title: Rails forms for has_many through association using cocoon
body: <p>i have used has-many-through relation between  user and skills with  bridge table skills_users. I have used cocoon gem. i am getting values from skills using dropdown box on each add_association. In adding new skill the dropdown shows all the values but i want only values that have not been selected. is there any way to reject values that have already been selected from dropbox of new skills</p>
  
  <pre><code>class SkillsUser &lt; ActiveRecord::Base
    belongs_to :skill
   belongs_to :user
  end
  
  class Skill &lt; ActiveRecord::Base
   has_many :skills_users
   has_many :users, through: :skills_users
   belongs_to :skill_type
  end
  
  class User &lt; ActiveRecord::Base
   has_many :skills_users
   has_many :skills, through: :skills_users
  end
  </code></pre>
  
  <p>here is my drop-down box code for skills</p>
  
  <pre><code> = f.input :skill_id, as: :select, collection: Skill.where(skill_type: skill_type.id), label_method: :name, value_method: :id,  label: false,  placeholder: 'Add Skills'
  </code></pre>