title: Set Django set Integer Field minimum to 1
body: <p>How can I create an integer field for a model such that the minimum allowed integer is 1?  I noticed Django has a <code>PositiveIntegerField</code> <a href="https://docs.djangoproject.com/en/1.7/ref/models/fields/#positiveintegerfield" rel="noreferrer">documented here</a> but it allows 0s.</p>