Use class attributes properly so that inheritance is respected

This fixes two bugs:

1. Subclasses were not properly inheriting their parents' Ransack
   aliases in Mongoid because each class defined its very own set of
   aliases.
2. In Active Record, Ransack aliases were defined in such a way that the
   parent's (and grandparent's, etc.) aliases were overwritten by the
   child, meaning that all aliases were ultimately kept on
   ActiveRecord::Base. This has the unfortunate effect of enforcing
   uniqueness of Ransack alias names across all models rather than per
   model. Depending on the load order of models, earlier definitions of
   an alias in other models would be clobbered.

lib/ransack/adapters/active_record/base.rb

@@ -23,7 +23,8 @@ module Ransack
         end
 
         def ransack_alias(new_name, old_name)
-          self._ransack_aliases.store(new_name.to_s, old_name.to_s)
+          self._ransack_aliases = _ransack_aliases.merge new_name.to_s =>
+            old_name.to_s
         end
 
         # Ransackable_attributes, by default, returns all column names

lib/ransack/adapters/mongoid/base.rb

@@ -8,6 +8,10 @@ module Ransack
         extend ActiveSupport::Concern
 
         included do
+          class_attribute :_ransackers
+          class_attribute :_ransack_aliases
+          self._ransackers ||= {}
+          self._ransack_aliases ||= {}
         end
 
         class ColumnWrapper < SimpleDelegator
@@ -33,22 +37,6 @@ module Ransack
         end
 
         module ClassMethods
-          def _ransack_aliases
-            @_ransack_aliases ||= {}
-          end
-
-          def _ransack_aliases=(value)
-            @_ransack_aliases = value
-          end
-
-          def _ransackers
-            @_ransackers ||= {}
-          end
-
-          def _ransackers=(value)
-            @_ransackers = value
-          end
-
           def ransack(params = {}, options = {})
             params = params.presence || {}
             Search.new(self, params ? params.delete_if {
@@ -58,7 +46,8 @@ module Ransack
           alias_method :search, :ransack
 
           def ransack_alias(new_name, old_name)
-            self._ransack_aliases.store(new_name.to_s, old_name.to_s)
+            self._ransack_aliases = _ransack_aliases.merge new_name.to_s =>
+              old_name.to_s
           end
 
           def ransacker(name, opts = {}, &block)

spec/mongoid/adapters/mongoid/base_spec.rb

@@ -65,6 +65,27 @@ module Ransack
             s = Person.ransack(term_cont: 'nomatch')
             expect(s.result.to_a).to eq []
           end
+
+          it 'makes aliases available to subclasses' do
+            yngwie = Musician.create!(name: 'Yngwie Malmsteen')
+
+            musicians = Musician.ransack(term_cont: 'ngw').result
+            expect(musicians).to eq([yngwie])
+          end
+
+          it 'handles naming collisions gracefully' do
+            frank = Person.create!(name: 'Frank Stallone')
+
+            people = Person.ransack(term_cont: 'allon').result
+            expect(people).to eq([frank])
+
+            Class.new(Article) do
+              ransack_alias :term, :title
+            end
+
+            people = Person.ransack(term_cont: 'allon').result
+            expect(people).to eq([frank])
+          end
         end
 
         describe '#ransacker' do

spec/mongoid/support/schema.rb

@@ -63,6 +63,9 @@ class Person
   end
 end
 
+class Musician < Person
+end
+
 class Article
   include Mongoid::Document
 

spec/ransack/adapters/active_record/base_spec.rb

@@ -190,6 +190,27 @@ module Ransack
             s = Person.ransack(daddy_eq: 'Drake')
             expect(s.result.to_a).to eq []
           end
+
+          it 'makes aliases available to subclasses' do
+            yngwie = Musician.create!(name: 'Yngwie Malmsteen')
+
+            musicians = Musician.ransack(term_cont: 'ngw').result
+            expect(musicians).to eq([yngwie])
+          end
+
+          it 'handles naming collisions gracefully' do
+            frank = Person.create!(name: 'Frank Stallone')
+
+            people = Person.ransack(term_cont: 'allon').result
+            expect(people).to eq([frank])
+
+            Class.new(Article) do
+              ransack_alias :term, :title
+            end
+
+            people = Person.ransack(term_cont: 'allon').result
+            expect(people).to eq([frank])
+          end
         end
 
         describe '#ransacker' do

spec/support/schema.rb

@@ -127,6 +127,9 @@ class Person < ActiveRecord::Base
   end
 end
 
+class Musician < Person
+end
+
 class Article < ActiveRecord::Base
   belongs_to :person
   has_many :comments