Fix django 2.0 deprecation warnings

Author: Ryan P Kilby

tests/testapp/models.py

@@ -6,44 +6,44 @@
 class Note(models.Model):
     title = models.CharField(max_length=100)
     content = models.TextField()
-    author = models.ForeignKey(User)
+    author = models.ForeignKey(User, on_delete=models.CASCADE)
 
 
 class Post(models.Model):
-    note = models.ForeignKey(Note)
+    note = models.ForeignKey(Note, on_delete=models.CASCADE)
     content = models.TextField()
     date_published = models.DateField(null=True)
 
 
 class Cover(models.Model):
     comment = models.CharField(max_length=100)
-    post = models.ForeignKey(Post)
+    post = models.ForeignKey(Post, on_delete=models.CASCADE)
 
 
 class Page(models.Model):
     title = models.CharField(max_length=100)
     content = models.TextField()
-    previous_page = models.ForeignKey('self', null=True)
+    previous_page = models.ForeignKey('self', null=True, on_delete=models.CASCADE)
 
 
 class A(models.Model):
     title = models.CharField(max_length=100)
-    b = models.ForeignKey('B', null=True)
+    b = models.ForeignKey('B', null=True, on_delete=models.CASCADE)
 
 
 class C(models.Model):
     title = models.CharField(max_length=100)
-    a = models.ForeignKey(A, null=True)
+    a = models.ForeignKey(A, null=True, on_delete=models.CASCADE)
 
 
 class B(models.Model):
     name = models.CharField(max_length=100)
-    c = models.ForeignKey(C, null=True)
+    c = models.ForeignKey(C, null=True, on_delete=models.CASCADE)
 
 
 class Person(models.Model):
     name = models.CharField(max_length=100)
-    best_friend = models.ForeignKey('self', null=True)
+    best_friend = models.ForeignKey('self', null=True, on_delete=models.CASCADE)
 
     date_joined = models.DateField(auto_now_add=True)
     time_joined = models.TimeField(auto_now_add=True)