diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 4523ebb..4237833 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -14,13 +14,25 @@ jobs:
       fail-fast: false
       matrix:
         gemfile:
-          - Gemfile.rails-6.1-stable
           - Gemfile.rails-7.0-stable
           - Gemfile.rails-7.2-stable
-        ruby-version: ['3.1', '3.0']
+          - Gemfile.rails-8.0-stable
+          - Gemfile.rails-8.1-stable
+        ruby-version: ['3.0', '3.1', '3.2', '3.3']
         exclude:
+          # Rails 7.2 doesn't work with Ruby 3.0 (requires Ruby 3.1+)
           - gemfile: Gemfile.rails-7.2-stable
-            ruby-version: "3.0"
+            ruby-version: '3.0'
+          # Rails 8.0 doesn't work with Ruby 3.0 or 3.1 (requires Ruby 3.2+)
+          - gemfile: Gemfile.rails-8.0-stable
+            ruby-version: '3.0'
+          - gemfile: Gemfile.rails-8.0-stable
+            ruby-version: '3.1'
+          # Rails 8.1 requires Ruby 3.2+
+          - gemfile: Gemfile.rails-8.1-stable
+            ruby-version: '3.0'
+          - gemfile: Gemfile.rails-8.1-stable
+            ruby-version: '3.1'
     env:
       BUNDLE_GEMFILE: gemfiles/${{ matrix.gemfile }}
     steps:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7e66bf5..517b404 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,11 @@
 # Changelog
 
+## v3.5.0 (2026-04-30)
+
+### Added
+
+- Add Rails 8.1 compatibility. Rails 8.1 changed `PredicateBuilder::PolymorphicArrayValue#initialize` from `(associated_table, values)` to `(reflection, values)`, dropping the `@associated_table` ivar. `type_to_ids_mapping` now reads `@reflection` when available and falls back to `@associated_table.send(:reflection)` for older Rails versions.
+
 ## v3.2.1 (2023-12-14)
 
 ### Fixed
@@ -26,3 +32,11 @@
 
 - Remove unsupported rails versions(5.0, 5.2, 6.0) and ruby version(2.7)
 
+## v3.4.0 (2024-XX-XX)
+
+### Added
+
+- Add Rails 8.0 compatibility (requires Ruby 3.2+)
+
+### Removed 
+- Remove unsupported rails versions 6.x
\ No newline at end of file
diff --git a/README.md b/README.md
index 5d72bd9..d01a0a4 100644
--- a/README.md
+++ b/README.md
@@ -138,16 +138,23 @@ Lastly, you will need to be careful of any place where you are doing raw SQL que
 
 ## Setup
 
-You'll need to have git, Ruby, and MySQL. Then get up and running with a few commands:
+You'll need to have git and Ruby. Then get up and running with a few commands:
 
 ```bash
 $ git clone ...
 $ bundle install
 $ vim spec/support/database.yml # Update username and password
-$ bin/setup
+$ bin/setup # Uses SQLite3 for testing (no additional setup required)
 $ bundle exec rspec
 ```
 
+## Database Compatibility
+
+This gem works with any database supported by ActiveRecord (SQLite3, MySQL, PostgreSQL, etc.). 
+The gem extends ActiveRecord's polymorphic associations and doesn't use database-specific features.
+
+Development and testing uses SQLite3 for simplicity.
+
 ## Contributing
 
