Transactions In .net core Web API Controller

DateTime today = helper.getISTDate();

 

model.forEach(obj=>{

});

 

{
if (res == "OK")
{
await db.SaveChangesAsync();
_ctxTransaction.Commit();
}
else
{
_ctxTransaction.Rollback();
}
}
catch (Exception e)
{
_ctxTransaction.Rollback();
throw e;
} 

 

return Ok();

}

 

 

 

updateInventory  method in helper class

public string updateInventory(ImsCoLine invObj){

 string res="";

var inv = db.ImsInvItem.FirstorDefault(y => y.ItemID== invObj.ItemID);

if(inv != null)

{

 inv.Qty=inv.Qty - invObj.Qty;
 if(inv.Qty<0)

   res="Negative Qty";

 else

   res="OK";

}

 if (res == "OK")
{
try
{
db.SaveChanges();
}
catch (Exception ex)
{
throw ex;
}
}

return res;

}

 
When an exception is raised in main method (await savechangesAsync()) the inner method changes are not rolled back
only main method changes are rolling back

if Qty is negative then also the changes has to be rolled back
If db.saveChanges() not included in inner method then the inner method data is not saving to the database.

Please advice If code is wrong