Handle duplicate migration names in multi db

In a multidatabse setup, migrations live in seperate directories. The check for duplicate migration names, does not take this into
account.
So you __can__ end up having multiple migration with the same name.

When running `rails db:migrate` from the console, this other migration
might be loaded __before__ running the migrations for "this" database.

Because they have the same name, we have to ensure that the
contents of this migration is not carried over from "the other"
migrations.

Author: anderslemke

activerecord/lib/active_record/migration.rb

@@ -1044,7 +1044,9 @@ def migration
       end
 
       def load_migration
-        require(File.expand_path(filename))
+        Object.send(:remove_const, name) rescue nil
+
+        load(File.expand_path(filename))
         name.constantize.new(name, version)
       end
   end

activerecord/test/cases/migration_test.rb

@@ -15,6 +15,12 @@
 require MIGRATIONS_ROOT + "/rename/2_rename_things"
 require MIGRATIONS_ROOT + "/decimal/1_give_me_big_numbers"
 
+class ValidPeopleHaveLastNames < ActiveRecord::Migration::Current
+  def change
+    drop_table :people
+  end
+end
+
 class BigNumber < ActiveRecord::Base
   unless current_adapter?(:PostgreSQLAdapter, :SQLite3Adapter)
     attribute :value_of_e, :integer
@@ -108,6 +114,14 @@ def test_migrator_versions
     assert_equal true, migrator.needs_migration?
   end
 
+  def test_name_collision_across_dbs
+    migrations_path = MIGRATIONS_ROOT + "/valid"
+    migrator = ActiveRecord::MigrationContext.new(migrations_path)
+    migrator.up
+
+    assert_column Person, :last_name
+  end
+
   def test_migration_detection_without_schema_migration_table
     ActiveRecord::Base.connection.drop_table "schema_migrations", if_exists: true