Replace id column with natural PK

postgres/queries/assignments.sql

@@ -22,4 +22,11 @@ SELECT assignments.date, categories.name as category, category_groups.name as gr
 FROM assignments
 INNER JOIN categories ON categories.id = assignments.category_id
 INNER JOIN category_groups ON categories.category_group_id = category_groups.id
-WHERE category_groups.budget_id = @budget_id;
+WHERE category_groups.budget_id = @budget_id;
+
+-- name: UpdateAssignment :exec
+INSERT INTO assignments (category_id, date, amount)
+VALUES($1, $2, $3) 
+ON CONFLICT (category_id, date) 
+DO 
+   UPDATE SET amount = $3;