```ruby
class User < ActiveRecord::Base

  # Include default devise modules. Others available are:
  # :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable, :confirmable

  has_many :tips, dependent: :destroy
  has_many :bookmarks, dependent: :destroy

  before_save { self.email = email.downcase }

  validates :first_name, presence: true, length: { maximum: 50 }
  validates :last_name, presence: true, length: { maximum: 50 }

  VALID_EMAIL_REGEX = /\A[\w+\-.]+@[a-z\d\-]+(?:\.[a-z\d\-]+)*\.[a-z]+\z/i
  validates :email, presence: true, format: { with: VALID_EMAIL_REGEX }, uniqueness: { case_sensitive: false }
  
  validates :password, presence: true, length: { minimum: 6 }
  validates :password_confirmation, presence: true

  def name 
    [first_name, last_name].join(' ')
  end

  def bookmarked_tips
    tips.sum(:bookmarks_count, :group => :user_id)
  end
end
```

Add additional validations for User model
user
```ruby
class User < ActiveRecord::Base

  # Include default devise modules. Others available are:
  # :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable, :confirmable

  has_many :tips, dependent: :destroy
  has_many :bookmarks, dependent: :destroy

  before_save { self.email = email.downcase }

  validates :first_name, presence: true, length: { maximum: 50 }
  validates :last_name, presence: true, length: { maximum: 50 }

  VALID_EMAIL_REGEX = /\A[\w+\-.]+@[a-z\d\-]+(?:\.[a-z\d\-]+)*\.[a-z]+\z/i
  validates :email, presence: true, format: { with: VALID_EMAIL_REGEX }, uniqueness: { case_sensitive: false }
  
  validates :password, presence: true, length: { minimum: 6 }, on: :create
  validates :password_confirmation, presence: true, on: :update, allow_blank: true

  def name 
    [first_name, last_name].join(' ')
  end

  def bookmarked_tips
    tips.sum(:bookmarks_count, :group => :user_id)
  end
end
```
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