title: Increment all records with fixed value in rails
body: <p>I want to do this</p>
  
  <pre><code>User.all.each do |user|
      user.salary = user.salary + 100
      user.save
  end
  </code></pre>
  
  <p>I tried this <code>User.update_all(salary: "salary + 100")</code> but it raised that exception <code>Mysql::Error: Incorrect integer value: 'salary + 100' for column 'salary' at row 1:</code></p>
  
  <p>What is the best way to increment all records in rails ? </p>