From 436277da88507f9aae0874e62f3e61a8546b9683 Mon Sep 17 00:00:00 2001
From: eileencodes <eileencodes@gmail.com>
Date: Fri, 26 Aug 2022 10:47:00 -0400
Subject: [PATCH] Move SchemaMigration to an independent object

Previously, SchemaMigration inherited from ActiveRecord::Base. This is
problematic for multiple databases and resulted in building the code in
AbstractAdapter that was previously there. Rather than hacking around
the fact that SchemaMigration inherits from Base, this PR makes
SchemaMigration an independent object. Then each connection can get it's
own SchemaMigration object. This change required defining the methods
that SchemaMigration was depending on ActiveRecord::Base for (ex
create!). I reimplemented only the methods called by the framework as
this class is no-doc's so it doesn't need to implement anything beyond
that. Now each connection gets it's own SchemaMigration object which
stores the connection. I also decided to update the method names (create
-> create_version, delete_by -> delete_version, delete_all ->
delete_all_versions) to be more explicit.

This change also required adding a NullSchemaMigraiton class for cases
when we don't have a connection yet but still need to copy migrations
from the MigrationContext. Ultimately I think this is a little weird -
we need to do so much work to pick up a set of files? Maybe something to
explore in the future.

Aside from removing the hack we added back in #36439 this change will
enable my work to stop clobbering and depending directly on
Base.connection in the rake tasks. While working on this I discovered
that we always have a `ActiveRecord::SchemaMigration` because the
connection is always on `Base` in the rake tasks. This will free us up
to do less hacky stuff in the migrations and tasks.
---
 activerecord/CHANGELOG.md                     |   6 +
 .../abstract/schema_statements.rb             |   2 +-
 .../connection_adapters/abstract_adapter.rb   |  16 +--
 activerecord/lib/active_record/migration.rb   |  35 +++---
 .../lib/active_record/schema_migration.rb     | 107 ++++++++++++------
 .../lib/active_record/tasks/database_tasks.rb |   2 +-
 .../test/cases/active_record_schema_test.rb   |   6 +-
 .../adapters/mysql2/schema_migrations_test.rb |   8 +-
 .../adapters/mysql2/table_options_test.rb     |   2 +-
 .../postgresql/extension_migration_test.rb    |   6 +-
 .../cases/adapters/postgresql/uuid_test.rb    |   4 +-
 .../cases/migration/compatibility_test.rb     |   6 +-
 .../test/cases/migration/logger_test.rb       |   2 +-
 activerecord/test/cases/migration_test.rb     |  57 ++++++----
 activerecord/test/cases/migrator_test.rb      |  44 +++----
 .../test/cases/multi_db_migrator_test.rb      |  26 +++--
 activerecord/test/cases/schema_dumper_test.rb |   9 +-
 .../test/cases/tasks/database_tasks_test.rb   |  28 ++---
 railties/test/application/loading_test.rb     |   4 +-
 railties/test/railties/engine_test.rb         |   2 +-
 20 files changed, 210 insertions(+), 162 deletions(-)

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 4a8c9314db..a0ca6d9c14 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Move `ActiveRecord::SchemaMigration` to an independent object.
+
+    `ActiveRecord::SchemaMigration` no longer inherits from `ActiveRecord::Base` and is now an independent object that should be instantiated with a `connection`. This class is private and should not be used by applications directly. If you want to interact with the schema migrations table, please access it on the connection directly, for example: `ActiveRecord::Base.connection.schema_migration`.
+
+    *Eileen M. Uchitelle*
+
 *   Deprecate `all_connection_pools` and make `connection_pool_list` more explicit.
 
     Following on #45924 `all_connection_pools` is now deprecated. `connection_pool_list` will either take an explicit role or applications can opt into the new behavior by passing `:all`.
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
index d4fcaaa399..50b09884dc 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
@@ -1270,7 +1270,7 @@ def check_constraint_exists?(table_name, **options)
       end
 
       def dump_schema_information # :nodoc:
-        versions = schema_migration.all_versions
+        versions = schema_migration.versions
         insert_versions_sql(versions) if versions.any?
       end
 
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
index e88b02544b..f32c1c0a97 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
@@ -193,21 +193,7 @@ def migration_context # :nodoc:
       end
 
       def schema_migration # :nodoc:
-        @schema_migration ||= begin
-                                conn = self
-                                connection_name = conn.pool.pool_config.connection_name
-
-                                return ActiveRecord::SchemaMigration if connection_name == "ActiveRecord::Base"
-
-                                schema_migration_name = "#{connection_name}::SchemaMigration"
-
-                                Class.new(ActiveRecord::SchemaMigration) do
-                                  define_singleton_method(:name) { schema_migration_name }
-                                  define_singleton_method(:to_s) { schema_migration_name }
-
-                                  self.connection_specification_name = connection_name
-                                end
-                              end
+        SchemaMigration.new(self)
       end
 
       def internal_metadata # :nodoc:
diff --git a/activerecord/lib/active_record/migration.rb b/activerecord/lib/active_record/migration.rb
index 2be7aff302..e152a110c4 100644
--- a/activerecord/lib/active_record/migration.rb
+++ b/activerecord/lib/active_record/migration.rb
@@ -950,14 +950,13 @@ def method_missing(method, *arguments, &block)
 
     def copy(destination, sources, options = {})
       copied = []
-      schema_migration = options[:schema_migration] || ActiveRecord::SchemaMigration
 
       FileUtils.mkdir_p(destination) unless File.exist?(destination)
 