 1. Fork it
@@ -155,3 +162,13 @@ $ bundle exec rspec
 3. Commit your changes (`git commit -am 'Add some feature'`)
 4. Push to the branch (`git push origin my-new-feature`)
 5. Create new Pull Request
+
+# FCM Annotations
+
+This fork of `clio/polymorphic_integer_type` should stay in sync with upstream. As of the Rails 8 upgrade (FCM-8671) it is based on upstream **v3.5.0** with a single intentional difference.
+
+**The only change from the fork** is a guard in `lib/polymorphic_integer_type/belongs_to_polymorphic_association_extension.rb`. Upstream unconditionally rewrites a polymorphic association's stored `_type` to the target's `base_class`, which clobbers a model's custom `polymorphic_name` on plain `polymorphic: true` associations. We scope that rewrite to this gem's integer-type associations only, so plain polymorphic associations respect `polymorphic_name` (the `rma` app relies on this, for example: `Enrollment.polymorphic_name => 'Patient::Segment'`).
+
+Regression coverage is in `spec/fcm_polymorphic_name_guard_spec.rb`, along with the `Country` / `legacy_source` test fixtures.
+
+**When re-syncing with upstream, re-apply the guard after merging.**
diff --git a/Rakefile b/Rakefile
index 4fcea63..39eb26e 100644
--- a/Rakefile
+++ b/Rakefile
@@ -1,53 +1,52 @@
-require "bundler/gem_tasks"
-require "yaml"
-require "active_record"
+# frozen_string_literal: true
+
+require 'bundler/gem_tasks'
+require 'yaml'
+require 'active_record'
 
 namespace :test do
   task :all do
-    Dir.glob("./gemfiles/Gemfile*").each do |gemfile|
-      next if gemfile.end_with?(".lock")
+    Dir.glob('./gemfiles/Gemfile*').each do |gemfile|
+      next if gemfile.end_with?('.lock')
+
       puts "Running specs for #{Pathname.new(gemfile).basename}"
       system("BUNDLE_GEMFILE=#{gemfile} bundle install > /dev/null && BUNDLE_GEMFILE=#{gemfile} bundle exec rspec")
-      puts ""
+      puts ''
     end
   end
 end
 
 namespace :db do
-  database_config = YAML.load(File.open("./spec/support/database.yml"))
-  admin_database_config = database_config.merge(database: "mysql")
-  migration_path = File.expand_path("./spec/support/migrations")
+  database_config = YAML.load(File.open('./spec/support/database.yml'))
+  migration_path = File.expand_path('./spec/support/migrations')
 
-  desc "Create the database"
+  desc 'Create the database'
   task :create do
-    ActiveRecord::Base.establish_connection(admin_database_config)
-    puts "Database created."
+    # SQLite3 creates the database file automatically, just ensure directory exists
+    db_file = database_config.fetch(:database)
+    FileUtils.mkdir_p(File.dirname(db_file)) unless File.dirname(db_file) == '.'
+    puts 'Database ready (SQLite3).'
   end
 
-  desc "Migrate the database"
+  desc 'Migrate the database'
   task :migrate do
     ActiveRecord::Base.establish_connection(database_config)
-    if defined?(ActiveRecord::MigrationContext)
-      migration_context = ActiveRecord::MigrationContext.new(migration_path, ActiveRecord::SchemaMigration)
-      migration_context.migrate
-    else
-      ActiveRecord::Migrator.migrate(migration_path)
-    end
-    Rake::Task["db:schema"].invoke
-    puts "Database migrated."
+    ActiveRecord::MigrationContext.new(migration_path).migrate
+    Rake::Task['db:schema'].invoke
+    puts 'Database migrated.'
   end
 
-  desc "Drop the database"
+  desc 'Drop the database'
   task :drop do
-    ActiveRecord::Base.establish_connection(admin_database_config)
+    # For SQLite3, just delete the file
     db_file = database_config.fetch(:database)
     File.delete(db_file) if File.exist?(db_file)
-    puts "Database deleted."
+    puts 'Database deleted.'
   end
 
-  desc "Reset the database"
+  desc 'Reset the database'
   task reset: [:drop, :create, :migrate]
-    desc 'Create a db/schema.rb file that is portable against any DB supported by AR'
+  desc 'Create a db/schema.rb file that is portable against any DB supported by AR'
 
