Disabling field in relationship

Hi all,

I am currently trying to disable a field on the relationshiptab when the item is at a certain state.

The method is current run under onFormPopulated.

Below is my code:

/////////////////////////////////////////////////////////////////////////////

var documentItem = document.thisItem;

documentItem.fetchRelationships("q_Quote Part");

var Xpath = "//Item/Relationships/Item";
var relationship = documentItem.getItemsByXPath(Xpath);
//alert(relationship.getItemCount());
if (documentItem.getProperty("state") == "Generate Quote") {
for (var i = 0; i < relationship.getItemCount(); i++) {

//var wrapper = relationship.getItemByIndex(i).getFieldByName("_quantity");
//var wrapper = getFieldByName("_quantity");
// wrapper.getElementsByTagName("input")[0].disabled = true;
}
}

///////////////////////////////////////////////////////////////////////////////////

I am currently having trouble targeting the field from the relationship tab. getFieldByName doesn't seem to be able to grab the wrapper if it is on the relationship tab. Any Input would greatly be appreciated!