Question

Is it possible to retrieve a count of distinct records based on a field value if the field needs to be interrogated (ideally, using ActiveRecord alone)?

For example, the following returns a count of unique records based on the ‘created_at’ field:

Record.count('created_at', :distinct => true)

However, is it possible to get a count of, say, unique days based on the ‘created_at’ field in a similar way?

A naive ActiveRecord example to explain my intent being:

Record.count('created_at'.day, :distinct => true)

(I know the string ‘created_at’ isn’t a ‘Time’, but that’s the sort of query I’d like to ask ActiveRecord.)

Answer 1

You need to group the records. For example

Record.group('DATE(created_at)').count('created_at')

tells you the number of rows created on each particular date, or

Record.group('DAYOFWEEK(created_at)').count('created_at')

would tell you the number of rows created on individual days of the week.

Beware that this with the usual active record setup this will do date calculations in UTC, if you want to do your calculations in a specific timezone you’ll have to add that conversion to the group statement.