Question

I already have a working solution, but I would really like to know why this doesn’t work:

ratings = Model.select(:rating).uniq
ratings.each { |r| puts r.rating }

It selects, but don’t print unique values, it prints all values, including the duplicates. And it’s in the documentation: http://guides.rubyonrails.org/active_record_querying.html#selecting-specific-fields

Answer 1

Model.select(:rating)

The result of this is a collection of Model objects. Not plain ratings. And from uniq‘s point of view, they are completely different. You can use this:

Model.select(:rating).map(&:rating).uniq

or this (most efficient):

Model.uniq.pluck(:rating)

Rails 5+

Model.distinct.pluck(:rating)

Update

Apparently, as of rails 5.0.0.1, it works only on "top level" queries, like above. Doesn’t work on collection proxies ("has_many" relations, for example).

Address.distinct.pluck(:city) # => ['Moscow']
user.addresses.distinct.pluck(:city) # => ['Moscow', 'Moscow', 'Moscow']

In this case, deduplicate after the query

user.addresses.pluck(:city).uniq # => ['Moscow']