Ability to like the post just one time

i'm working on the new ability on my RoR application. I've implemented the like/dislike system and now i'd like to add the ability to like the post just one time( User 1 can like or dislike the Post 2 just one time). How can i make this. I thought, that i can make it just like this

class User < ActiveRecord::Base
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable
 # validates :avatar, attachment_presence: true
  has_attached_file :avatar, styles: { medium: "300x300>", thumb: "100x100>" }
  validates_attachment_content_type :avatar, content_type: /\Aimage\/.*\Z/

  has_and_belongs_to_many :posts

  def admin?
    admin
  end

end

 class Post < ActiveRecord::Base
  has_and_belongs_to_many :users
end

but besides of that i need one more field this has_and_belongs_to_many model, which will be the boolean field, that tells me if true - it is liked, false - it is disliked. BUt how can i access this field in this model. i do not have any idea ? Could you help me ?