<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_migration_files(self):
 """Find migrations files."""

        migrations = (re.match(MigrationFile.PATTERN, filename)
                      for filename in os.listdir(self.directory))
        migrations = (MigrationFile(m.group('id'), m.group(0))
                      for m in migrations if m)
        return sorted(migrations, key=lambda m: m.id)