Aggregate value of usrField SOLine not working - acumatica

Question

I'm a bit lost here on what approach to take to get an aggregate value from my usr field on SOLineExt. My goal is a simple aggregation of the user field which is based on a this value from the SOLine:
itemExt.UsrMinutes * (int)row.OrderQty;
In the SOLineExt the value above is stored in UsrMultByQuant it gets the value fine but I can't seem to aggregate the the SOLineExt value to get the total of the UsrMultByQuant into my SOOrder field called UsrTotalWorkMinutes
[PXDBInt]
[PXUIField(DisplayName="TotalWorkMinutes")]
[PXFormula(null, typeof(SumCalc<SOLineExt.usrMultByQuant>))]
public virtual int? UsrTotalWorkMinutes{get;set;}
public abstract class usrTotalWorkMinutes : IBqlField{}
I was referencing T200 Lesson 7, also I attempted to implement a fieldselecting (as instructed on Lesson 11.8 in t300) to try that approach for getting my total usr field value from the SOLineExt table with no success.
Any advice is appreciated, Thank you
Edit:
I removed the null from the PXformula and i am receiving this error now.
I also modified the dac slightly to have the PXParent attribute to select the current SOLine
[PXDBInt]
[PXUIField(DisplayName="TotalWorkMinutes")]
[PXFormula(typeof(SumCalc<SOLineExt.usrMultByQuant>))]
[PXParent(typeof(Select<SOLine,
Where<SOLine.orderNbr, Equal<Current<SOOrder.orderNbr>>>>))]
public virtual int? UsrCuryWorkMinutes{get;set;}
public abstract class usrCuryWorkMinutes : IBqlField{}
Still does not total my SOLineExt value and I am recieving the error above when null removed.

Answer 1

The 2 extension classes from below should be enough to achieve the requested behavior (notice, fields UsrCuryWorkMinutes and UsrMultByQuant are of the decimal type):
public class SOOrderExt : PXCacheExtension<SOOrder>
{
public abstract class usrCuryWorkMinutes : IBqlField { }
[PXDBDecimal(2)]
[PXUIField(DisplayName = "Total Work Minutes")]
public virtual decimal? UsrCuryWorkMinutes { get; set; }
}
public class SOLineExt : PXCacheExtension<SOLine>
{
public abstract class usrMinutes : IBqlField { }
[PXDBInt]
[PXUIField(DisplayName = "Work Minutes")]
public virtual int? UsrMinutes { get; set; }
public abstract class usrMultByQuant : IBqlField { }
[PXDBDecimal(2)]
[PXUIField(DisplayName = "Total Work Minutes", Enabled = false)]
[PXFormula(typeof(Mult<IsNull<SOLine.orderQty, decimal0>, SOLineExt.usrMinutes>),
typeof(SumCalc<SOOrderExt.usrCuryWorkMinutes>))]
public virtual decimal? UsrMultByQuant { get; set; }
}
There is no need for PXParentAttribute in SOLineExt because of the original SOLine.OrderNbr field already decorated with PXParentAttribute to aggregated other values from SOLine to SOOrder:
[System.SerializableAttribute()]
[PXCacheName(Messages.SOLine)]
public partial class SOLine : PX.Data.IBqlTable, ILSPrimary, IDiscountable, ISortOrder
{
...
#region OrderNbr
public abstract class orderNbr : PX.Data.IBqlField
{
}
protected String _OrderNbr;
[PXDBString(15, IsUnicode = true, IsKey = true, InputMask = "")]
[PXDBDefault(typeof(SOOrder.orderNbr), DefaultForUpdate = false)]
[PXParent(typeof(Select<SOOrder, Where<SOOrder.orderType, Equal<Current<SOLine.orderType>>, And<SOOrder.orderNbr, Equal<Current<SOLine.orderNbr>>>>>))]
[PXUIField(DisplayName = "Order Nbr.", Visible = false, Enabled = false)]
public virtual String OrderNbr
{
...
}
#endregion
...
}