Add validations and hooks to User model

app/models/user.rb

@@ -1,2 +1,36 @@
 class User < ApplicationRecord
+    before_save { self.email = email.downcase }
+
+    ##### ActiveRecord Validations
+    validates :username,
+              presence: true,
+              length: { minimum: 3, maximum: 26 }
+
+    validates :email,
+              case_sensitive: false,
+              presence: true,
+              uniqueness: true,
+              email: true
+    
+    validates :password,
+              length: { minimum: 8 },
+              presence: true,
+              on: :create
+
+    validates :password,
+              length: { minimum: 8 },
+              on: :update,
+              if: :password_digest_changed?
+
+    validates :password_confirmation,
+              presence: true,
+              on: :create
+
+    validates :password_confirmation,
+              presence: true,
+              on: :update,
+              if: :password_digest_changed?
+
+##### ActiveModel Security
+    has_secure_password
 end