Extending InventoryItem DAC with field from another DAC

In the  code I’m trying to Extend InventoryItem DAC to include the AlternateID from INItemXRef

 I created a customization project, and added the required code, the new field showing in the InventoryItem fields,
Code:
 

  public class InventoryItemExt : PXCacheExtension<InventoryItem>
    {
        #region CustomAlternateID
        [PXString(50, IsUnicode = true)]
        [PXUIField(DisplayName = "GTIN/EAN/UPC/ISBN")]
        [PXSelector(
            typeof(Search<INItemXRef.alternateID,
                          Where<INItemXRef.inventoryID, Equal<Current<InventoryItem.inventoryID>>,
                                And<INItemXRef.alternateType, Equal<INAlternateType.gIN>>>>),
            typeof(INItemXRef.alternateID),
            SubstituteKey = typeof(INItemXRef.alternateID))]
        public virtual string CustomAlternateID { get; set; }
        public abstract class customAlternateID : PX.Data.BQL.BqlString.Field<customAlternateID> { }
        #endregion
    }

But it always return empty when I use it in the GI or view in screen

 

Hope if someone can advise on what is missing in the code , thanks in advance