It was (I think) possible in Rails prior to 2.1 to perform calculations like this:
total = Donation.sum(:amount)
I never used this functionality, so it’s news to me! But in any case, it’s quite useful; under the hood, it’s similar to a query like SELECT SUM(amount) FROM Donation d, albeit in a database-independent fashion.
In any case, Rails 2.1 takes this a step farther; you can specify a complex calculation for an aggregate function. For example, to calculate the aggregate sum of donations, you can do this:
total = Donation.sum("amount * amount")
…and Rails will calculate it for you! Something as complex as the standard deviation becomes a snap. Incidentally, Rails provides the following aggregate queries:
- ActiveRecord.average
- ActiveRecord.count (like COUNT(*))
- ActiveRecord.maximum
- ActiveRecord.minimum
- ActiveRecord.sum
You can find the full details at the ActiveRecord/Calculations API.
This is one of several topics discussed in the Rails 2.1 eBook.
