on_delete is required for django 2.0 and later (#1405)

docs/guide/usage.txt

@@ -23,7 +23,7 @@ Let's start with our model::
         price = models.DecimalField(max_digits=5, decimal_places=2)
         description = models.TextField()
         release_date = models.DateField()
-        manufacturer = models.ForeignKey(Manufacturer)
+        manufacturer = models.ForeignKey(Manufacturer, on_delete=models.CASCADE)
 
 The filter
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