ambition/app/models/missions_user.rb

class Missions_user <ActiveRecord:: Base
  
    validates :mission_id, presence: true
    validates :user_id, presence: true
    
    validate :mission_exist, on: :create
    validate :user_exist, on: :create
    
    private 
        def mission_exist
            
            mission = Mission.find_by(id: mission_id)
            if mission == nil
                errors[:mission] = "mission belong to Missions_user is not exist"
            end
        
        
        end
        
        def user_exist
            
            user =  User.find_by(id: user_id)
            if user == nil
                errors[:user] = "user belong to Missions_user is not exist"
            end
        
        end
        
    
end