ruby - Rails forms for has_many through association using cocoon -

i have used has-many-through relation between user , skills bridge table skills_users. have used cocoon gem. getting values skills using dropdown box on each add_association. in adding new skill dropdown shows values want values have not been selected. there way reject values have been selected dropbox of new skills

class skillsuser < activerecord::base   belongs_to :skill  belongs_to :user end  class skill < activerecord::base  has_many :skills_users  has_many :users, through: :skills_users  belongs_to :skill_type end  class user < activerecord::base  has_many :skills_users  has_many :skills, through: :skills_users end 

here drop-down box code skills

 = 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' 

in controller add:

@available_skills = skill.where(skill_type: skill_type).where("`id` not in ?", @user.skills.pluck(:id)) 

then in view:

 = f.input :skill_id, as: :select, collection: @available_skills, label_method: :name, value_method: :id,  label: false,  placeholder: 'add skills'