title: How to write a where.not query with multiple conditions in rails 4
body: <p>What I want to express is this:</p>
  
  <pre><code>Model.where("a IS NOT NULL `**`OR`**` b IS NOT NULL")
  </code></pre>
  
  <p>In Rails 4, <code>where.not</code> clause is enabled:</p>
  
  <pre><code>Model.where.not(a: nil, b: nil)
  </code></pre>
  
  <p>However, This expression equals</p>
  
  <pre><code>Model.where("a IS NOT NULL `**`AND`**` b IS NOT NULL")
  </code></pre>
  
  <p>How can I express</p>
  
  <pre><code>"a IS NOT NULL `**`OR`**` b IS NOT NULL"
  </code></pre>
  
  <p>with <code>where.not</code> clause?</p>