Pare back default `index` option for the migration generator

- Using `references` or `belongs_to` in migrations will always add index
  for the referenced column by default, without adding `index:true` option
  to generated migration file.
- Users can opt out of this by passing `index: false`.
- Legacy migrations won't be affected by this change. They will continue
  to run as they were before.
- Fixes #18146

7 files changed, 51 insertions, 12 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 81799b65d6..f3dc26ddb3 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,12 @@
+*   Using `references` or `belongs_to` in migrations will always add index
+    for the referenced column by default, without adding `index: true` option
+    to generated migration file. Users can opt out of this by passing
+    `index: false`.
+
+    Fixes #18146.
+
+    *Matthew Draper*, *Prathamesh Sonpatki*
+
 *   Run `type` attributes through attributes API type-casting before
     instantiating the corresponding subclass. This makes it possible to define
     custom STI mappings.
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
index 1cda23dc1d..690e0ba957 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
@@ -69,7 +69,7 @@ module ActiveRecord
       def initialize(
         name,
         polymorphic: false,
-        index: false,
+        index: true,
         foreign_key: false,
         type: :integer,
         **options
diff --git a/activerecord/lib/active_record/migration/compatibility.rb b/activerecord/lib/active_record/migration/compatibility.rb
index 1b94573870..5d742b523b 100644
--- a/activerecord/lib/active_record/migration/compatibility.rb
+++ b/activerecord/lib/active_record/migration/compatibility.rb
@@ -5,6 +5,12 @@ module ActiveRecord
 
       module FourTwoShared
         module TableDefinition
+          def references(*, **options)
+            options[:index] ||= false
+            super
+          end
+          alias :belongs_to :references
+
           def timestamps(*, **options)
             options[:null] = true if options[:null].nil?
             super
@@ -24,6 +30,12 @@ module ActiveRecord
           end
         end
 
+        def add_reference(*, **options)
+          options[:index] ||= false
+          super
+        end
+        alias :add_belongs_to :add_reference
+
         def add_timestamps(*, **options)
           options[:null] = true if options[:null].nil?
           super
diff --git a/activerecord/test/cases/migration/compatibility_test.rb b/activerecord/test/cases/migration/compatibility_test.rb
index b1e1d72944..6a9cdd9d29 100644
--- a/activerecord/test/cases/migration/compatibility_test.rb
+++ b/activerecord/test/cases/migration/compatibility_test.rb
@@ -53,6 +53,24 @@ module ActiveRecord
         ActiveRecord::Migrator.new(:up, [migration]).migrate
         assert_not connection.index_exists?(:testings, :bar)
       end
+
+      def test_references_does_not_add_index_by_default
+        migration = Class.new(ActiveRecord::Migration) {
+          def migrate(x)
+            create_table :more_testings do |t|
+              t.references :foo
+              t.belongs_to :bar, index: false
+            end
+          end
+        }.new
+
+        ActiveRecord::Migrator.new(:up, [migration]).migrate
+
+        assert_not connection.index_exists?(:more_testings, :foo_id)
+        assert_not connection.index_exists?(:more_testings, :bar_id)
+      ensure
+        connection.drop_table :more_testings rescue nil
+      end
     end
   end
 end
diff --git a/activerecord/test/cases/migration/references_foreign_key_test.rb b/activerecord/test/cases/migration/references_foreign_key_test.rb
index edbc8abe4d..b01415afb2 100644
--- a/activerecord/test/cases/migration/references_foreign_key_test.rb
+++ b/activerecord/test/cases/migration/references_foreign_key_test.rb
@@ -32,10 +32,10 @@ module ActiveRecord
         assert_equal [], @connection.foreign_keys("testings")
       end
 
-      test "foreign keys can be created in one query" do
+      test "foreign keys can be created in one query when index is not added" do
         assert_queries(1) do
           @connection.create_table :testings do |t|
-            t.references :testing_parent, foreign_key: true
+            t.references :testing_parent, foreign_key: true, index: false
           end
         end
       end
diff --git a/activerecord/test/cases/migration/references_index_test.rb b/activerecord/test/cases/migration/references_index_test.rb
index ad6b828d0b..a9a7f0f4c4 100644
--- a/activerecord/test/cases/migration/references_index_test.rb
+++ b/activerecord/test/cases/migration/references_index_test.rb
@@ -23,12 +23,12 @@ module ActiveRecord
         assert connection.index_exists?(table_name, :foo_id, :name => :index_testings_on_foo_id)
       end
 
-      def test_does_not_create_index
+      def test_creates_index_by_default_even_if_index_option_is_not_passed
         connection.create_table table_name do |t|
           t.references :foo
         end
 
-        assert_not connection.index_exists?(table_name, :foo_id, :name => :index_testings_on_foo_id)
+        assert connection.index_exists?(table_name, :foo_id, :name => :index_testings_on_foo_id)
       end
 
       def test_does_not_create_index_explicit
@@ -68,13 +68,13 @@ module ActiveRecord
         assert connection.index_exists?(table_name, :foo_id, :name => :index_testings_on_foo_id)
       end
 
-      def test_does_not_create_index_for_existing_table
+      def test_creates_index_for_existing_table_even_if_index_option_is_not_passed
         connection.create_table table_name
         connection.change_table table_name do |t|
           t.references :foo
         end
 
-        assert_not connection.index_exists?(table_name, :foo_id, :name => :index_testings_on_foo_id)
+        assert connection.index_exists?(table_name, :foo_id, :name => :index_testings_on_foo_id)
       end
 
       def test_does_not_create_index_for_existing_table_explicit
diff --git a/activerecord/test/cases/migration/references_statements_test.rb b/activerecord/test/cases/migration/references_statements_test.rb
index f613fd66c3..b9ce6bbc55 100644
--- a/activerecord/test/cases/migration/references_statements_test.rb
+++ b/activerecord/test/cases/migration/references_statements_test.rb
@@ -30,14 +30,14 @@ module ActiveRecord
         assert column_exists?(table_name, :taggable_type, :string)
       end
 
-      def test_creates_reference_id_index
-        add_reference table_name, :user, index: true
-        assert index_exists?(table_name, :user_id)
+      def test_does_not_create_reference_id_index_if_index_is_false
+        add_reference table_name, :user, index: false
+        assert_not index_exists?(table_name, :user_id)
       end
 
-      def test_does_not_create_reference_id_index
+      def test_create_reference_id_index_even_if_index_option_is_passed
         add_reference table_name, :user
-        assert_not index_exists?(table_name, :user_id)
+        assert index_exists?(table_name, :user_id)
       end
 
       def test_creates_polymorphic_index