select vs distinct vs uniq? - sql

Question

I am confused by Ruby's ActiveRecord uniq method. I am using it to try to get back an array of complete objects, not just a particular field.
In my Padrino app script, which saves newspaper names and scores as Score objects, the uniq method by attribute on an ActiveRecord Relation is not working, and neither is distinct, with or without SQL syntax. Can anyone explain what is going on?
class Score < ActiveRecord::Base
def self.from_today
self.where('created_at > ?', Date.today)
end
end
scores = Score.from_today
scores.class
=> Score::ActiveRecord_Relation
scores.first
=> #<Score id: 123, score: -2.55, source: "Mail", created_at: "2016-08-11 04:29:24", updated_at: "2016-08-11 04:29:24">
scores.map(&:source)
=> ["Mail", "Guardian", "Telegraph", "Independent", "Express", "Mail"]
scores.uniq(:source).count
=> 6
scores.distinct(:source).count
=> 6
scores.select('distinct (SOURCE)').count
=> 5 #AHA, it works!
scores.select(:source).distinct
=> #<ActiveRecord::Relation [#<Score id: nil, source: "Telegraph">, #<Score id: nil, source: "Mail">, #<Score id: nil, source: "Independent">, #<Score id: nil, source: "Express">, #<Score id: nil, source: "Guardian">]>
#oops, no it doesn't

Answer 1

In Rails 5 distinct has no parameter. In Rails 4.2 the parameter is only true or false. When true, return distinct records, when false return none distinct records. uniq is in this case only an alias for distinct
So for Rails 4
scores.select(:source).distinct.count is what you want. This restricts distinct to column source