getYear() function not allowed in criteria - what's the alternative?

getYear() function not allowed in criteria - what's the alternative?

In my form1 I have a dateField and salePriceField. In my form2 I have a yearField and totalSalesField.
I'm trying to insert records into form2 like below, but getting the "not allowed in left side of criteria error.

dateList = form1[ID !=0].distinct(dateField);
for each  saleDate in dateList
{
anualSales = form1[dateField.toYear() = saleDate.toYear()].sum();
                        x = insert into form2
[
yearField=saleDate.toYear()
totalSalesField=anualSales

                                          etc....