jqGrid Change the value of a field on beforeShowForm

I have a grid with a value in a predefined field, I want to do it is to click edit a row want to change the value of one of the fields in the FormEdit.

The ColModel is:

    colModel:[
                {name:'id',index:'ID', width:50, hidden:true, search:false, editable:false,resizable:false},
                {name:'product',index:'product', align:"center", width:20, search:false, editable:false,resizable:false},
                {name:'volume',index:'volume', align:"center", hidden:false, width:15, search:false, editable:true}
                ],

I would establish volume = 1 when open the edit form, and i do like this...

 ondblClickRow: function(id){
                jQuery("#products").jqGrid('editGridRow', id, {
                  recreateForm:true,  
                  beforeShowForm: function(form) { 
                      $('#tr_volume',form).show(); 
                      $("#tr_volume",form).val('1');
                      },
                  reloadAfterSubmit:false, closeOnEscape:true, closeAfterAdd:true, closeAfterEdit:true,
                  editable:true, editrules:{edithidden:true,required:true,number:true,minValue:1}                    
               })

But it's not working for me, what is wrong?

You don't use hidden: true property of volume column. So the setting $('#tr_volume',form).show() is unneeded. The setting of $("#tr_volume",form).val('1') is wrong, because the <tr> element have no value. I think that you need jusu use $("#volume",form).val('1') or $("#volume").val('1') instead. The <input> field with the id="volume" (the same id like the column name) will be created in the form. It's the field which you probably try to change.