title: Having error null value in column "publicatithon_date" violates not-null constraint in django
body: <p>Hello I'm new to django and I'm learning django with this book:
  djangobook.com
  and I stuck in chapter 5 &amp; 6 because when I try to check my work(in <a href="http://127.0.0.1:8000/admin/books/book/add/" rel="nofollow noreferrer">http://127.0.0.1:8000/admin/books/book/add/</a>) I get some errors and I was trying to fix those errors and after that they gone but I got a new error and I tried to figure out how to solve it but I still don't know how.
  This is my error:</p>
  <blockquote>
  <p>IntegrityError at /admin/books/book/add/</p>
  <p>null value in column &quot;publicatithon_date&quot; violates not-null constraint</p>
  </blockquote>
  <p>And you can see all of my codes here:
  <a href="https://github.com/Anahitahf/mysite2" rel="nofollow noreferrer">https://github.com/Anahitahf/mysite2</a></p>
  <p>How can I solve this error?
  Thanks in advance for any feedbacks.</p>