title: mongoid update all documents with conditions
body: <p>I have a model</p>
  
  <pre><code>class Employee
    include Mongoid::Document
    field :first_name
    field :last_name
    field :address1
    field :address2
    field :salary
  end
  </code></pre>
  
  <p>Now I need to update all Employee's salary to 10000 whose address1 is "Calgary"</p>
  
  <p>Now I tried this query</p>
  
  <pre><code>Employee.update_all "salary = 10000", "address1 = 'Calgary'"
  </code></pre>
  
  <p>But this query gave me error as:</p>
  
  <pre><code>NoMethodError: undefined method `update_all' for Employee:Class
  </code></pre>
  
  <p>Thanks</p>