put improved foreign key constraints on DB to fix bug where when I delete a cset, that was being used to compare_to, it breaks

main.py

@@ -41,7 +41,10 @@ def index():
         depletion_date=depletion_date.date(); # just show date
 
     # if we are comparing...(compare_to will be 0 / None to start with, and then COMP will be None
-    COMP=get_comp_set_data(finance_data['compare_to'])
+    if finance_data['compare_to']:
+        COMP=get_comp_set_data(finance_data['compare_to'])
+    else:
+        COMP={}
 
     DISP=[]
     # Row 1
@@ -122,6 +125,9 @@ def update():
 
     old_finance_data = get_finance_data()
 
+    raw_compare_to = request.form.get('compare_to')
+    compare_to_value = None if raw_compare_to == "0" else int(raw_compare_to)
+
     finance_data = (
         request.form['D_Salary'],
         request.form['D_Num_fortnights_pay'],
@@ -147,9 +153,10 @@ def update():
         request.form['Mark_reno_date'],
         request.form['Car_buyout_date'],
         request.form['Sell_shares'],
-        request.form['compare_to'],
+        compare_to_value,
         request.form['Ioniq6_future']
     )
+
     update_finance(finance_data)
     new_finance_data = get_finance_data()
     # changed Ioniq6_future, Car_buyout_date or D_Num_fortnights_pay, so lets force recalc key_dates, and therefore estimated bills