add extra function to model but cakephp thinks it's query???

I just add an function to model and call that function from another
controller.
but what i get is the following error. What happen?????

In Model( department.php ),

class Department extends AppModel {
var $useDbConfig = 'default_account';
function get_child_dep( $dep_id, $child ){
}
}

In Controll( orgmap_controller.php ),

$this -> loadModel( 'Department' );
$this -> Department -> get_child_dep( 1 )

error message ----
Warning (512): SQL Error: 1064: You have an error in your SQL syntax;
check the manual that corresponds to your MySQL server version for the
right syntax to use near 'get_child_dep' at line 1 [CORE/cake/libs/
model/datasources/dbo_source.php, line 535]

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