active_record'
+
+module Draper
+  module QueryMethods
+    describe LoadStrategy do
+      describe '#new' do
+        subject { described_class.new(:active_record) }
+
+        it { is_expected.to be_an_instance_of(LoadStrategy::ActiveRecord) }
+      end
+    end
+
+    describe LoadStrategy::ActiveRecord do
+      describe '#allowed?' do
+        it 'checks whether or not ActiveRecord::Relation::VALUE_METHODS has the given method' do
+          allow(::ActiveRecord::Relation::VALUE_METHODS).to receive(:include?)
+
+          described_class.new.allowed? :foo
+
+          expect(::ActiveRecord::Relation::VALUE_METHODS).to have_received(:include?).with(:foo)
+        end
+      end
+    end
+  end
+end

spec/draper/query_methods_spec.rb

@@ -0,0 +1,39 @@
+require 'spec_helper'
+require_relative '../dummy/app/decorators/post_decorator'
+
+Post = Struct.new(:id) { }
+
+module Draper
+  describe QueryMethods do
+    describe '#method_missing' do
+      let(:collection) { [ Post.new, Post.new ] }
+      let(:collection_decorator) { PostDecorator.decorate_collection(collection) }
+      let(:fake_strategy) { instance_double(QueryMethods::LoadStrategy::ActiveRecord) }
+
+      before { allow(QueryMethods::LoadStrategy).to receive(:new).and_return(fake_strategy) }
+
+      context 'when strategy allows collection to call the method' do
+        let(:results) { spy(:results) }
+
+        before do
+          allow(fake_strategy).to receive(:allowed?).with(:some_query_method).and_return(true)
+          allow(collection).to receive(:send).with(:some_query_method).and_return(results)
+        end
+
+        it 'calls the method on the collection and decorate it results' do
+          collection_decorator.some_query_method
+
+          expect(results).to have_received(:decorate)
+        end
+      end
+
+      context 'when strategy does not allow collection to call the method' do
+        before { allow(fake_strategy).to receive(:allowed?).with(:some_query_method).and_return(false) }
+
+        it 'raises NoMethodError' do
+          expect { collection_decorator.some_query_method }.to raise_exception(NoMethodError)
+        end
+      end
+    end
+  end
+end