Rails：选择特定列唯一的所有记录

Question

I have a Recommendation model and I would like to select all recommendations that are distinct by their movie_id column. I've tried a few methods but none that have worked yet:

scope :not_duplicate, -> {
    group_by(&:movie_id)
  }

==> doesn't work on a class

scope :not_duplicate, -> {
        uniq_by(&:movie_id)
      }

==> doesn't work on a class

scope :not_duplicate, -> {
        select(:movie_id).group(:movie_id, "recommendations.id")
      }

==> only returns movie_id values and I need entire record

scope :not_duplicate, -> {
        select("recommendations.*").group(:movie_id)
      }

==> PG::GroupingError: ERROR:  column "recommendations.id" must appear in the GROUP BY clause or be used in an aggregate function

I'm sure this is a pretty regular occurrence but for some reason I can't find the solution. Any ideas?

Answer 1

如果我正确理解了您的问题，可以尝试一下

Recommendation.select {|rec| rec.movie_id == movie_id).uniq

Answer 2

How about:

scope :not_duplicate, -> {
        group(:id, :movie_id).distinct
      }