-      destination_migrations = ActiveRecord::MigrationContext.new(destination, schema_migration).migrations
+      destination_migrations = ActiveRecord::MigrationContext.new(destination, SchemaMigration::NullSchemaMigration.new).migrations
       last = destination_migrations.last
       sources.each do |scope, path|
-        source_migrations = ActiveRecord::MigrationContext.new(path, schema_migration).migrations
+        source_migrations = ActiveRecord::MigrationContext.new(path, SchemaMigration::NullSchemaMigration.new).migrations
 
         source_migrations.each do |migration|
           source = File.binread(migration.filename)
@@ -1018,7 +1017,7 @@ def next_migration_number(number)
       if ActiveRecord.timestamped_migrations
         [Time.now.utc.strftime("%Y%m%d%H%M%S"), "%.14d" % number].max
       else
-        SchemaMigration.normalize_migration_number(number)
+        "%.3d" % number.to_i
       end
     end
 
@@ -1076,15 +1075,21 @@ def load_migration
   #
   # A migration context requires the path to the migrations is set
   # in the +migrations_paths+ parameter. Optionally a +schema_migration+
-  # class can be provided. For most applications, +SchemaMigration+ is
-  # sufficient. Multiple database applications need a +SchemaMigration+
-  # per primary database.
+  # class can be provided. Multiple database applications will instantiate
+  # a +SchemaMigration+ object per database. From the Rake tasks, Rails will
+  # handle this for you.
   class MigrationContext
     attr_reader :migrations_paths, :schema_migration, :internal_metadata
 
-    def initialize(migrations_paths, schema_migration = SchemaMigration, internal_metadata = InternalMetadata)
+    def initialize(migrations_paths, schema_migration = nil, internal_metadata = InternalMetadata)
+      if schema_migration == SchemaMigration
+        schema_migration = SchemaMigration.new(ActiveRecord::Base.connection)
+
+        ActiveSupport::Deprecation.warn("SchemaMigration no longer inherits from ActiveRecord::Base. Please instaniate a new SchemaMigration object with the desired connection, ie `ActiveRecord::SchemaMigration.new(ActiveRecord::Base.connection)`.")
+      end
+
       @migrations_paths = migrations_paths
-      @schema_migration = schema_migration
+      @schema_migration = schema_migration || SchemaMigration.new(ActiveRecord::Base.connection)
       @internal_metadata = internal_metadata
     end
 
@@ -1152,7 +1157,7 @@ def open # :nodoc:
 
     def get_all_versions # :nodoc:
       if schema_migration.table_exists?
-        schema_migration.all_versions.map(&:to_i)
+        schema_migration.integer_versions
       else
         []
       end
@@ -1257,7 +1262,9 @@ class << self
 
       # For cases where a table doesn't exist like loading from schema cache
       def current_version
-        MigrationContext.new(migrations_paths, SchemaMigration, InternalMetadata).current_version
+        schema_migration = SchemaMigration.new(ActiveRecord::Base.connection)
+
+        MigrationContext.new(migrations_paths, schema_migration, InternalMetadata).current_version
       end
     end
 
@@ -1327,7 +1334,7 @@ def migrated
     end
 
     def load_migrated
-      @migrated_versions = Set.new(@schema_migration.all_versions.map(&:to_i))
+      @migrated_versions = Set.new(@schema_migration.integer_versions)
     end
 
     private
@@ -1406,10 +1413,10 @@ def validate(migrations)
       def record_version_state_after_migrating(version)
         if down?
           migrated.delete(version)
-          @schema_migration.delete_by(version: version.to_s)
+          @schema_migration.delete_version(version.to_s)
         else
           migrated << version
-          @schema_migration.create!(version: version.to_s)
+          @schema_migration.create_version(version.to_s)
         end
       end
 
diff --git a/activerecord/lib/active_record/schema_migration.rb b/activerecord/lib/active_record/schema_migration.rb
index 624b49586f..7b977f005f 100644
--- a/activerecord/lib/active_record/schema_migration.rb
+++ b/activerecord/lib/active_record/schema_migration.rb
@@ -1,54 +1,87 @@
 # frozen_string_literal: true
 
-require "active_record/scoping/default"
-require "active_record/scoping/named"
-
 module ActiveRecord
   # This class is used to create a table that keeps track of which migrations
   # have been applied to a given database. When a migration is run, its schema
-  # number is inserted in to the `SchemaMigration.table_name` so it doesn't need
+  # number is inserted in to the schema migrations table so it doesn't need
   # to be executed the next time.
-  class SchemaMigration < ActiveRecord::Base # :nodoc:
-    class << self
-      def primary_key
-        "version"
-      end
+  class SchemaMigration # :nodoc:
+    attr_reader :connection, :arel_table
 
-      def table_name
-        "#{table_name_prefix}#{schema_migrations_table_name}#{table_name_suffix}"
-      end
+    def initialize(connection)
+      @connection = connection
+      @arel_table = Arel::Table.new(table_name)
+    end
 
-      def create_table
-        unless connection.table_exists?(table_name)
-          connection.create_table(table_name, id: false) do |t|
-            t.string :version, **connection.internal_string_options_for_primary_key
-          end
-        end
-      end
+    def create_version(version)
+      im = Arel::InsertManager.new(arel_table)
+      im.insert(arel_table[primary_key] => version)
+      connection.insert(im, "#{self} Create", primary_key, version)
+    end
 
-      def drop_table
-        connection.drop_table table_name, if_exists: true
-      end
+    def delete_version(version)
+      dm = Arel::DeleteManager.new(arel_table)
+      dm.wheres = [arel_table[primary_key].eq(version)]
 
-      def normalize_migration_number(number)
-        "%.3d" % number.to_i
-      end
+      connection.delete(dm, "#{self} Destroy")
+    end
 
