TypeError: __init__() missing 1 required positional argument: 'on_delete'

[a-maryam]

Location 1

File "/srv/venv/lib/python3.5/site-packages/flatcontent/models.py", line 13, in FlatContent
site = models.ForeignKey(Site, blank=True, null=True)
TypeError: init() missing 1 required positional argument: 'on_delete'

Location 2

File "/srv/venv/lib/python3.5/site-packages/flatcontent/migrations/0001_initial.py", line 20, in Migration
('site', models.ForeignKey(to='sites.Site', null=True, blank=True)),
TypeError: init() missing 1 required positional argument: 'on_delete'

Location 3

File "/srv/venv/lib/python3.5/site-packages/flatcontent/migrations/0001_initial.py", line 20, in Migration
('site', models.ForeignKey(to='sites.Site', null=True, blank=True)),
TypeError: init() missing 1 required positional argument: 'on_delete'

Fix (Disclaimer: Not acquainted with the possible repercussions)

add on_delete=models.PROTECT to the foreign key arguments

Source