jQuery Grid Plugin - jqGrid

I have a custom validation function which I would like to validate against 2 defined columns, that is: if column "locked_flag" is "Y" AND there is no value for "locked_date", then validation would fail with an error message. 

How can I do this? (refer to a different column from the one I am validating)

Something like: 

colModel: [

{name:'locked_flag',index:'locked_flag' editrules:{ custom:true,custom_func:lockedvalid } ...

{name:'locked_date',index:'locked_date', .....

]

function lockedvalid(value){
    
        if (value == 'Y' && locked_date.length == 0 ) {
            return [false,'Please enter a date to lock this record...',''];
        }
        return [true,'',''];
    }