Angular - TypeError Cannot set _id to Null Value - node.js

Question

Has a transaction function that worked in first pass and at 2nd pass, got "TypeError: Cannot set _id to Null value. Both passes were to create a new transaction. Besides, system seemed to indicate that there was value for variable that was being used to assign to_id. In this case,
print"this.selectedLeave_id" and saw value as expected. However, Angular right away in next statement complaining that Null value was set to "this.SLTran.leave_id".
Below pls find code and any help is appreciated.
onSubmitLeaveTran()
{
if (this.selectedLeaveTran_id != 0) // means first loaded all trans and then select from table to modify
{
this.sLTran = this.tempSLTran; // tempSLTran was from row selected to be modified
this.sLTran.leaveType = this.tempLeaveType; // from dialog box edited data
this.sLTran.leaveStartDate = this.tempLeaveStartDate; // from dialog box edited data
this.sLTran.leaveEndDate = this.tempLeaveEndDate; // from dialog box edited data
this.sLTran.numDaysRequested = this.tempNumDaysRequested; // from dialog box edited data
console.log('2-2.5 inside onSubmit Leave Tran for update :',this.sLTran);
this.staffLeaveDataSvc.updateSLTran(this.sLTran).subscribe((sLTran:staffLeaveTran) => {this.sLTran = sLTran});
}
else
{ // a new tran
console.log('2-2.4 inside onSubmit Leave Tran selectedLeave_id for new tran:',this.selectedLeave_id);
this.sLTran.leave_id = this.selectedLeave_id; // first established the leave_id associated with this new tran
this.sLTran.leaveType = this.tempLeaveType;
this.sLTran.leaveStartDate = this.tempLeaveStartDate;
this.sLTran.leaveEndDate = this.tempLeaveEndDate;
this.sLTran.numDaysRequested = this.tempNumDaysRequested;
this.staffLeaveDataSvc.addSLTran(this.sLTran).subscribe(
(sLTran:staffLeaveTran) => {
this.sLTran = sLTran
});
}
};