   task :schema do
     # Noop to make ActiveRecord happy
diff --git a/gemfiles/Gemfile.rails-6.1-stable b/gemfiles/Gemfile.rails-6.1-stable
deleted file mode 100644
index 5056859..0000000
--- a/gemfiles/Gemfile.rails-6.1-stable
+++ /dev/null
@@ -1,8 +0,0 @@
-# frozen_string_literal: true
-
-source "https://rubygems.org"
-
-gemspec path: ".."
-
-gem "activerecord", github: "rails/rails", branch: "6-1-stable"
-gem "sqlite3", "~> 1.4"
diff --git a/gemfiles/Gemfile.rails-8.0-stable b/gemfiles/Gemfile.rails-8.0-stable
new file mode 100644
index 0000000..c486345
--- /dev/null
+++ b/gemfiles/Gemfile.rails-8.0-stable
@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+gemspec path: ".."
+
+gem "activerecord", github: "rails/rails", branch: "8-0-stable"
diff --git a/gemfiles/Gemfile.rails-8.1-stable b/gemfiles/Gemfile.rails-8.1-stable
new file mode 100644
index 0000000..b6f721a
--- /dev/null
+++ b/gemfiles/Gemfile.rails-8.1-stable
@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+gemspec path: ".."
+
+gem "activerecord", github: "rails/rails", branch: "8-1-stable"
diff --git a/lib/polymorphic_integer_type.rb b/lib/polymorphic_integer_type.rb
index 5dbcb40..b5bf33d 100644
--- a/lib/polymorphic_integer_type.rb
+++ b/lib/polymorphic_integer_type.rb
@@ -4,7 +4,6 @@
 require "polymorphic_integer_type/extensions"
 require "polymorphic_integer_type/mapping"
 require "polymorphic_integer_type/module_generator"
-require "polymorphic_integer_type/belongs_to_valid_options_extension"
 require "polymorphic_integer_type/belongs_to_polymorphic_association_extension"
 require "polymorphic_integer_type/activerecord_5_0_0/polymorphic_array_value_extension"
 require "polymorphic_integer_type/polymorphic_foreign_association_extension"
diff --git a/lib/polymorphic_integer_type/activerecord_5_0_0/polymorphic_array_value_extension.rb b/lib/polymorphic_integer_type/activerecord_5_0_0/polymorphic_array_value_extension.rb
index df57836..b45df40 100644
--- a/lib/polymorphic_integer_type/activerecord_5_0_0/polymorphic_array_value_extension.rb
+++ b/lib/polymorphic_integer_type/activerecord_5_0_0/polymorphic_array_value_extension.rb
@@ -10,10 +10,14 @@ module PolymorphicArrayValueExtension
     # end
 
     def type_to_ids_mapping
-      if ACTIVE_RECORD_VERSION < Gem::Version.new("6.1")
-        association = @associated_table.send(:association)
+      # Rails 8.1 changed `PredicateBuilder::PolymorphicArrayValue#initialize`
+      # from `(associated_table, values)` to `(reflection, values)`, dropping
+      # the `@associated_table` ivar. Read whichever ivar is available so this
+      # gem works across Rails 6.1–8.1.
+      association = if instance_variable_defined?(:@reflection) && @reflection
+        @reflection
       else
-        association = @associated_table.send(:reflection)
+        @associated_table.send(:reflection)
       end
 
       name = association.name
diff --git a/lib/polymorphic_integer_type/belongs_to_polymorphic_association_extension.rb b/lib/polymorphic_integer_type/belongs_to_polymorphic_association_extension.rb
index 8d4b7c8..a7930ba 100644
--- a/lib/polymorphic_integer_type/belongs_to_polymorphic_association_extension.rb
+++ b/lib/polymorphic_integer_type/belongs_to_polymorphic_association_extension.rb
@@ -1,25 +1,37 @@
-module PolymorphicIntegerType
-  module BelongsToPolymorphicAssociationExtension
-    private
+module ActiveRecord
+  module Associations
+    class BelongsToPolymorphicAssociation < BelongsToAssociation
+      private
 
-    if Gem::Version.new(ActiveRecord::VERSION::STRING) < Gem::Version.new("6.1")
-      def replace_keys(record)
-        super
-
-        if reflection.options[:integer_type] || reflection.options[:polymorphic].is_a?(Hash)
-          owner[reflection.foreign_type] = record.class.base_class unless record.nil?
-        end
-      end
-    else
       def replace_keys(record, force: false)
         super
+        return if record.nil?
 
