error

 Showing D:/studmed/studmed/app/views/illnesses/_form.html.erb where line #49 raised:

undefined method `id' for #<Illness::ActiveRecord_Relation:0x0000000c9c1f60>
Did you mean?  ids

method.rb

    def get_differential_diagnoses
      differential_diagnoses = []
      CATEGORIES.each do |category|
        differential_diagnoses << Illness.where(category: category)
      end
      differential_diagnoses
    end

view.html.erb

<%= select_tag 'differential_id', options_from_collection_for_select(@differential_diagnoses, 'id', 'name'), prompt: "Vælg differential diagnoser", class: 'form-control', multiple: true %>