title: Understanding rails migration statement (:null => false)
body: <p>I am trying to understand the following statement, it is from a rails migration file:</p>
  
  <pre><code>x.datetime "new",     :null =&gt; false
  x.datetime "update",  :null =&gt; false
  </code></pre>
  
  <p>I understand the the first part of both statements (everything before the comma) but I am unsure on the null portion</p>
  
  <pre><code>:null =&gt; false
  </code></pre>
  
  <p>Is this basically saying "if it does not exist, then it is false?"  The logic just seems a bit strange, any clarification on this would be greatly helpful.</p>