-        if reflection.options[:integer_type] || reflection.options[:polymorphic].is_a?(Hash)
-          owner[reflection.foreign_type] = record.class.base_class unless record.nil?
-        end
+        # FCM fork delta -- RE-APPLY after any upstream sync!
+        #
+        # This gem *reopens* (does not prepend) `BelongsToPolymorphicAssociation`,
+        # so the `super` above is BelongsToAssociation#replace_keys, which sets
+        # the foreign key but NOT the polymorphic type, so this method is
+        # responsible for the `_type`.
+        #
+        # Upstream sets it unconditionally to the target's `base_class`, which
+        # clobbers a model's custom `polymorphic_name` on plain `polymorphic: true`
+        # associations.
+        # 
+        # The rma app relies on custom `polymorphic_name` (e.g.
+        # Enrollment => 'Patient::Segment'), so use `base_class` ONLY for this
+        # gem's integer-type associations (detected via the generated
+        # `<foreign_type>_mapping` class method) and fall back to Rails'
+        # `polymorphic_name` for plain polymorphic associations.
+        # 
+        # Regression is tested in spec/fcm_polymorphic_name_guard_spec.rb.
+        owner[reflection.foreign_type] =
+          if owner.class.respond_to?("#{reflection.foreign_type}_mapping")
+            record.class.base_class
+          else
+            record.class.polymorphic_name
+          end
       end
     end
   end
 end
-
-ActiveRecord::Associations::BelongsToPolymorphicAssociation.prepend(PolymorphicIntegerType::BelongsToPolymorphicAssociationExtension)
diff --git a/lib/polymorphic_integer_type/belongs_to_valid_options_extension.rb b/lib/polymorphic_integer_type/belongs_to_valid_options_extension.rb
deleted file mode 100644
index 67dd805..0000000
--- a/lib/polymorphic_integer_type/belongs_to_valid_options_extension.rb
+++ /dev/null
@@ -1,11 +0,0 @@
-module PolymorphicIntegerType
-  module BelongsToValidOptionsExtension
-    def valid_options(options)
-      valid = super
-      valid << :integer_type if options[:polymorphic]
-      valid
-    end
-  end
-end
-
-ActiveRecord::Associations::Builder::BelongsTo.singleton_class.prepend(PolymorphicIntegerType::BelongsToValidOptionsExtension)
diff --git a/lib/polymorphic_integer_type/extensions.rb b/lib/polymorphic_integer_type/extensions.rb
index 47f3611..8e2144f 100644
--- a/lib/polymorphic_integer_type/extensions.rb
+++ b/lib/polymorphic_integer_type/extensions.rb
@@ -9,14 +9,15 @@ module ClassMethods
 
       def belongs_to(name, scope = nil, **options)
         options = scope if scope.kind_of? Hash
+        integer_type = options.delete :integer_type
         super
-        if options[:polymorphic] && (options[:integer_type] || options[:polymorphic].is_a?(Hash))
+        if options[:polymorphic] && (integer_type || options[:polymorphic].is_a?(Hash))
           mapping =
-            case options[:integer_type]
+            case integer_type
             when true then PolymorphicIntegerType::Mapping[name]
             when nil then options[:polymorphic]
             else
-              raise ArgumentError, "Unknown integer_type value: #{options[:integer_type].inspect}"
+              raise ArgumentError, "Unknown integer_type value: #{integer_type.inspect}"
             end.dup
 
           foreign_type = reflections[name.to_s].foreign_type
@@ -117,12 +118,7 @@ def remove_integer_type_and_set_attributes_and_extension(integer_type_values, re
         if is_polymorphic_integer
           reflection.foreign_integer_type = foreign_integer_type
           reflection.integer_type = integer_type
-
-          if Gem::Version.new(ActiveRecord::VERSION::STRING) < Gem::Version.new("6.1")
-            ActiveRecord::Associations::Association.prepend(PolymorphicIntegerType::PolymorphicForeignAssociationExtension)
-          else
-            ActiveRecord::Associations::ForeignAssociation.prepend(PolymorphicIntegerType::PolymorphicForeignAssociationExtension)
-          end
+          ActiveRecord::Associations::ForeignAssociation.prepend(PolymorphicIntegerType::PolymorphicForeignAssociationExtension)
         end
       end
 
