Allow `distinct_on` to be able to handle multiple fields

Right now the `distinct_on` call only allows for a single column to be defined. We had an issue in our project today that caused us to have to go well out of our way to be able to get the data we wanted. If `distinct_on` supported the multiple columns, it would have been a very quick change. 

https://www.postgresql.org/docs/current/sql-select.html#SQL-DISTINCT