Update multiple columns using django F() object

Question

I have a model that stores stats on a one integer stat per column basis. I have a view that handles the update of said stats, like so:

class PlayerStats(models.Model):
    #In game stats - these represent the actual keys sent by the game
    NumberOfJumps = models.IntegerField(default=0)
    NumberOfDoubleJumps = models.IntegerField(default=0)
    NumberOfSilverPickups = models.IntegerField(default=0)
    NumberOfGoldPickups = models.IntegerField(default=0)
    NumberOfHealthPickups = models.IntegerField(default=0)

I basically get a dicitonary of stats that I need to to add to the current stats stored in the database.

I really don´t want to pull all of the data out of the model and then update it again, as I would like to do this on the database level, if possible.

A colleague suggested that I use django´s F() object in order to push this out of the view code, mostly in order to keep it thread-safe and avoid any mysql deadlocks (the stats table is potentially being updated continually by different threads)

The dictionary contains keys that mirror those used in the database, so at the moment I´m doing it like this:

def update_stats(new_stats):
    player_stats = PlayerStats(user=user, **new_stats)
    old_stats = player_stats.values()[0]
    updated_stats = {}
    for stat in new_stats:
        if old_stat[stat]:
            updated_stats[stat] = old_stats[stat] + new_stats[stat]
    PlayerStats.objects.filter(user=user).update(**updated_stats)

Anybody have any pointers as to how to achieve this by using the F() object?

okm · Accepted Answer · 2013-03-25 12:53:52Z

4

+50

To update using models.F, you need to construct something like

qs.update(field_1=models.F('field_1')+field_1_delta,
          field_2=models.F('field_2')+field_2_delta, 
          ...)

For you code, it might be

new_stats = {
    'NumberOfHealthPickups': 99
    # ...
}
updated_stats = {}
for stat in new_stats:
    updated_stats[stat] = models.F(stat) + new_stats[stat]
PlayerStats.objects.filter(user=user).update(**updated_stats)

answered Mar 25, 2013 at 12:53

okm

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1 Comment

Gissur Þórhallsson Over a year ago

This is exactly what I was looking for, since I need to touch the database as little as possible.

shx2 · Accepted Answer · 2013-03-25 10:55:36Z

2

One option is to update the fields one by one.

This code will not update all fields at once (so it might be slow, db-access-wise), but it is safe (no deadlocks, no lost updates).

user_stats = PlayerStats.objects.get(user=user)
for stat, increment in new_stats.iteritems():
    user_stats.update(**{ stat: F(stat) + increment })

answered Mar 25, 2013 at 10:55

shx2

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1 Comment

Gissur Þórhallsson Over a year ago

That level of db access is not acceptable in my case, but thanks anyway.

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