-      def normalized_versions
-        all_versions.map { |v| normalize_migration_number v }
-      end
-
-      def all_versions
-        order(:version).pluck(:version)
-      end
-
-      def table_exists?
-        connection.data_source_exists?(table_name)
+    def delete_all_versions
+      versions.each do |version|
+        delete_version(version)
       end
     end
 
-    def version
-      super.to_i
+    def primary_key
+      "version"
+    end
+
+    def table_name
+      "#{ActiveRecord::Base.table_name_prefix}#{ActiveRecord::Base.schema_migrations_table_name}#{ActiveRecord::Base.table_name_suffix}"
+    end
+
+    def create_table
+      unless connection.table_exists?(table_name)
+        connection.create_table(table_name, id: false) do |t|
+          t.string :version, **connection.internal_string_options_for_primary_key
+        end
+      end
+    end
+
+    def drop_table
+      connection.drop_table table_name, if_exists: true
+    end
+
+    def normalize_migration_number(number)
+      "%.3d" % number.to_i
+    end
+
+    def normalized_versions
+      versions.map { |v| normalize_migration_number v }
+    end
+
+    def versions
+      sm = Arel::SelectManager.new(arel_table)
+      sm.project(arel_table[primary_key])
+      sm.order(arel_table[primary_key].asc)
+      connection.select_values(sm)
+    end
+
+    def integer_versions
+      versions.map(&:to_i)
+    end
+
+    def count
+      sm = Arel::SelectManager.new(arel_table)
+      sm.project(*Arel::Nodes::Count.new([Arel.star]))
+      connection.select_values(sm).first
+    end
+
+    def table_exists?
+      connection.data_source_exists?(table_name)
+    end
+
+    class NullSchemaMigration
     end
   end
 end
diff --git a/activerecord/lib/active_record/tasks/database_tasks.rb b/activerecord/lib/active_record/tasks/database_tasks.rb
index bbb7494bef..e8485adbf4 100644
--- a/activerecord/lib/active_record/tasks/database_tasks.rb
+++ b/activerecord/lib/active_record/tasks/database_tasks.rb
@@ -192,7 +192,7 @@ def prepare_all
           ActiveRecord::Base.establish_connection(db_config)
 
           begin
-            database_initialized = ActiveRecord::SchemaMigration.table_exists?
+            database_initialized = ActiveRecord::Base.connection.schema_migration.table_exists?
           rescue ActiveRecord::NoDatabaseError
             create(db_config)
             retry
diff --git a/activerecord/test/cases/active_record_schema_test.rb b/activerecord/test/cases/active_record_schema_test.rb
index cffe187bb1..6c7b5cc65c 100644
--- a/activerecord/test/cases/active_record_schema_test.rb
+++ b/activerecord/test/cases/active_record_schema_test.rb
@@ -20,7 +20,7 @@ class ActiveRecordSchemaTest < ActiveRecord::TestCase
     @connection.drop_table :nep_schema_migrations rescue nil
     @connection.drop_table :has_timestamps rescue nil
     @connection.drop_table :multiple_indexes rescue nil
-    @schema_migration.delete_all rescue nil
+    @schema_migration.delete_all_versions rescue nil
     ActiveRecord::Migration.verbose = @original_verbose
   end
 
@@ -31,7 +31,7 @@ def test_has_primary_key
 
     @schema_migration.create_table
     assert_difference "@schema_migration.count", 1 do
-      @schema_migration.create version: 12
+      @schema_migration.create_version(12)
     end
   ensure
     @schema_migration.drop_table
@@ -69,7 +69,6 @@ def test_schema_define
   def test_schema_define_with_table_name_prefix
     old_table_name_prefix = ActiveRecord::Base.table_name_prefix
     ActiveRecord::Base.table_name_prefix = "nep_"
-    @schema_migration.reset_table_name
     @internal_metadata.reset_table_name
     ActiveRecord::Schema.define(version: 7) do
       create_table :fruits do |t|
@@ -82,7 +81,6 @@ def test_schema_define_with_table_name_prefix
     assert_equal 7, @connection.migration_context.current_version
   ensure
     ActiveRecord::Base.table_name_prefix = old_table_name_prefix
-    @schema_migration.reset_table_name
     @internal_metadata.reset_table_name
   end
 
diff --git a/activerecord/test/cases/adapters/mysql2/schema_migrations_test.rb b/activerecord/test/cases/adapters/mysql2/schema_migrations_test.rb
index 4b8c00fa3c..436bda9258 100644
--- a/activerecord/test/cases/adapters/mysql2/schema_migrations_test.rb
+++ b/activerecord/test/cases/adapters/mysql2/schema_migrations_test.rb
@@ -5,6 +5,10 @@
 class SchemaMigrationsTest < ActiveRecord::Mysql2TestCase
   self.use_transactional_tests = false
 
+  def setup
+    @schema_migration = ActiveRecord::Base.connection.schema_migration
+  end
+
   def test_renaming_index_on_foreign_key
     connection.add_index "engines", "car_id"
     connection.add_foreign_key :engines, :cars, name: "fk_engines_cars"
@@ -17,10 +21,10 @@ def test_renaming_index_on_foreign_key
 
   def test_initializes_schema_migrations_for_encoding_utf8mb4
     with_encoding_utf8mb4 do
-      table_name = ActiveRecord::SchemaMigration.table_name
+      table_name = @schema_migration.table_name
       connection.drop_table table_name, if_exists: true
 
