```ruby
class User < ActiveRecord::Base
  has_secure_password
  validates :username, :email, presence: true
  validates :email, uniqueness: true

  has_many :questions
  has_many :answers
  has_many :votes
  has_many :comments
  
end
```

Add validation to put a format constraint on email attribute in User class
user
```ruby
class User < ActiveRecord::Base
  has_secure_password
  validates :username, :email, presence: true
  validates :email, uniqueness: true
  validates_format_of :email, :with => /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\Z/i, :on => :create

  has_many :questions
  has_many :answers
  has_many :votes
  has_many :comments

end
```
assistant