Ubuntu 14.04 hangs on boot after starting CUPS

Question

Given these models:

class Places(models.Model):
    #...

class Tour(models.Model):
    places = ManyToManyField(A, through='TourPlaces')

class TourPlaces(models.Model):
    tour = ForeignKey(Tour, on_delete=models.CASCADE)
    place = ForeignKey(Place, on_delete=models.CASCADE)
    #...

How can I build a Queryset so that I can get the Tours a Places belongs to on the SQL side without hitting the database repeatedly? Pseudo example:

for place in Places.objects.(some kind of annotation maybe?):
    print(place.tours[0])

Instead of

for place in Places.objects.all():
    print(TourPlaces.objects.get(place=place).tour[0])

Context: My REST Framework ModelSerializer for Places, using the code above in a SerializerMethodField, is too slow.

Solution

The problem turned out to be with lightdm after an upgrade.

I was able to get it working with the following commands:

sudo apt-get install ubuntu-session (I'm not sure if this is needed, I ended up not using lightdm anyway)

sudo apt-get install gdm (Select gdm from the list)

sudo dpkg-reconfigure lightdm (Select gdm from the list)

reboot

This solution ends up with GDM as the display manager for Ubuntu, but I like the look and it runs better on my machine.

Some links that may be useful:

General "Computer boots to black screen" problem troubleshooting

Comments

[Sonickyle27]

Just a note, using the remove command with apt-get will not delete the config files and user files (at least I think with xkb-data). You need to use purge instead.

[dinosaur]

I had a similar problem (boot hanging after the CUPS line, but I could drop into a shel with ctrl+alt+f4) and my solution involved sudo apt-get remove lightdm and sudo apt-get install lightdm. Also one or two rounds of booting into recovery mode and choosing the dpkg option to fix broken packages (though not sure if that was the problem)