-      ActiveRecord::SchemaMigration.create_table
+      @schema_migration.create_table
 
       assert connection.column_exists?(table_name, :version, :string)
     end
diff --git a/activerecord/test/cases/adapters/mysql2/table_options_test.rb b/activerecord/test/cases/adapters/mysql2/table_options_test.rb
index 6edbe769e7..101dd76ff4 100644
--- a/activerecord/test/cases/adapters/mysql2/table_options_test.rb
+++ b/activerecord/test/cases/adapters/mysql2/table_options_test.rb
@@ -94,7 +94,7 @@ def teardown
     ActiveRecord::Base.logger       = @logger_was
     ActiveRecord::Migration.verbose = @verbose_was
     ActiveRecord::Base.connection.drop_table "mysql_table_options", if_exists: true
-    ActiveRecord::SchemaMigration.delete_all rescue nil
+    ActiveRecord::Base.connection.schema_migration.delete_all_versions rescue nil
   end
 
   test "new migrations do not contain default ENGINE=InnoDB option" do
diff --git a/activerecord/test/cases/adapters/postgresql/extension_migration_test.rb b/activerecord/test/cases/adapters/postgresql/extension_migration_test.rb
index 908a1d2f2b..21fae532d1 100644
--- a/activerecord/test/cases/adapters/postgresql/extension_migration_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/extension_migration_test.rb
@@ -27,20 +27,18 @@ def setup
 
     ActiveRecord::Base.table_name_prefix = "p_"
     ActiveRecord::Base.table_name_suffix = "_s"
-    @connection.schema_migration.reset_table_name
     @connection.internal_metadata.reset_table_name
 
-    @connection.schema_migration.delete_all rescue nil
+    @connection.schema_migration.delete_all_versions rescue nil
     ActiveRecord::Migration.verbose = false
   end
 
   def teardown
-    @connection.schema_migration.delete_all rescue nil
+    @connection.schema_migration.delete_all_versions rescue nil
     ActiveRecord::Migration.verbose = true
 
     ActiveRecord::Base.table_name_prefix = @old_table_name_prefix
     ActiveRecord::Base.table_name_suffix = @old_table_name_suffix
-    @connection.schema_migration.reset_table_name
     @connection.internal_metadata.reset_table_name
 
     super
diff --git a/activerecord/test/cases/adapters/postgresql/uuid_test.rb b/activerecord/test/cases/adapters/postgresql/uuid_test.rb
index 82041a5f23..ea34014a6d 100644
--- a/activerecord/test/cases/adapters/postgresql/uuid_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/uuid_test.rb
@@ -308,7 +308,7 @@ def migrate(x)
   ensure
     drop_table "pg_uuids_4"
     ActiveRecord::Migration.verbose = @verbose_was
-    ActiveRecord::Base.connection.schema_migration.delete_all
+    ActiveRecord::Base.connection.schema_migration.delete_all_versions
   end
   uses_transaction :test_schema_dumper_for_uuid_primary_key_default_in_legacy_migration
 end
@@ -360,7 +360,7 @@ def migrate(x)
   ensure
     drop_table "pg_uuids_4"
     ActiveRecord::Migration.verbose = @verbose_was
-    ActiveRecord::Base.connection.schema_migration.delete_all
+    ActiveRecord::Base.connection.schema_migration.delete_all_versions
   end
   uses_transaction :test_schema_dumper_for_uuid_primary_key_with_default_nil_in_legacy_migration
 end
diff --git a/activerecord/test/cases/migration/compatibility_test.rb b/activerecord/test/cases/migration/compatibility_test.rb
index 1fbd0adf5d..49026d38ff 100644
--- a/activerecord/test/cases/migration/compatibility_test.rb
+++ b/activerecord/test/cases/migration/compatibility_test.rb
@@ -30,7 +30,7 @@ def setup
       teardown do
         connection.drop_table :testings rescue nil
         ActiveRecord::Migration.verbose = @verbose_was
-        @schema_migration.delete_all rescue nil
+        @schema_migration.delete_all_versions rescue nil
       end
 
       def test_migration_doesnt_remove_named_index
@@ -685,7 +685,7 @@ def setup
 
   def teardown
     ActiveRecord::Migration.verbose = @verbose_was
-    @schema_migration.delete_all rescue nil
+    @schema_migration.delete_all_versions rescue nil
     connection.drop_table :testings rescue nil
   end
 
@@ -812,7 +812,7 @@ def setup
   def teardown
     @migration.migrate(:down) if @migration
     ActiveRecord::Migration.verbose = @verbose_was
-    ActiveRecord::SchemaMigration.delete_all rescue nil
+    ActiveRecord::Base.connection.schema_migration.delete_all_versions rescue nil
     LegacyPrimaryKey.reset_column_information
   end
 
diff --git a/activerecord/test/cases/migration/logger_test.rb b/activerecord/test/cases/migration/logger_test.rb
index ed4e8d853c..92836f69e6 100644
--- a/activerecord/test/cases/migration/logger_test.rb
+++ b/activerecord/test/cases/migration/logger_test.rb
@@ -19,7 +19,7 @@ def setup
         super
         @schema_migration = ActiveRecord::Base.connection.schema_migration
         @schema_migration.create_table
-        @schema_migration.delete_all
+        @schema_migration.delete_all_versions
         @internal_metadata = ActiveRecord::Base.connection.internal_metadata
       end
 
diff --git a/activerecord/test/cases/migration_test.rb b/activerecord/test/cases/migration_test.rb
index 915f6fa32c..471ff885bb 100644
--- a/activerecord/test/cases/migration_test.rb
+++ b/activerecord/test/cases/migration_test.rb
@@ -53,8 +53,8 @@ def setup
     ActiveRecord::Base.table_name_prefix = ""
     ActiveRecord::Base.table_name_suffix = ""
 
-    ActiveRecord::SchemaMigration.create_table
-    ActiveRecord::SchemaMigration.delete_all
+    @schema_migration.create_table
+    @schema_migration.delete_all_versions
 
