diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 99d6e0b6..0d157429 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -11,7 +11,7 @@ on:
 jobs:
   test:
     runs-on: ubuntu-latest
-    
+
     services:
       postgres:
         image: postgres:17-alpine
@@ -26,7 +26,7 @@ jobs:
           --health-interval 10s
           --health-timeout 5s
           --health-retries 5
-          
+
       mysql:
         image: mysql:8
         ports:
@@ -47,6 +47,7 @@ jobs:
           - '3.4'
         rails:
           - '8.0'
+          - '8.1.0.rc1'
 
     steps:
       - name: Checkout
diff --git a/Gemfile b/Gemfile
index 80b33c68..82cdc1a3 100644
--- a/Gemfile
+++ b/Gemfile
@@ -5,10 +5,11 @@ source 'https://rubygems.org'
 gemspec
 
 gem 'dotenv'
-gem 'railties'
 gem 'with_advisory_lock', '>= 7'
 
-gem 'activerecord', "~> #{ENV['RAILS_VERSION'] || '8.0'}"
+rails_version = ENV['RAILS_VERSION'] || '8.0'
+gem 'activerecord', "~> #{rails_version}"
+gem 'railties', "~> #{rails_version}"
 
 platforms :mri, :truffleruby do
   # Database adapters
diff --git a/lib/closure_tree/support.rb b/lib/closure_tree/support.rb
index c13676d0..3a0a8ae7 100644
--- a/lib/closure_tree/support.rb
+++ b/lib/closure_tree/support.rb
@@ -33,6 +33,9 @@ def hierarchy_class_for_model
       hierarchy_class = parent_class.const_set(short_hierarchy_class_name, Class.new(model_class.superclass))
       model_class_name = model_class.to_s
       hierarchy_class.class_eval do
+        # Rails 8.1+ requires an implicit_order_column for models without a primary key
+        self.implicit_order_column = 'ancestor_id'
+
         belongs_to :ancestor, class_name: model_class_name
         belongs_to :descendant, class_name: model_class_name
         def ==(other)