Select com simple_fields_for

_goal.html.slim

.main-home
  - @goals.each do |goal|
    h1.text-center = goal.title

    = simple_form_for goal do |f|
      = f.error_notification

      .form-group.text-center
        = f.simple_fields_for :goal_options do |g|
          = g.simple_fields_for :user_goal_options do |u|
            = u.input :goal_option_id do
              = g.select :goal_option_id

      .form-group.text-center
        = f.submit "Avançar", class: 'btn btn-success btn-lg'

hr

goal.rb

class Goal < ActiveRecord::Base

  belongs_to :page
  has_many :goal_options

  validates :title, presence: true
  accepts_nested_attributes_for :goal_options

end

goal_option.rb

class GoalOption < ActiveRecord::Base

  belongs_to :goal
  # Relacionamento muitos pra muitos com User
  has_many :user_goal_options
  has_many :users,
    :through => :user_goal_options

  accepts_nested_attributes_for :user_goal_options

end

user.rb

class User < ActiveRecord::Base

  has_many :answers
  # Relacionamento muitos pra muitos com Goal
  has_many :user_goal_options
  has_many :goal_optons,
    :through => :user_goal_options

end

user_goal_option.rb

class UserGoalOption < ActiveRecord::Base
  belongs_to :user
  belongs_to :goal_option
end