diff --git a/lib/polymorphic_integer_type/version.rb b/lib/polymorphic_integer_type/version.rb
index febae78..27b1ad4 100644
--- a/lib/polymorphic_integer_type/version.rb
+++ b/lib/polymorphic_integer_type/version.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module PolymorphicIntegerType
-  VERSION = "3.3.0"
+  VERSION = '3.5.0'
 end
diff --git a/polymorphic_integer_type.gemspec b/polymorphic_integer_type.gemspec
index 52bb61a..babacda 100644
--- a/polymorphic_integer_type.gemspec
+++ b/polymorphic_integer_type.gemspec
@@ -1,27 +1,30 @@
-# coding: utf-8
-lib = File.expand_path('../lib', __FILE__)
+# frozen_string_literal: true
+
+lib = File.expand_path('lib', __dir__)
 $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 require 'polymorphic_integer_type/version'
 
 Gem::Specification.new do |spec|
-  spec.name          = "polymorphic_integer_type"
+  spec.name          = 'polymorphic_integer_type'
   spec.version       = PolymorphicIntegerType::VERSION
-  spec.authors       = ["Kyle d'Oliveira"]
-  spec.email         = ["kyle@goclio.com"]
-  spec.description   = %q{Allows the *_type field in the DB to be an integer rather than a string}
-  spec.summary       = %q{Use integers rather than strings for the _type field}
-  spec.homepage      = ""
-  spec.license       = "MIT"
+  spec.authors       = ['Kyle d\'Oliveira']
+  spec.email         = ['kyle@goclio.com']
+  spec.description   = 'Allows the *_type field in the DB to be an integer rather than a string'
+  spec.summary       = 'Use integers rather than strings for the _type field'
+  spec.homepage      = ''
+  spec.license       = 'MIT'
+
+  spec.required_ruby_version = '>= 3.0'
 
   spec.files         = `git ls-files -- . ':!.github/'`.split($/)
   spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
   spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
-  spec.require_paths = ["lib"]
+  spec.require_paths = ['lib']
 
-  spec.add_dependency "activerecord", "< 8"
-  spec.add_development_dependency "bundler"
-  spec.add_development_dependency "rake"
-  spec.add_development_dependency "rspec"
-  spec.add_development_dependency "sqlite3"
-  spec.add_development_dependency "pry-byebug"
+  spec.add_dependency 'activerecord', '< 9.0'
+  spec.add_development_dependency 'bundler'
+  spec.add_development_dependency 'pry-byebug'
+  spec.add_development_dependency 'rake'
+  spec.add_development_dependency 'rspec'
+  spec.add_development_dependency 'sqlite3'
 end
diff --git a/spec/fcm_polymorphic_name_guard_spec.rb b/spec/fcm_polymorphic_name_guard_spec.rb
new file mode 100644
index 0000000..0fa94bf
--- /dev/null
+++ b/spec/fcm_polymorphic_name_guard_spec.rb
@@ -0,0 +1,37 @@
+require 'spec_helper'
+
+# FCM fork regression test
+#
+# This fork of clio/polymorphic_integer_type carries exactly ONE intentional
+# difference over upstream (see the "FCM Annotations" section of README.md and
+# the guard in lib/polymorphic_integer_type/belongs_to_polymorphic_association_extension.rb):
+#
+# Upstream unconditionally rewrites a polymorphic association's stored `_type`
+# to the target's `base_class`. That clobbers a model's custom `polymorphic_name`
+# on PLAIN `polymorphic: true` associations. The rma app depends on custom
+# `polymorphic_name` (e.g. Enrollment.polymorphic_name => 'Patient::Segment'),
+# so the guard restricts that rewrite to this gem's integer-type associations only.
+#
+# If a future upstream re-sync drops the guard, these specs fail - which is the point.
+RSpec.describe 'FCM: polymorphic_name on plain polymorphic associations' do
+  let(:country) { Country.create!(name: 'Canada') }
+
+  it "stores the target's polymorphic_name (not its base_class) on a plain polymorphic belongs_to" do
+    link = Link.create!(legacy_source: country)
+
+    # Pure upstream (no guard) would store 'Country' (base_class) here.
+    expect(link.legacy_source_type).to eq('Nation')
+  end
+
+  it 'reads the plain polymorphic association back via polymorphic_class_for' do
+    link = Link.create!(legacy_source: country)
+
+    expect(link.reload.legacy_source).to eq(country)
+  end
+
+  it 'guards plain polymorphic associations only, leaving integer-type ones to the gem' do
+    # `source` is integer-type (the gem defines `source_type_mapping`); `legacy_source` is plain.
+    expect(Link).to respond_to(:source_type_mapping)
+    expect(Link).not_to respond_to(:legacy_source_type_mapping)
+  end
+end
diff --git a/spec/polymorphic_integer_type_spec.rb b/spec/polymorphic_integer_type_spec.rb
index 08a25c0..73a4cdd 100644
--- a/spec/polymorphic_integer_type_spec.rb
+++ b/spec/polymorphic_integer_type_spec.rb
@@ -136,6 +136,11 @@
       expect(Link.where(source: [source])).to eq [link]
     end
 
