filter() and order_by()

[chris_brand@…]

I suspect that this should be multiple tickets, but here goes...

This is to document where I had problems getting filter() and order_by() to do what I want. I'm using MySQL.
Excerpts from my model :
class Cadet(models.Model):

sqn = models.ForeignKey(Squadron)
enrollment_date = models.DateField(help_text="YYYY-MM-DD")
date_of_birth = models.DateField(help_text="YYYY-MM-DD")

class Camp(models.Model):

name = models.CharField(maxlength=40)
min_training_level = models.PositiveIntegerField("Minimum training level", default=0)
min_age = models.FloatField("Minimum age", max_digits=3, decimal_places=1, default=0)

class Application(models.Model):

cadet = models.ForeignKey(Cadet, edit_inline=models.STACKED, num_extra_on_change=1, num_in_admin=1)
year = models.IntegerField(maxlength=4, core=True, help_text="4-digit year", validator_list=[year_validator])
camp = models.ForeignKey(Camp, core=True)
training_level = models.PositiveIntegerField("current training level")

The only places I can find where I couldn't get filtering to work are where I'm doing something that is probably too complex to do with SQL (although I'm no SQL expert).

Here's the function I ended up with after trying to get it going with
filter() :
def exception_age(request, year_filter):

"""Lists cadets that might not be the right age for the course they applied for."""
this_june = date(int(year_filter), 6, 1)
this_sept = date(int(year_filter), 9, 1)
apps = list(Application.objects.filter(year=year_filter))
# filter out any that are definitely the right age
apps = [a for a in apps if (a.cadet.age_on(this_june) <

float(a.camp.min_age)) or (a.cadet.ages_out() < this_sept)]

I don't have the filter() versions that I tried along the way, but it was based on comparing the application.cadet.date_of_birth field.

This one involves comparing the values in two tables :
def exception_eligibility(request, year_filter):

"""Lists cadets not in the right level for the course they applied for."""
apps = list(Application.objects.filter(year=year_filter))
# filter out any that are in the right level
apps = [a for a in apps if a.training_level < a.camp.min_training_level]
# Sort the list by squadron number
apps.sort(key=sqn_of_app);
return render_to_response('camps/exception_eligibility.html',

{'object_list' : apps,})

and this one requires doing a calculation on the difference between two fields (cadet.date_of_birth and cadet.enrollment_date) :
def exception_enrollment(request, year_filter):

"""Lists cadets enrolled too young who've applied this year."""
apps = list(Application.objects.filter(year=year_filter))
# filter out any that joined old enough
cdts = [a.cadet for a in apps if a.cadet.age_at_enrollment() < 12]
# Sort the list by squadron number
cdts.sort(key=sqn_of_cdt);
return render_to_response('camps/exception_enrollment.html',

{'object_list' : cdts,})

I do have examples of where order_by doesn't work. Here's one :
def wing(app):

return app.cadet.sqn.wing.name

def camp_csv(request, year_filter, object_id):

[...]
app_list = list(camp.application_set.filter(year=year_filter))
# Sort by wing
app_list.sort(key=wing)

# app_list =
camp.application_set.filter(year=year_filter).order_by('camps_applicationc
adet.sqn.wing.name')

[...]

You can see where I've commented-out what I wanted to do. The commented-out version gives :
Exception Type: OperationalError
Exception Value: (1054, "Unknown column
'camps_applicationcadet.sqn.wing.name' in 'order clause'")

This is following a series of foreign keys (application belongs to cadet, belongs to sqn, belongs to wing).