<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def validate_identifier(self, field):
 """Validate field identifier."""

        if field.data:
            field.data = field.data.lower()
            if Community.get(field.data, with_deleted=True):
                raise validators.ValidationError(
                    _('The identifier already exists. '
                      'Please choose a different one.'))