diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index d6dc685..cb8bf9c 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -3,9 +3,22 @@ name: CI RSpec Test
 on: [push, pull_request]
 
 jobs:
+  rubocop:
+    runs-on: ubuntu-latest
+    env:
+      CI: true
+    steps:
+      - uses: actions/checkout@v3
+      - name: Set up Ruby 3.4
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: 3.4
+          bundler-cache: true
+      - name: Run RuboCop
+        run: bundle exec rubocop --parallel
+
   build:
-    name: >-
-      ${{ matrix.ruby }}
+    name: "${{ matrix.ruby }} mongoid-${{ matrix.mongoid }}"
     env:
       CI: true
       TESTOPTS: -v
@@ -14,18 +27,28 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [2.7, 3.0, 3.1, 3.2, truffleruby]
-        mongoid: [7, 8]
+        ruby:
+          - ruby-2.7
+          - ruby-3.0
+          - ruby-3.1
+          - ruby-3.2
+          - ruby-3.3
+          - ruby-3.4
+          - truffleruby
+        mongoid:
+          - '8.0'
+          - '8.1'
+          - '9.0'
         experimental: [false]
         include:
-          - ruby: 3.2
-            mongoid: HEAD
+          - ruby: ruby-3.2
+            mongoid: head
             experimental: true
-          - ruby: head
+          - ruby: ruby-head
             mongoid: 7
             experimental: true
-          - ruby: head
-            mongoid: HEAD
+          - ruby: ruby-head
+            mongoid: head
             experimental: true
           - ruby: jruby
             mongoid: 8
@@ -58,13 +81,6 @@ jobs:
         env:
           MONGOID_VERSION: ${{ matrix.mongoid }}
 
-      - name: rubocop
-        timeout-minutes: 5
-        run: bundle exec rubocop
-        continue-on-error: ${{ matrix.experimental }}
-        env:
-          MONGOID_VERSION: ${{ matrix.mongoid }}
-
       - name: test
         timeout-minutes: 10
         run: bundle exec rake spec
diff --git a/.rubocop.yml b/.rubocop.yml
index d9984b6..411beaf 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -29,6 +29,9 @@ Style/Documentation:
 Style/DoubleNegation:
   Enabled: false
 
+Style/FetchEnvVar:
+  Enabled: false
+
 Style/ModuleFunction:
   EnforcedStyle: extend_self
 
diff --git a/Gemfile b/Gemfile
index 17ccaec..c4faa6b 100644
--- a/Gemfile
+++ b/Gemfile
@@ -4,12 +4,12 @@ source 'https://rubygems.org'
 
 gemspec name: 'mongoid-slug'
 
-case (version = ENV['MONGOID_VERSION'] || '8')
-when 'HEAD'
+case (version = ENV['MONGOID_VERSION'])&.downcase
+when 'head'
   gem 'mongoid', github: 'mongodb/mongoid'
-when /\A\d+\z/
+when /\A\d+(?:\.\d+)?\z/
   gem 'mongoid', "~> #{version}.0"
-else
+when String
   gem 'mongoid', version
 end
 
diff --git a/lib/mongoid/slug.rb b/lib/mongoid/slug.rb
index 1f34b49..81234d3 100644
--- a/lib/mongoid/slug.rb
+++ b/lib/mongoid/slug.rb
@@ -79,7 +79,7 @@ def slug(*fields, &block)
         options = fields.extract_options!
 
         self.slug_scope            = options[:scope]
-        self.slug_index            = options[:index].nil? ? true : options[:index]
+        self.slug_index            = options[:index].nil? || 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]
diff --git a/lib/mongoid/slug/criteria.rb b/lib/mongoid/slug/criteria.rb
index 09b7de4..80ca7fd 100644
--- a/lib/mongoid/slug/criteria.rb
+++ b/lib/mongoid/slug/criteria.rb
@@ -26,11 +26,13 @@ class Criteria < Mongoid::Criteria
       # @example Find by multiple slugs.
       #   criteria.find([ 'some-slug', 'some-other-slug' ])
       #
-      # @param [ Array<Object> ] args The ids or slugs to search for.
+      # @param [ Array<Object> ] slugs The ids or slugs to search for.
       #
       # @return [ Array<Document>, Document ] The matching document(s).
-      def find(*args)
-        look_like_slugs?(args.__find_args__) ? find_by_slug!(*args) : super
+      def find(*slugs)
+        return find_by_slug!(*slugs) if look_like_slugs?(*slugs)
+
+        super
       end
 
       # Find the matchind document(s) in the criteria for the provided slugs.
@@ -41,21 +43,22 @@ def find(*args)
       # @example Find by multiple slugs.
       #   criteria.find([ 'some-slug', 'some-other-slug' ])
       #
-      # @param [ Array<Object> ] args The slugs to search for.
+      # @param [ Array<Object>... ] slugs The slugs to search for.
       #
       # @return [ Array<Document>, Document ] The matching document(s).
-      def find_by_slug!(*args)
-        slugs = args.__find_args__
+      def find_by_slug!(*slugs)
+        slugs = find_args(slugs)
         raise_invalid if slugs.any?(&:nil?)
-        for_slugs(slugs).execute_or_raise_for_slugs(slugs, args.multi_arged?)
+        for_slugs(slugs).execute_or_raise_for_slugs(slugs)
       end
 
-      def look_like_slugs?(args)
-        return false unless args.all? { |id| id.is_a?(String) }
+      def look_like_slugs?(*slugs)
+        slugs = find_args(slugs)
+        return false unless slugs.all?(String)
 
         id_field = @klass.fields['_id']
         @slug_strategy ||= id_field.options[:slug_id_strategy] || build_slug_strategy(id_field.type)
-        args.none? { |id| @slug_strategy.call(id) }
+        slugs.none? { |slug| @slug_strategy.call(slug) }
       end
 
       protected
@@ -94,10 +97,16 @@ def for_slugs(slugs)
         end
       end
 
-      def execute_or_raise_for_slugs(slugs, multi)
+      def find_args(args)
+        args = args.flatten
+        args.uniq!(&:to_s)
+        args
+      end
+
+      def execute_or_raise_for_slugs(slugs)
         result = uniq
         check_for_missing_documents_for_slugs!(result, slugs)
-        multi ? result : result.first
+        slugs.size == 1 ? result.first : result
       end
 
       def check_for_missing_documents_for_slugs!(result, slugs)
diff --git a/lib/mongoid/slug/unique_slug.rb b/lib/mongoid/slug/unique_slug.rb
index 3132129..f79bd8f 100644
--- a/lib/mongoid/slug/unique_slug.rb
+++ b/lib/mongoid/slug/unique_slug.rb
@@ -44,9 +44,8 @@ def highest_existing_counter
 
         def sort_existing_slugs
           # remove the slug part and leave the absolute integer part and sort
-          re = /^#{Regexp.escape(@slug)}/
           @sorted_existing = existing_slugs.map do |s|
-            s.sub(re, '').to_i.abs
+            s.delete_prefix(@slug).to_i.abs
           end.sort
         end