Merge alias actions when merging abilities (#538)

Closes #468
Closes #280

This changes the behaviour of `Ability#merge` to also include defined
`alias_actions`.

If a collision between the defined aliases occurs the actions on +self+
will be overwritten with those on the passed object.

The inline documentation has been updated in order to reflect those
changes.

lib/cancan/ability.rb

@@ -202,12 +202,13 @@ def has_raw_sql?(action, subject)
       relevant_rules(action, subject).any?(&:only_raw_sql?)
     end
 
-    # Copies all rules of the given +CanCan::Ability+ and adds them to +self+.
+    # Copies all rules and aliased actions of the given +CanCan::Ability+ and adds them to +self+.
     #   class ReadAbility
     #     include CanCan::Ability
     #
     #     def initialize
     #       can :read, User
+    #       alias_action :show, :index, to: :see
     #     end
     #   end
     #
@@ -216,18 +217,43 @@ def has_raw_sql?(action, subject)
     #
     #     def initialize
     #       can :edit, User
+    #       alias_action :create, :update, to: :modify
     #     end
     #   end
     #
     #   read_ability = ReadAbility.new
     #   read_ability.can? :edit, User.new #=> false
     #   read_ability.merge(WritingAbility.new)
     #   read_ability.can? :edit, User.new #=> true
+    #   read_ability.aliased_actions #=> [:see => [:show, :index], :modify => [:create, :update]]
     #
+    # If there are collisions when merging the +aliased_actions+, the actions on +self+ will be
+    # overwritten.
+    #
+    # class ReadAbility
+    #   include CanCan::Ability
+    #
+    #   def initialize
+    #     alias_action :show, :index, to: :see
+    #   end
+    # end
+    #
+    # class ShowAbility
+    #   include CanCan::Ability
+    #
+    #   def initialize
+    #     alias_action :show, to: :see
+    #   end
+    # end
+    #
+    # read_ability = ReadAbility.new
+    # read_ability.merge(ShowAbility)
+    # read_ability.aliased_actions #=> [:see => [:show]]
     def merge(ability)
       ability.rules.each do |rule|
         add_rule(rule.dup)
       end
+      @aliased_actions = aliased_actions.merge(ability.aliased_actions)
       self
     end
 

spec/cancan/ability_spec.rb

@@ -564,6 +564,35 @@ class Container < Hash
       expect(@ability.send(:rules).size).to eq(2)
     end
 
+    it 'adds the aliased actions from the given ability' do
+      @ability.alias_action :show, to: :see
+      (another_ability = double).extend(CanCan::Ability)
+      another_ability.alias_action :create, :update, to: :manage
+
+      @ability.merge(another_ability)
+      expect(@ability.aliased_actions).to eq(
+        read: %i[index show],
+        create: %i[new],
+        update: %i[edit],
+        manage: %i[create update],
+        see: %i[show]
+      )
+    end
+
+    it 'overwrittes the aliased actions with the value from the given ability' do
+      @ability.alias_action :show, :index, to: :see
+      (another_ability = double).extend(CanCan::Ability)
+      another_ability.alias_action :show, to: :see
+
+      @ability.merge(another_ability)
+      expect(@ability.aliased_actions).to eq(
+        read: %i[index show],
+        create: %i[new],
+        update: %i[edit],
+        see: %i[show]
+      )
+    end
+
     it 'can add an empty ability' do
       (another_ability = double).extend(CanCan::Ability)