+    it "properly finds objects when passing an array of mixed-type sources" do
+      link_for_dog = Link.create(source: dog)
+      expect(Link.where(source: [cat, dog])).to match_array [link, link_for_dog]
+    end
+
     it "properly finds the object with a find_by" do
       expect(Link.find_by(source: source, id: link.id)).to eql link
     end
@@ -179,7 +184,6 @@
       end
     end
   end
-
   context "When a link is given polymorphic record" do
     let(:link) { Link.create(source: source) }
     let(:source) { cat }
@@ -194,17 +198,6 @@
     end
   end
 
-  context "When a link is given polymorphic record via a non-integer type association" do
-    let(:link) { Link.create(legacy_source: source) }
-    let(:source) { Place.create(name: "Main Street") }
-
-    it "appropriately sets the source_id and source_type to the polymorphic_name" do
-      expect(link.legacy_source_id).to eql source.id
-      expect(link.legacy_source_type).to eql Place.polymorphic_name
-      expect(link.legacy_source).to eql source
-    end
-  end
-
   context "When a link is given polymorphic id and type" do
     let(:link) { Link.create(source_id: source.id, source_type: source.class.to_s) }
     let(:source) { cat }
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index c802cb9..26774ac 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -3,6 +3,7 @@
 require 'polymorphic_integer_type'
 require 'support/configuration'
 require 'support/link'
+require 'support/country'
 require 'support/animal'
 require 'support/namespaced_animal'
 require 'support/namespaced_plant'
@@ -13,7 +14,6 @@
 require 'support/profile'
 require 'support/profile_history'
 require 'support/namespaced_activity'
-require 'support/place'
 require 'byebug'
 require 'pry'
 
@@ -21,17 +21,10 @@
   config.before(:suite) do
     database_config = YAML.load(File.open("#{File.dirname(__FILE__)}/support/database.yml"))
     migrations_path = "#{File.dirname(__FILE__)}/support/migrations"
-    active_record_version = Gem::Version.new(ActiveRecord::VERSION::STRING)
 
     ActiveRecord::Base.establish_connection(database_config)
-    
-    if active_record_version >= Gem::Version.new("6.1") && active_record_version < Gem::Version.new("7.0")
-      ActiveRecord::MigrationContext.new(migrations_path, ActiveRecord::SchemaMigration).migrate      
-    end
 
