Rails 2.1: Aggregate Expressions

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.

Tags: activerecord, rails 2.1, sql     Posted in Development

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