Query model for all possible values of a column - Ruby Rails [duplicate]

This question already has an answer here:

• Rails: how can I get unique values from column 5 answers

So I'm learning programming in general, and Ruby specifically; in the course we are developing an app like rottentomatoes.

I need to query the model from the controller, to render in the view the distinct values in that specific column.

I have this piece of haml to embed the result on the template:

  = form_tag movies_path, :method => :get do
    Include:
    - @all_ratings.each do |rating|
      = rating
      = check_box_tag "ratings[#{rating}]"
    = submit_tag 'Refresh'

I have @all_ratings in the controller, I have tried movie[attr_rating] , Movie(:rating)... And I don't know what else to throw to it.

I do not want to hardcode the values (['G','PG','PG-13','R']) in the controller, but learn how to query the model.