-    if active_record_version >= Gem::Version.new("7.0")
-      ActiveRecord::MigrationContext.new(migrations_path).migrate
-    end
+    ActiveRecord::MigrationContext.new(migrations_path).migrate
   end
 
   config.around do |example|
diff --git a/spec/support/country.rb b/spec/support/country.rb
new file mode 100644
index 0000000..15f36ac
--- /dev/null
+++ b/spec/support/country.rb
@@ -0,0 +1,12 @@
+# FCM regression fixture (see spec/fcm_polymorphic_name_guard_spec.rb).
+#
+# A plain (non-integer-type) polymorphic target whose `polymorphic_name` differs
+# from its class name. This mirrors the rma app's real case (Enrollment.polymorphic_name =>
+# 'Patient::Segment') and is used to prove the fork's guard leaves plain
+# polymorphic associations to Rails so the custom polymorphic_name is stored -
+# rather than upstream's unconditional base_class rewrite.
+class Country < ActiveRecord::Base
+  def self.polymorphic_name
+    'Nation'
+  end
+end
diff --git a/spec/support/link.rb b/spec/support/link.rb
index 00deb6f..3be89ca 100644
--- a/spec/support/link.rb
+++ b/spec/support/link.rb
@@ -1,12 +1,17 @@
 class Link < ActiveRecord::Base
   include PolymorphicIntegerType::Extensions
 
-  def self.polymorphic_class_for(name)
-    name == 'Country' ? Place : super
-  end
-
   belongs_to :source, polymorphic: true, integer_type: true
   belongs_to :target, polymorphic: true, integer_type: true
 
+  # FCM regression fixture (see spec/fcm_polymorphic_name_guard_spec.rb): a
+  # plain (non-integer-type) polymorphic association whose target (Country)
+  # overrides `polymorphic_name`. Exercises the fork's guard that leaves plain
+  # polymorphic associations to Rails instead of forcing the target's base_class.
   belongs_to :legacy_source, polymorphic: true
+
+  # Resolve the custom polymorphic_name ('Nation') back to Country on read.
+  def self.polymorphic_class_for(name)
+    name == 'Nation' ? Country : super
+  end
 end
diff --git a/spec/support/migrations/10_create_country_table_and_legacy_source_reference.rb b/spec/support/migrations/10_create_country_table_and_legacy_source_reference.rb
new file mode 100644
index 0000000..893164b
--- /dev/null
+++ b/spec/support/migrations/10_create_country_table_and_legacy_source_reference.rb
@@ -0,0 +1,23 @@
+# FCM regression fixture - supports spec/fcm_polymorphic_name_guard_spec.rb and
+# the fork delta documented in lib/polymorphic_integer_type/belongs_to_polymorphic_association_extension.rb.
+#
+# Adds a plain (string-typed) polymorphic `legacy_source` to `links`, plus a
+# `countries` table whose model (Country) overrides `polymorphic_name`.
+class CreateCountryTableAndLegacySourceReference < ActiveRecord::Migration[5.0]
+
+  def up
+    create_table :countries do |t|
+      t.string :name
+    end
+
+    add_column :links, :legacy_source_id, :integer
+    add_column :links, :legacy_source_type, :string
+  end
+
+  def down
+    remove_column :links, :legacy_source_type
+    remove_column :links, :legacy_source_id
+    drop_table :countries
+  end
+
+end
diff --git a/spec/support/migrations/11_add_legacy_source_reference_to_links_table.rb b/spec/support/migrations/11_add_legacy_source_reference_to_links_table.rb
deleted file mode 100644
index 5f67911..0000000
--- a/spec/support/migrations/11_add_legacy_source_reference_to_links_table.rb
+++ /dev/null
@@ -1,7 +0,0 @@
-class AddLegacySourceReferenceToLinksTable < ActiveRecord::Migration[5.0]
-  def change
-    add_reference :links, :legacy_source, polymorphic: true
-  end
-end
-
-
diff --git a/spec/support/place.rb b/spec/support/place.rb
deleted file mode 100644
index fc6b981..0000000
--- a/spec/support/place.rb
+++ /dev/null
@@ -1,7 +0,0 @@
-class Place < ActiveRecord::Base
-  has_many :source_links, as: :target, inverse_of: :legacy_source, class_name: "Link"
-
-  def self.polymorphic_name
-    'Country'
-  end
-end