Refactor schema migration on connection

This method was jumping through extra hoops to find the name of the
class the connection is stored on when we can get it from the connection
itself. Since we already have the connection we don't need to loop through the
pools.

In addition, this was using the wrong class name. The class name for the
schema migration should come from the connection owner class, not from
the `db_config.name`. In this case, `db_config.name` is the name of the
configuration in the database.yml. Rails uses the class name to lookup
connections, not the db config name, so we should be consistent here.

While working on this I noticed that we were generating an extra schema
migration class for `ActiveRecord::Base`. Since `ActiveRecord::Base` can
and should use the default and we don't want to create a new one for
single db applications, we should skip creating this if the spec name is
`ActiveRecord::Base`. I added an additional test that ensures the class
generation is correct.

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -130,19 +130,17 @@ module ActiveRecord
       def schema_migration # :nodoc:
         @schema_migration ||= begin
                                 conn = self
-                                name = conn.pool.db_config.name
-                                schema_migration_name = "#{name}::SchemaMigration"
+                                spec_name = conn.pool.pool_config.connection_specification_name
+
+                                return ActiveRecord::SchemaMigration if spec_name == "ActiveRecord::Base"
+
+                                schema_migration_name = "#{spec_name}::SchemaMigration"
 
                                 Class.new(ActiveRecord::SchemaMigration) do
                                   define_singleton_method(:name) { schema_migration_name }
                                   define_singleton_method(:to_s) { schema_migration_name }
 
-                                  connection_handler.connection_pool_names.each do |pool_name|
-                                    if conn.pool == connection_handler.retrieve_connection_pool(pool_name)
-                                      self.connection_specification_name = pool_name
-                                      break
-                                    end
-                                  end
+                                  self.connection_specification_name = spec_name
                                 end
                               end
       end

activerecord/test/cases/multi_db_migrator_test.rb

@@ -67,6 +67,11 @@ class MultiDbMigratorTest < ActiveRecord::TestCase
     end
   end
 
+  def test_schema_migration_class_names
+    assert_equal "ActiveRecord::SchemaMigration", @schema_migration_a.name
+    assert_equal "ARUnit2Model::SchemaMigration", @schema_migration_b.name
+  end
+
   def test_finds_migrations
     @migrations_a_list.each_with_index do |pair, i|
       assert_equal @migrations_a[i].version, pair.first

railties/test/application/loading_test.rb

@@ -133,12 +133,12 @@ class LoadingTest < ActiveSupport::TestCase
     require "#{rails_root}/config/environment"
     setup_ar!
 
-    initial = [ActiveStorage::Blob, ActiveStorage::Attachment, ActiveRecord::SchemaMigration, ActiveRecord::InternalMetadata, ApplicationRecord, "primary::SchemaMigration"].collect(&:to_s).sort
+    initial = [ActiveStorage::Blob, ActiveStorage::Attachment, ActiveRecord::SchemaMigration, 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", "primary::SchemaMigration"], ActiveRecord::Base.descendants.collect(&:to_s).sort.uniq
+    assert_equal ["ActiveRecord::InternalMetadata", "ActiveRecord::SchemaMigration"], ActiveRecord::Base.descendants.collect(&:to_s).sort.uniq
   end
 
   test "initialize cant be called twice" do

railties/test/application/zeitwerk_integration_test.rb

@@ -101,7 +101,7 @@ class ZeitwerkIntegrationTest < ActiveSupport::TestCase
   test "autoloaded? and overridden class names" do
     invalid_constant_name = Module.new do
       def self.name
-        "primary::SchemaMigration"
+        "MyModule::SchemaMigration"
       end
     end
     assert_not deps.autoloaded?(invalid_constant_name)