Workflow last approver by using po recId

 public UserId getWFLastApprover(PurchPurchaseOrderHeader _purchaseOrderHeader)

    {

        WorkflowTrackingTable           workflowTrackingTable;

        WorkflowTrackingStatusTable     workflowTrackingStatusTable;

        PurchTable purchTable =  PurchTable::find(_purchaseOrderHeader.PurchId);

        select firstonly RecId from workflowTrackingStatusTable

            where

                workflowTrackingStatusTable.ContextTableId      == purchTable.TableId

               && workflowTrackingStatusTable.ContextRecId        == purchTable.RecId

            join firstonly user from workflowTrackingTable order by CreatedDateTime desc

            where workflowTrackingTable.WorkflowTrackingStatusTable == workflowTrackingStatusTable.RecId

            &&    (workflowTrackingTable.TrackingType == WorkflowTrackingType::Approval   ||

            workflowTrackingTable.TrackingType == WorkflowTrackingType::Completion)

            &&    workflowTrackingTable.TrackingContext == WorkflowTrackingContext::WorkItem;

        if(!workflowTrackingTable.User)

        {

        PurchReqTable purchReq = PurchReqTable::findPurchReqId(_purchaseOrderHeader.CMCPurchReqId);

        select firstonly RecId from workflowTrackingStatusTable

            where

                workflowTrackingStatusTable.ContextTableId      == purchReq.TableId

               && workflowTrackingStatusTable.ContextRecId        == purchReq.RecId

            join firstonly user from workflowTrackingTable order by CreatedDateTime desc

            where workflowTrackingTable.WorkflowTrackingStatusTable == workflowTrackingStatusTable.RecId

            &&(workflowTrackingTable.TrackingType == WorkflowTrackingType::Approval   ||

            workflowTrackingTable.TrackingType == WorkflowTrackingType::Completion)

            &&    workflowTrackingTable.TrackingContext == WorkflowTrackingContext::WorkItem;

        }

        return workflowTrackingTable.User;

    }