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_application__c
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_application__cadet.sqn.wing.name' in 'order clause'")

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

[anonymous]

For the order_by() one, I was pointed to the correct syntax :

camp.application_set.select_related().filter(

year=year_filter).order_by('camps_wing.name')

The documentation implies that selected_Related() was a performance optimisation, but it seems to be required here.

[Michael Radziej <mir@…>]

Putting the code into braces brings better readability:

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')

[Michael Radziej <mir@…>]

Args ... that didn't really improve the readability.

Can you please repost this with proper line endings and with the code in { { { and } } } (remove the spaces between the braces)? Try the preview button before you finally submit.

Regarding order_by:

I don't know how this folklore about the "table.field" syntax with the dot goes around -- this might work by chance (but not in all circumstances), but the documented way is "model__field". Unfortunately, the documented syntax "model__field" doesn't work, either. Ticket #2076 is about this.

Regarding filter:

I cannot really make sense of your code in this format, but you seem to have left out what actually went wrong.

I would normally close the ticket, but it seems that Jacob wants to have a look on this.

[cbrand@…]

More readable version of the code follows (sorry about that)

Models :

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")

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,}) 

As for the dot syntax, ​http://www.djangoproject.com/documentation/db_api/#order-by-fields says :
To order by a field in a different table, add the other table's name and a dot, like so
which lends the idea some legitimacy...

[Malcolm Tredinnick]

Fixed formatting in summary.