title: rails set datetime db column to be the same value as updated_at
body: <p>I'm generating a document from a model, this document always needs to match the lastest version of the model, and I can't recreate it every time I load it, because it's heavy and it's sent to docusign.</p>
  <p>I figured I could use the <code>updated_at</code> field with and additional <code>document_version:datetime</code> db column, so that the model can be updated, but when appropriate I can check if <code>updated_at == document_version</code> and know if I need to reproduce the document.</p>
  <p>in my code I have:</p>
  <pre><code>model.update(document_version: model.updated_at)
  </code></pre>
  <p>this doesn't work, I'm just setting the document version to the previous value of updated_at, the <code>update</code> method changes it.</p>
  <pre><code>model.update(document_version: model.updated_at, updated_at: model.updated_at)
  </code></pre>
  <p>the explicit setting of updated_at seems to be ignored.</p>
  <p>my best option would be to update the <code>document_version</code> to whatever <code>updated_at</code> is going to be updated to.</p>
  <p>I'm not sure how to do that</p>