How can I add Alternate ID in SO303000 (Invoice) in detail grid

​@FinnSystemAG 

If you want to display the Alternate ID in the Invoice Detail grid:

First, create a custom field in the DAC (e.g., UsrAlternateID). Then, use the FieldSelecting event in the graph to fetch and assign the corresponding Alternate ID for the selected Inventory Item.

This will allow the value to be shown dynamically in the UI. Below is the sample code.

In DAC

#region UsrAlternateID
[PXString(30, IsUnicode = true)]
[PXUIField(DisplayName = "Alternate ID", Enabled = false)]
public virtual string UsrAlternateID { get; set; }
public abstract class usrAlternateID : PX.Data.BQL.BqlString.Field<usrAlternateID> { }
#endregion

In GraphExtension 

protected void ARTran_UsrAlternateID_FieldSelecting(PXCache sender, PXFieldSelectingEventArgs e)
    {
        var row = (ARTran)e.Row;
        if (row?.InventoryID == null || Base.Document.Current?.CustomerID == null)
            return;

        var altID = PXSelectJoin<INItemXRef,
            InnerJoin<InventoryItem, On<INItemXRef.inventoryID, Equal<InventoryItem.inventoryID>>>,
            Where<INItemXRef.inventoryID, Equal<Required<INItemXRef.inventoryID>>,
                  And<INItemXRef.bAccountID, Equal<Required<INItemXRef.bAccountID>>>>>
            .Select(Base, row.InventoryID, Base.Document.Current.CustomerID)
            .RowCast<INItemXRef>()
            .FirstOrDefault();

        if (altID != null)
        {
            e.ReturnValue = altID.AlternateID;
        }
    }

Hope this helps!