     %w(things awesome_things prefix_things_suffix p_awesome_things_s).each do |table|
       Thing.connection.drop_table(table) rescue nil
@@ -79,6 +79,22 @@ def setup
     ActiveRecord::Migration.verbose = @verbose_was
   end
 
+  def test_passing_a_schema_migration_class_to_migration_context_is_deprecated
+    migrations_path = MIGRATIONS_ROOT + "/valid"
+    migrator = assert_deprecated { ActiveRecord::MigrationContext.new(migrations_path, ActiveRecord::SchemaMigration) }
+    migrator.up
+
+    assert_equal 3, migrator.current_version
+    assert_equal false, migrator.needs_migration?
+
+    migrator.down
+    assert_equal 0, migrator.current_version
+    assert_equal true, migrator.needs_migration?
+
+    @schema_migration.create_version(3)
+    assert_equal true, migrator.needs_migration?
+  end
+
   def test_migration_context_with_default_schema_migration
     migrations_path = MIGRATIONS_ROOT + "/valid"
     migrator = ActiveRecord::MigrationContext.new(migrations_path)
@@ -91,7 +107,7 @@ def test_migration_context_with_default_schema_migration
     assert_equal 0, migrator.current_version
     assert_equal true, migrator.needs_migration?
 
-    ActiveRecord::SchemaMigration.create!(version: 3)
+    @schema_migration.create_version(3)
     assert_equal true, migrator.needs_migration?
   end
 
@@ -111,7 +127,7 @@ def test_migrator_versions
     assert_equal 0, migrator.current_version
     assert_equal true, migrator.needs_migration?
 
-    ActiveRecord::SchemaMigration.create!(version: 3)
+    @schema_migration.create_version(3)
     assert_equal true, migrator.needs_migration?
   end
 
@@ -635,21 +651,20 @@ def migrate(x)
   def test_schema_migrations_table_name
     original_schema_migrations_table_name = ActiveRecord::Base.schema_migrations_table_name
 
-    assert_equal "schema_migrations", ActiveRecord::SchemaMigration.table_name
+    assert_equal "schema_migrations", @schema_migration.table_name
     ActiveRecord::Base.table_name_prefix = "prefix_"
     ActiveRecord::Base.table_name_suffix = "_suffix"
     Reminder.reset_table_name
-    assert_equal "prefix_schema_migrations_suffix", ActiveRecord::SchemaMigration.table_name
+    assert_equal "prefix_schema_migrations_suffix", @schema_migration.table_name
     ActiveRecord::Base.schema_migrations_table_name = "changed"
     Reminder.reset_table_name
-    assert_equal "prefix_changed_suffix", ActiveRecord::SchemaMigration.table_name
+    assert_equal "prefix_changed_suffix", @schema_migration.table_name
     ActiveRecord::Base.table_name_prefix = ""
     ActiveRecord::Base.table_name_suffix = ""
     Reminder.reset_table_name
-    assert_equal "changed", ActiveRecord::SchemaMigration.table_name
+    assert_equal "changed", @schema_migration.table_name
   ensure
     ActiveRecord::Base.schema_migrations_table_name = original_schema_migrations_table_name
-    ActiveRecord::SchemaMigration.reset_table_name
     Reminder.reset_table_name
   end
 
@@ -774,28 +789,26 @@ def test_internal_metadata_create_table_wont_be_affected_by_schema_cache
   end
 
   def test_schema_migration_create_table_wont_be_affected_by_schema_cache
-    ActiveRecord::SchemaMigration.drop_table
-    assert_not_predicate ActiveRecord::SchemaMigration, :table_exists?
+    @schema_migration.drop_table
+    assert_not_predicate @schema_migration, :table_exists?
 
-    ActiveRecord::SchemaMigration.connection.transaction do
-      ActiveRecord::SchemaMigration.create_table
-      assert_predicate ActiveRecord::SchemaMigration, :table_exists?
+    @schema_migration.connection.transaction do
+      @schema_migration.create_table
+      assert_predicate @schema_migration, :table_exists?
 
-      schema_migration = ActiveRecord::SchemaMigration.create!(version: "foo")
-      assert_equal "foo", schema_migration[:version]
+      assert_equal "foo", @schema_migration.create_version("foo")
       raise ActiveRecord::Rollback
     end
 
-    ActiveRecord::SchemaMigration.connection.transaction do
-      ActiveRecord::SchemaMigration.create_table
-      assert_predicate ActiveRecord::SchemaMigration, :table_exists?
+    @schema_migration.connection.transaction do
+      @schema_migration.create_table
+      assert_predicate @schema_migration, :table_exists?
 
-      schema_migration = ActiveRecord::SchemaMigration.create!(version: "bar")
-      assert_equal "bar", schema_migration[:version]
+      assert_equal "bar", @schema_migration.create_version("bar")
       raise ActiveRecord::Rollback
     end
   ensure
-    ActiveRecord::SchemaMigration.create_table
+    @schema_migration.create_table
   end
 
   def test_proper_table_name_on_migration
diff --git a/activerecord/test/cases/migrator_test.rb b/activerecord/test/cases/migrator_test.rb
index 3652355a74..8e49dc68f1 100644
--- a/activerecord/test/cases/migrator_test.rb
+++ b/activerecord/test/cases/migrator_test.rb
@@ -25,7 +25,7 @@ def setup
     super
     @schema_migration = ActiveRecord::Base.connection.schema_migration
     @schema_migration.create_table
-    @schema_migration.delete_all rescue nil
+    @schema_migration.delete_all_versions rescue nil
     @internal_metadata = ActiveRecord::Base.connection.internal_metadata
     @verbose_was = ActiveRecord::Migration.verbose
     ActiveRecord::Migration.message_count = 0
@@ -38,7 +38,7 @@ def puts(*)
   end
 
