Model.select（：评分）
 

这个结果是型号对象的数组。不是普通的收视率。而从 uniq的的角度来看，他们是完全不同。您可以使用此：

  Model.select（：等级）.MAP（安培;：评级）.uniq
 

或此（最有效的）

  Model.uniq.pluck（：评分）
 

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

Model.select(:rating)

Result of this is an array 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)