user.rb

class User < ActiveRecord::Base

  validates :username, presence: :true
  validates :password_digest, presence: :true

  has_secure_password

end

user_spec.rb

require 'spec/helper'

describe User do
  subject do
    User.new :username => 'dmitrynix', :password_digest => "any string"
  end

  it { should be_valid }

  it 'should not be valid without username' do
    subject.username = nil
    expect(subject).to_not be_valid
  end

  it 'should not be valid without password_digest' do
    subject.password_digest = nil
    expect(subject).to_not be_valid
  end
end