   teardown do
-    @schema_migration.delete_all rescue nil
+    @schema_migration.delete_all_versions rescue nil
     ActiveRecord::Migration.verbose = @verbose_was
     ActiveRecord::Migration.class_eval do
       undef :puts
@@ -145,7 +145,7 @@ def test_relative_migrations
   end
 
   def test_finds_pending_migrations
-    @schema_migration.create!(version: "1")
+    @schema_migration.create_version("1")
     migration_list = [ActiveRecord::Migration.new("foo", 1), ActiveRecord::Migration.new("bar", 3)]
     migrations = ActiveRecord::Migrator.new(:up, migration_list, @schema_migration, @internal_metadata).pending_migrations
 
@@ -157,8 +157,8 @@ def test_migrations_status
     path = MIGRATIONS_ROOT + "/valid"
     schema_migration = ActiveRecord::Base.connection.schema_migration
 
-    @schema_migration.create(version: 2)
-    @schema_migration.create(version: 10)
+    @schema_migration.create_version(2)
+    @schema_migration.create_version(10)
 
     assert_equal [
       ["down", "001", "Valid people have last names"],
@@ -172,10 +172,10 @@ def test_migrations_status_order_new_and_old_version
     path = MIGRATIONS_ROOT + "/old_and_new_versions"
     schema_migration = ActiveRecord::Base.connection.schema_migration
 
-    @schema_migration.create(version: 230)
-    @schema_migration.create(version: 231)
-    @schema_migration.create(version: 20210716122844)
-    @schema_migration.create(version: 20210716123013)
+    @schema_migration.create_version(230)
+    @schema_migration.create_version(231)
+    @schema_migration.create_version(20210716122844)
+    @schema_migration.create_version(20210716123013)
 
     assert_equal [
       ["up", "230", "Add people hobby"],
@@ -189,14 +189,14 @@ def test_migrations_status_order_new_and_old_version_applied_out_of_order
     path = MIGRATIONS_ROOT + "/old_and_new_versions"
     schema_migration = ActiveRecord::Base.connection.schema_migration
 
-    @schema_migration.create(version: 230)
-    @schema_migration.create(version: 231)
+    @schema_migration.create_version(230)
+    @schema_migration.create_version(231)
 
     # "Apply" a newer migration and not an older to simulate out-of-order
     # migration application which should not affect ordering in status and is
     # possible if a branch is merged which contains a migration which has an
     # earlier version but is judged to be compatible with existing migrations.
-    @schema_migration.create(version: 20210716123013)
+    @schema_migration.create_version(20210716123013)
 
     assert_equal [
       ["up", "230", "Add people hobby"],
@@ -210,8 +210,8 @@ def test_migrations_status_in_subdirectories
     path = MIGRATIONS_ROOT + "/valid_with_subdirectories"
     schema_migration = ActiveRecord::Base.connection.schema_migration
 
-    @schema_migration.create(version: 2)
-    @schema_migration.create(version: 10)
+    @schema_migration.create_version(2)
+    @schema_migration.create_version(10)
 
     assert_equal [
       ["down", "001", "Valid people have last names"],
@@ -243,8 +243,8 @@ def test_migrations_status_from_two_directories
     paths = [MIGRATIONS_ROOT + "/valid_with_timestamps", MIGRATIONS_ROOT + "/to_copy_with_timestamps"]
     schema_migration = ActiveRecord::Base.connection.schema_migration
 
-    @schema_migration.create(version: "20100101010101")
-    @schema_migration.create(version: "20160528010101")
+    @schema_migration.create_version("20100101010101")
+    @schema_migration.create_version("20160528010101")
 
     assert_equal [
       ["down", "20090101010101", "People have hobbies"],
@@ -300,7 +300,7 @@ def test_down_calls_down
   end
 
   def test_current_version
-    @schema_migration.create!(version: "1000")
+    @schema_migration.create_version("1000")
     schema_migration = ActiveRecord::Base.connection.schema_migration
     migrator = ActiveRecord::MigrationContext.new("db/migrate", schema_migration)
     assert_equal 1000, migrator.current_version
@@ -455,7 +455,7 @@ def test_migrator_output_when_running_single_migration
 
     result = migrator.run(:up, 1)
 
-    assert_equal(1, result.version)
+    assert_equal("1", result)
   end
 
   def test_migrator_rollback
@@ -484,10 +484,10 @@ def test_migrator_db_has_no_schema_migrations_table
     _, migrator = migrator_class(3)
     migrator = migrator.new("valid", schema_migration)
 
-    ActiveRecord::SchemaMigration.drop_table
-    assert_not_predicate ActiveRecord::SchemaMigration, :table_exists?
+    @schema_migration.drop_table
+    assert_not_predicate @schema_migration, :table_exists?
     migrator.migrate(1)
-    assert_predicate ActiveRecord::SchemaMigration, :table_exists?
+    assert_predicate @schema_migration, :table_exists?
   end
 
   def test_migrator_forward
@@ -506,7 +506,7 @@ def test_migrator_forward
 
   def test_only_loads_pending_migrations
     # migrate up to 1
-    @schema_migration.create!(version: "1")
+    @schema_migration.create_version("1")
 
     schema_migration = ActiveRecord::Base.connection.schema_migration
     calls, migrator = migrator_class(3)
diff --git a/activerecord/test/cases/multi_db_migrator_test.rb b/activerecord/test/cases/multi_db_migrator_test.rb
index b85661c8f8..6f211388c1 100644
--- a/activerecord/test/cases/multi_db_migrator_test.rb
+++ b/activerecord/test/cases/multi_db_migrator_test.rb
@@ -29,8 +29,8 @@ def setup
     @connection_a.schema_migration.create_table
     @connection_b.schema_migration.create_table
 
