do not hold on to a stale connection object. fixes #15998

activerecord/lib/active_record/migration.rb

@@ -366,22 +366,27 @@ module ActiveRecord
     # This class is used to verify that all migrations have been run before
     # loading a web page if config.active_record.migration_error is set to :page_load
     class CheckPending
-      def initialize(app, connection = Base.connection)
+      def initialize(app)
         @app = app
-        @connection = connection
         @last_check = 0
       end
 
       def call(env)
-        if @connection.supports_migrations?
+        if connection.supports_migrations?
           mtime = ActiveRecord::Migrator.last_migration.mtime.to_i
           if @last_check < mtime
-            ActiveRecord::Migration.check_pending!(@connection)
+            ActiveRecord::Migration.check_pending!(connection)
             @last_check = mtime
           end
         end
         @app.call(env)
       end
+
+      private
+
+      def connection
+        ActiveRecord::Base.connection
+      end
     end
 
     class << self

activerecord/test/cases/migration/pending_migrations_test.rb

@@ -8,14 +8,17 @@ module ActiveRecord
         super
         @connection = MiniTest::Mock.new
         @app = MiniTest::Mock.new
-        @pending = CheckPending.new(@app, @connection)
+        conn = @connection
+        @pending = Class.new(CheckPending) {
+          define_method(:connection) { conn }
+        }.new(@app)
         @pending.instance_variable_set :@last_check, -1 # Force checking
       end
 
       def teardown
-        super
         assert @connection.verify
         assert @app.verify
+        super
       end
 
       def test_errors_if_pending