How can I count the number of records that have a unique value in a particular field in ROR?

I have a record set that includes a date field, and want to determine how many unique dates are represented in the record set.

Something like:

Record.find(:all).date.unique.count 

but of course, that doesn't seem to work.

This has changed slightly in rails 4 and above :distinct => true is now deprecated. Use:

Record.distinct.count('date')

Or if you want the date and the number:

Record.group(:date).distinct.count(:date)

What you're going for is the following SQL:

SELECT COUNT(DISTINCT date) FROM records

ActiveRecord has this built in:

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

Outside of SQL:

Record.find(:all).group_by(&:date).count

ActiveSupport's Enumerable#group_by is indispensable.

the latest #count on rails source code only accept 1 parameter. see: http://api.rubyonrails.org/classes/ActiveRecord/Calculations.html#method-i-count

so I achieved the requirement by

Record.count('DISTINCT date')

Detailing the answer:

Post.create(:user_id => 1, :created_on => '2010-09-29')
Post.create(:user_id => 1, :created_on => '2010-09-29')
Post.create(:user_id => 2, :created_on => '2010-09-29')
Post.create(:user_id => null, :created_on => '2010-09-29')

Post.group(:created_on).count
# => {'2010-09-29' => 4}

Post.group(:created_on).count(:user_id)
# => {'2010-09-29' => 3}

Post.group(:created_on).count(:user_id, :distinct => true) # Rails <= 3
Post.group(:created_on).distinct.count(:user_id) # Rails = 4
# => {'2010-09-29' => 2}

As I mentioned here, in Rails 4, using (...).uniq.count(:user_id) as mentioned in other answers (for this question and elsewhere on SO) will actually lead to an extra DISTINCT being in the query:

SELECT DISTINCT COUNT(DISTINCT user_id) FROM ...

What we actually have to do is use a SQL string ourselves:

(...).count("DISTINCT user_id")

Which gives us:

SELECT COUNT(DISTINCT user_id) FROM ...

Also, make sure you have an index on the field in your db, or else that query will quickly become sloooow.

(It's much better to do this in SQL, otherwise you pull the entire db table into memory just to answer the count.)