ambition/app/models/projects_user.rb

class Projects_user < ActiveRecord:: Base
    
    validates :project_id, presence: true
    validates :user_id, presence: true
    
    validate :user_exists , on: :create
    validate :project_exists, on: :create
    
    private
        
        def project_exists
            
            project = Project.find_by(id: project_id)
            if project == nil
                errors[:project] = "project belong to projects_user is not exist"
            end
        
        end
        
        def user_exists
            
            user = User.find_by(id: user_id)
            if user == nil
                errors[:user] = "user belong to projects_user is not exist"
            end
        
        end
    
end