Find rows with multiple duplicate fields with Active Record, Rails & Postgres

Question

What is the best way to find records with duplicate values across multiple columns using Postgres, and Activerecord?

I found this solution here:

User.f

Answer 1

Tested & Working Version

User.select(:first,:email).group(:first,:email).having("count(*) > 1")

Also, this is a little unrelated but handy. If you want to see how times each combination was found, put .size at the end:

User.select(:first,:email).group(:first,:email).having("count(*) > 1").size

and you'll get a result set back that looks like this:

{[nil, nil]=>512,
 ["Joe", "test@test.com"]=>23,
 ["Jim", "email2@gmail.com"]=>36,
 ["John", "email3@gmail.com"]=>21}

Thought that was pretty cool and hadn't seen it before.

Credit to Taryn, this is just a tweaked version of her answer.