Grails "Cannot set readonly property: programId"

Question

I got thrown into an existing grails project and one of the issues i'm having is that when saving something a batch, i get the error: Cannot set readonly property: programId

Here's my save snippet that causes the error

// Create a batch
def batch = new Batch()
batch.name = session.batch_name
batch.startDate = new Date()
batch.endDate = new Date()
batch.programId = 120
if(batch.save()) {
  ...
}

Here's my batch domain class

class Batch  extends AbstractDomainObject{
    String name
    Date startDate
    Date endDate
    String comments
    StatusType currentStatus

    static belongsTo = [program:Program]    

    static constraints = {
        name(blank:false,maxSize:100)
        startDate()
        endDate()
        comments (nullable:true, maxSize:DEFAULT_SIZE_OF_COMMENTS)
        currentStatus(nullable:true)
    }
    static transients= ["currentStatus"]

    static mapping = {
        id column:'batch_id', generator:'sequence', params:[sequence:'sq_batch']
        currentStatus column:'status_type_id'
        program column:'program_id'
        statuses sort:'statusDate'
        startDate type:'date'
        endDate type:'date'
    }

    public String toString(){
        return name
    }


    public Date createdDate(){
        Date date=null
        if(this?.statuses?.size()>0){
            this?.statuses.each{
                if(it.status.value==SystemConstants.STATUS_PENDING){
                    date = it.statusDate
                }
            }
        }
        return date

    }
}

Why does it not let me set the programId?

Answer 1

programId is a dynamic property that gives you the id of the program instance without loading it from the database, but it's a getter without a setter. If you want to set the reference without incurring the cost of loading the whole instance, use this instead:

batch.program = Program.load(120)

load uses a proxy that only has the id stored, and doesn't go to the database unless you call a method or access a property other than id. So it will work here since Hibernate only needs its id for the SQL insert that eventually gets run.