-    @connection_a.schema_migration.delete_all rescue nil
-    @connection_b.schema_migration.delete_all rescue nil
+    @connection_a.schema_migration.delete_all_versions rescue nil
+    @connection_b.schema_migration.delete_all_versions rescue nil
 
     @path_a = MIGRATIONS_ROOT + "/valid"
     @path_b = MIGRATIONS_ROOT + "/to_copy"
@@ -57,8 +57,8 @@ def puts(*)
   end
 
   teardown do
-    @connection_a.schema_migration.delete_all rescue nil
-    @connection_b.schema_migration.delete_all rescue nil
+    @connection_a.schema_migration.delete_all_versions rescue nil
+    @connection_b.schema_migration.delete_all_versions rescue nil
 
     ActiveRecord::Migration.verbose = @verbose_was
     ActiveRecord::Migration.class_eval do
@@ -69,9 +69,11 @@ def puts(*)
     end
   end
 
-  def test_schema_migration_class_names
-    assert_equal "ActiveRecord::SchemaMigration", @schema_migration_a.name
-    assert_equal "ARUnit2Model::SchemaMigration", @schema_migration_b.name
+  def test_schema_migration_is_different_for_different_connections
+    assert_not_equal @schema_migration_a, @schema_migration_b
+    assert_not_equal @schema_migration_a.connection, @schema_migration_b.connection
+    assert "ActiveRecord::Base", @schema_migration_a.connection.pool.pool_config.connection_name
+    assert "Arunit2", @schema_migration_b.connection.pool.pool_config.connection_name
   end
 
   def test_finds_migrations
@@ -87,8 +89,8 @@ def test_finds_migrations
   end
 
   def test_migrations_status
-    @schema_migration_a.create(version: 2)
-    @schema_migration_a.create(version: 10)
+    @schema_migration_a.create_version(2)
+    @schema_migration_a.create_version(10)
 
     assert_equal [
       ["down", "001", "Valid people have last names"],
@@ -97,7 +99,7 @@ def test_migrations_status
       ["up",   "010", "********** NO FILE **********"],
     ], ActiveRecord::MigrationContext.new(@path_a, @schema_migration_a).migrations_status
 
-    @schema_migration_b.create(version: 4)
+    @schema_migration_b.create_version(4)
 
     assert_equal [
       ["down", "001", "People have hobbies"],
@@ -136,14 +138,14 @@ def test_get_all_versions
   end
 
   def test_finds_pending_migrations
-    @schema_migration_a.create!(version: "1")
+    @schema_migration_a.create_version("1")
     migration_list_a = [ActiveRecord::Migration.new("foo", 1), ActiveRecord::Migration.new("bar", 3)]
     migrations_a = ActiveRecord::Migrator.new(:up, migration_list_a, @schema_migration_a, @internal_metadata_a).pending_migrations
 
     assert_equal 1, migrations_a.size
     assert_equal migration_list_a.last, migrations_a.first
 
-    @schema_migration_b.create!(version: "1")
+    @schema_migration_b.create_version("1")
     migration_list_b = [ActiveRecord::Migration.new("foo", 1), ActiveRecord::Migration.new("bar", 3)]
     migrations_b = ActiveRecord::Migrator.new(:up, migration_list_b, @schema_migration_b, @internal_metadata_b).pending_migrations
 
diff --git a/activerecord/test/cases/schema_dumper_test.rb b/activerecord/test/cases/schema_dumper_test.rb
index 964f50940f..b4f7707424 100644
--- a/activerecord/test/cases/schema_dumper_test.rb
+++ b/activerecord/test/cases/schema_dumper_test.rb
@@ -8,7 +8,8 @@ class SchemaDumperTest < ActiveRecord::TestCase
   self.use_transactional_tests = false
 
   setup do
-    ActiveRecord::SchemaMigration.create_table
+    @schema_migration = ActiveRecord::Base.connection.schema_migration
+    @schema_migration.create_table
   end
 
   def standard_dump
@@ -16,7 +17,7 @@ def standard_dump
   end
 
   def test_dump_schema_information_with_empty_versions
-    ActiveRecord::SchemaMigration.delete_all
+    @schema_migration.delete_all_versions
     schema_info = ActiveRecord::Base.connection.dump_schema_information
     assert_no_match(/INSERT INTO/, schema_info)
   end
@@ -24,7 +25,7 @@ def test_dump_schema_information_with_empty_versions
   def test_dump_schema_information_outputs_lexically_reverse_ordered_versions_regardless_of_database_order
     versions = %w{ 20100101010101 20100201010101 20100301010101 }
     versions.shuffle.each do |v|
-      ActiveRecord::SchemaMigration.create!(version: v)
+      @schema_migration.create_version(v)
     end
 
     schema_info = ActiveRecord::Base.connection.dump_schema_information
@@ -37,7 +38,7 @@ def test_dump_schema_information_outputs_lexically_reverse_ordered_versions_rega
     STR
     assert_equal expected, schema_info
   ensure
-    ActiveRecord::SchemaMigration.delete_all
+    @schema_migration.delete_all_versions
   end
 
   def test_schema_dump_include_migration_version
diff --git a/activerecord/test/cases/tasks/database_tasks_test.rb b/activerecord/test/cases/tasks/database_tasks_test.rb
index 25f39e518f..64888093aa 100644
--- a/activerecord/test/cases/tasks/database_tasks_test.rb
+++ b/activerecord/test/cases/tasks/database_tasks_test.rb
@@ -1031,14 +1031,18 @@ def test_migrate_using_empty_scope_and_verbose_mode
     end
 
