Make add_reference cop accept a hash for :index

It might happen you want to make the reference column have a unique
value, or you want to create partial indexes. So instead of only
accepting a `true` value, also accept a hash of options.

rubocop/cop/migration/add_reference.rb

@@ -8,7 +8,7 @@ module RuboCop
       class AddReference < RuboCop::Cop::Cop
         include MigrationHelpers
 
-        MSG = '`add_reference` requires `index: true`'
+        MSG = '`add_reference` requires `index: true` or `index: { options... }`'
 
         def on_send(node)
           return unless in_migration?(node)
@@ -33,7 +33,12 @@ module RuboCop
         private
 
         def index_enabled?(pair)
-          hash_key_type(pair) == :sym && hash_key_name(pair) == :index && pair.children[1].true_type?
+          return unless hash_key_type(pair) == :sym
+          return unless hash_key_name(pair) == :index
+
+          index = pair.children[1]
+
+          index.true_type? || index.hash_type?
         end
 
         def hash_key_type(pair)

spec/rubocop/cop/migration/add_reference_spec.rb

@@ -29,7 +29,7 @@ describe RuboCop::Cop::Migration::AddReference do
       expect_offense(<<~RUBY)
         call do
           add_reference(:projects, :users)
-          ^^^^^^^^^^^^^ `add_reference` requires `index: true`
+          ^^^^^^^^^^^^^ `add_reference` requires `index: true` or `index: { options... }`
         end
       RUBY
     end
@@ -38,7 +38,7 @@ describe RuboCop::Cop::Migration::AddReference do
       expect_offense(<<~RUBY)
         def up
           add_reference(:projects, :users, index: false)
-          ^^^^^^^^^^^^^ `add_reference` requires `index: true`
+          ^^^^^^^^^^^^^ `add_reference` requires `index: true` or `index: { options... }`
         end
       RUBY
     end
@@ -50,5 +50,13 @@ describe RuboCop::Cop::Migration::AddReference do
         end
       RUBY
     end
+
+    it 'does not register an offense when the index is unique' do
+      expect_no_offenses(<<~RUBY)
+        def up
+          add_reference(:projects, :users, index: { unique: true } )
+        end
+      RUBY
+    end
   end
 end