title: Unique Invoice numbers in Django Views for Models
body: <p>I have a django model for my invoices.</p>
  
  <p>Now I need to generate unique invoice numbers for each day (in my Django Views function). </p>
  
  <p>Example, invoice no, for 7th April, 2016 shall be like: 16040701, 16040702, 16040703, etc.</p>
  
  <p>How can this be achieved?</p>