     class DatabaseTasksMigrateStatusTest < DatabaseTasksMigrationTestCase
+      def setup
+        @schema_migration = ActiveRecord::Base.connection.schema_migration
+      end
+
       def test_migrate_status_table
-        ActiveRecord::SchemaMigration.create_table
+        @schema_migration.create_table
         output = capture_migration_status
         assert_match(/database: :memory:/, output)
         assert_match(/down    001             Valid people have last names/, output)
         assert_match(/down    002             We need reminders/, output)
         assert_match(/down    003             Innocent jointable/, output)
-        ActiveRecord::SchemaMigration.drop_table
+        @schema_migration.drop_table
       end
 
       private
@@ -1169,25 +1173,27 @@ class DatabaseTasksTruncateAllTest < ActiveRecord::TestCase
       fixtures :authors, :author_addresses
 
       def setup
-        SchemaMigration.create_table
-        SchemaMigration.create!(version: SchemaMigration.table_name)
+        @schema_migration = ActiveRecord::Base.connection.schema_migration
+        @schema_migration.create_table
+        @schema_migration.create_version(@schema_migration.table_name)
         InternalMetadata.create_table
         InternalMetadata.create!(key: InternalMetadata.table_name)
+        @old_configurations = ActiveRecord::Base.configurations
       end
 
       def teardown
-        SchemaMigration.delete_all
+        @schema_migration.delete_all_versions
         InternalMetadata.delete_all
         clean_up_connection_handler
+        ActiveRecord::Base.configurations = @old_configurations
       end
 
       def test_truncate_tables
-        assert_operator SchemaMigration.count, :>, 0
+        assert_operator @schema_migration.count, :>, 0
         assert_operator InternalMetadata.count, :>, 0
         assert_operator Author.count, :>, 0
         assert_operator AuthorAddress.count, :>, 0
 
-        old_configurations = ActiveRecord::Base.configurations
         db_config = ActiveRecord::Base.configurations.configs_for(env_name: "arunit", name: "primary")
         configurations = { development: db_config.configuration_hash }
         ActiveRecord::Base.configurations = configurations
@@ -1198,12 +1204,10 @@ def test_truncate_tables
           )
         end
 
-        assert_operator SchemaMigration.count, :>, 0
+        assert_operator @schema_migration.count, :>, 0
         assert_operator InternalMetadata.count, :>, 0
         assert_equal 0, Author.count
         assert_equal 0, AuthorAddress.count
-      ensure
-        ActiveRecord::Base.configurations = old_configurations
       end
     end
 
@@ -1211,14 +1215,12 @@ class DatabaseTasksTruncateAllWithPrefixTest < DatabaseTasksTruncateAllTest
       setup do
         ActiveRecord::Base.table_name_prefix = "p_"
 
-        SchemaMigration.reset_table_name
         InternalMetadata.reset_table_name
       end
 
       teardown do
         ActiveRecord::Base.table_name_prefix = nil
 
-        SchemaMigration.reset_table_name
         InternalMetadata.reset_table_name
       end
     end
@@ -1227,14 +1229,12 @@ class DatabaseTasksTruncateAllWithSuffixTest < DatabaseTasksTruncateAllTest
       setup do
         ActiveRecord::Base.table_name_suffix = "_s"
 
-        SchemaMigration.reset_table_name
         InternalMetadata.reset_table_name
       end
 
       teardown do
         ActiveRecord::Base.table_name_suffix = nil
 
-        SchemaMigration.reset_table_name
         InternalMetadata.reset_table_name
       end
     end
diff --git a/railties/test/application/loading_test.rb b/railties/test/application/loading_test.rb
index 2015171385..af11368b53 100644
--- a/railties/test/application/loading_test.rb
+++ b/railties/test/application/loading_test.rb
@@ -135,14 +135,14 @@ class Post < ApplicationRecord
 
     initial = [
       ActiveStorage::Record, ActiveStorage::Blob, ActiveStorage::Attachment,
-      ActiveRecord::SchemaMigration, ActiveRecord::InternalMetadata, ApplicationRecord
+      ActiveRecord::InternalMetadata, ApplicationRecord
     ].collect(&:to_s).sort
 
     assert_equal initial, ActiveRecord::Base.descendants.collect(&:to_s).sort.uniq
     get "/load"
     assert_equal [Post].collect(&:to_s).sort, ActiveRecord::Base.descendants.collect(&:to_s).sort - initial
     get "/unload"
-    assert_equal ["ActiveRecord::InternalMetadata", "ActiveRecord::SchemaMigration"], ActiveRecord::Base.descendants.collect(&:to_s).sort.uniq
+    assert_equal ["ActiveRecord::InternalMetadata"], ActiveRecord::Base.descendants.collect(&:to_s).sort.uniq
   end
 
   test "initialize can't be called twice" do
diff --git a/railties/test/railties/engine_test.rb b/railties/test/railties/engine_test.rb
index 5c16a18ae0..8bc60d5349 100644
--- a/railties/test/railties/engine_test.rb
+++ b/railties/test/railties/engine_test.rb
@@ -34,7 +34,7 @@ def boot_rails
 
     def migrations
       migration_root = File.expand_path(ActiveRecord::Migrator.migrations_paths.first, app_path)
-      ActiveRecord::MigrationContext.new(migration_root, ActiveRecord::SchemaMigration).migrations
+      ActiveRecord::MigrationContext.new(migration_root, ActiveRecord::SchemaMigration::NullSchemaMigration.new).migrations
     end
 
     test "serving sprocket's assets" do