Question

I’m making a walkathon rails app. Each walker is sponsored by people who pledge money per lap up to a maximum amount.

I have a table called sponsorships that has columns walker_id, amount_per_lap, and max_amount. I want to write a SQL query to determine how much money a walker has raised.

There is also a walkers table that has id, name, and laps columns.

I know this isn’t valid SQL, but I wanted something like this, but don’t know the best way to do it. The walker_id and laps could be provided before executing the query.

SELECT SUM(MIN(Laps * sponsorships.amount_per_lap, sponsorships.max_amount)) FROM sponsorships
where sponsorships.walker_id = 1;

I’m making this in rails, so I was trying to figure out how to do something like this in Arel, but couldn’t figure it out.

Any help would be greatly appreciated.

Edit: clarifying the walkers table.

Edit2: Accidentally had max instead of min in the pseudo code

Answer 1

I think the SQL you’re looking for is this:

select w.id, w.name, sum(least(w.laps * s.amount_per_lap, s.max_amount))
from walkers w
join sponsorships s on w.id = s.walker_id
group by w.id, w.name

The least function is what applies your “no more than max_amount” condition. Translating that to AR should be a simple matter for you now that you know what to SELECT; I tend to go straight to SQL for anything like this.