title: Need help in Django 2x ,( FOREIGN KEY constraint failed )
body: <p>hi everyone i made a simple modeland my problem is when i try to make a FOREIGN KEY in my model it shows me this error message , it would be awesome if you can help 
  here is the code </p>
  
  <p>Model.py:</p>
  
  <pre><code>from django.db import models
  from django.utils import timezone
  
  # Create your models here.
  class category(models.Model):
      title=models.CharField(max_length=100,default='')
      def __str__(self):
          return self.title
  class gigpost(models.Model):
  
      title=models.CharField(default='',max_length=100)
      Category=models.OneToOneField(category,on_delete=True,default='')
      published_at=models.DateTimeField(auto_now_add=True)
      description=models.TextField(default='',max_length=None)
      mainphoto=models.ImageField(default='')
      photo=models.FileField()
      def __str__(self):
          return self.title
  </code></pre>
  
  <p>Admin.py</p>
  
  <pre><code>from django.contrib import admin
  from .models import gigpost , category
  
  # Register your models here.
  admin.site.register(gigpost)
  admin.site.register(category)
  </code></pre>
  
  <p>the error message : </p>
  
  <pre><code>IntegrityError at /admin/gigpost/category/add/
  FOREIGN KEY constraint failed
  Request Method: POST
  Request URL:    http://127.0.0.1:8000/admin/gigpost/category/add/
  Django Version: 2.2.5
  Exception Type: IntegrityError
  Exception Value:    
  FOREIGN KEY constraint failed
  Exception Location: C:\Users\Madara\Miniconda3\envs\myEv\lib\site-packages\django\db\backends\base\base.py in _commit, line 240
  Python Executable:  C:\Users\Madara\Miniconda3\envs\myEv\python.exe
  Python Version: 3.7.6
  Python Path:    
  ['C:\\Users\\Madara\\Desktop\\empliya\\empliya',
   'C:\\Users\\Madara\\Miniconda3\\envs\\myEv\\python37.zip',
   'C:\\Users\\Madara\\Miniconda3\\envs\\myEv\\DLLs',
   'C:\\Users\\Madara\\Miniconda3\\envs\\myEv\\lib',
   'C:\\Users\\Madara\\Miniconda3\\envs\\myEv',
   'C:\\Users\\Madara\\Miniconda3\\envs\\myEv\\lib\\site-packages']
  Server time:    Tue, 18 Feb 2020 19:34:39 +0000
  </code></pre>