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

Get all duplicates with a single query if you use PostgreSQL:

def duplicated_users
  duplicated_ids = User
    .group(:first, :email)
    .having("COUNT(*) > 1")
    .select('unnest((array_agg("id"))[2:])')

  User.where(id: duplicated_ids)
end

irb> duplicated_users

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", "[email protected]"]=>23,
 ["Jim", "[email protected]"]=>36,
 ["John", "[email protected]"]=>21}

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

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

That error occurs because POSTGRES requires you to put grouping columns in the SELECT clause.

try:

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

(note: not tested, you may need to tweak it)

EDITED to remove id column

If you need the full models, try the following (based on @newUserNameHere's answer).

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

This will return the rows where the email address of the row is not unique.

I'm not aware of a way to do this over multiple attributes.