Profile.rb

class Profile < ActiveRecord::Base

  belongs_to :user
  has_many :collections

  attr_accessible :avatar, :name, :title, :description

  validates :name, presence: true, length: { minimum: 1, maximum: 60 }
  validates :description, presence: true, length: { minimum: 1, maximum: 60 }
  validates :title, presence: true, length: { minimum: 1, maximum: 60 }

  has_attached_file :avatar, :styles => { :medium => "300x300#", :thumb => "100x100#" }, :default_url => "/assets/generic.png"

  before_save :default_values

  private

  def default_values
    name ||= "Tell us your name!"
    title ||= "What do you do?"
    description ||= "Tell us about yourself!"
  end
end

rails c output

[10] pry(main)> a.save
   (0.2ms)  begin transaction
  User Exists (0.3ms)  SELECT 1 AS one FROM "users" WHERE "users"."email" = '[email protected]' LIMIT 1
  User Exists (0.1ms)  SELECT 1 AS one FROM "users" WHERE "users"."username" = 'username' LIMIT 1
   (0.0ms)  rollback transaction
=> false

[11] pry(main)> a.errors
=> #<ActiveModel::Errors:0x0000000726fed8
 @base=
  #<User id: nil, email: "[email protected]", encrypted_password: "$2a$10$ntsSfoqVtaMX24sIdS488OI4ZM.uJZFlx.j6Xy8y4zFo...", reset_password_token: nil, reset_password_sent_at: nil, remember_created_at: nil, sign_in_count: 0, current_sign_in_at: nil, last_sign_in_at: nil, current_sign_in_ip: nil, last_sign_in_ip: nil, created_at: nil, updated_at: nil, username: "username", confirmation_token: nil, confirmed_at: nil, confirmation_sent_at: nil, unconfirmed_email: nil>,
 @messages=
  {:"profile.name"=>
    ["can't be blank", "is too short (minimum is 1 characters)"],
   :"profile.description"=>
    ["can't be blank", "is too short (minimum is 1 characters)"],
   :"profile.title"=>
    ["can't be blank", "is too short (minimum is 1 characters)"]}>

User.rb

class User < ActiveRecord::Base
  # Include default devise modules. Others available are:
  # :token_authenticatable, :confirmable,
  # :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable, :confirmable,
         :recoverable, :rememberable, :trackable, :validatable

  # Setup accessible (or protected) attributes for your model
  attr_accessible :email, :password, :password_confirmation, :remember_me,
                  :username, :profiles

  attr_accessor :name

  has_one :profile
  accepts_nested_attributes_for :profile

  validates :username, uniqueness: true, presence: true, length: { minimum: 1, maximum: 1000 }

  before_validation :set_profile

  private

  def set_profile
    unless self.profile
      build_profile
    end
  end
end