title: What is the correct format to use Unique in _sql_constraints in OpenERP?
body: <p>I have tried unique in sql_constaints in OpenERP(Odoo) using two different methods using flower brackets {} or square brackets [].
  Both works fine.
  Which one is correct?</p>
  
  <pre class="lang-py prettyprint-override"><code>_sql_constraints = {
      ('email_uniq', 'unique(email)', ' Please enter Unique Email id.')
      }
  </code></pre>
  
  <p>(or)</p>
  
  <pre class="lang-py prettyprint-override"><code>_sql_constraints = [
      ('email_uniq', 'unique(email)', ' Please enter Unique Email id.')
      ]
  </code></pre>
  
  <p>P.S: But if I want to use more than a constraint it accepting only square brackets [] like this example.</p>
  
  <pre class="lang-py prettyprint-override"><code>_sql_constraints = [
      ('email_uniq', 'unique(email)', ' Please enter Unique Email id.'),
      ('contact_uniq', 'unique(contact)', ' Please enter Unique Mobile no.')
      ]
  </code></pre>
  
  <p>What is the reason behind it?</p>