Count The Number Of Records By Attribute

In Count How Many Records There Are Of Each Type, I walked through how to use SQL (in PostgreSQL) to get a count of how many records there are with each unique value in a given column. This is something I tend to do with a type or status column.

We can ask the same question with Rails, with very little code. It produces a nearly identical query and the same results.

> Book.group(:status).count
#=> { nil => 123, "published" => 611, "draft" => 364, "review" => 239 }

We've picked the Book model and we want it to group books by their status. Tacking on the #count at the end tells it to apply the count aggregate. The result is a hash of each unique value of the specified attribute (status) paired with the count.