Add option for index: false (#271)

lib/mongoid/slug.rb

@@ -18,6 +18,7 @@ module Slug
     included do
       cattr_accessor :slug_reserved_words,
                      :slug_scope,
+                     :slug_index,
                      :slugged_attributes,
                      :slug_url_builder,
                      :slug_history,
@@ -77,6 +78,7 @@ def slug(*fields, &block)
         options = fields.extract_options!
 
         self.slug_scope            = options[:scope]
+        self.slug_index            = options[:index].nil? ? true : options[:index]
         self.slug_reserved_words   = options[:reserve] || Set.new(%w[new edit])
         self.slugged_attributes    = fields.map(&:to_s)
         self.slug_history          = options[:history]
@@ -87,7 +89,7 @@ def slug(*fields, &block)
         alias_attribute :slugs, :_slugs
 
         # Set indexes
-        unless embedded?
+        if slug_index && !embedded?
           Mongoid::Slug::IndexBuilder.build_indexes(self, slug_scope_key, slug_by_model_type,
                                                     options[:localize])
         end

spec/models/no_index.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class NoIndex
+  include Mongoid::Document
+  include Mongoid::Slug
+  field :title
+
+  slug :title, index: false
+end

spec/mongoid/slug_spec.rb

@@ -598,6 +598,11 @@ class Person
       it_should_behave_like 'has an index', { _slugs: 1 }, unique: true, sparse: true
     end
 
+    context 'when slug index is skipped' do
+      subject { NoIndex }
+      it_should_behave_like 'does not have an index', _slugs: 1
+    end
+
     context 'when slug is scoped by a reference association' do
       subject { Author }
       it_should_behave_like 'does not have an index', _slugs: 1