app/models/user.rb
class User < ApplicationRecord
# Include default devise modules. Others available are:
# :confirmable, :lockable, :timeoutable, :trackable and :omniauthable
validates_presence_of :user_name # Ensures user name can't be blank.
devise :database_authenticatable, :registerable,
:recoverable, :rememberable, :validatable
has_many :posts, dependent: :destroy
end