Aggregation across foreign keys / complex aggregation and annotation

[Christopher Mutel]

Following the general rule that no good deed goes unpunished, I have a
question about complex aggregation / annotation or annotation across
foreign keys.

I have the following simplified model:

class Unit(models.Model):
unit = models.TextField()

class Process(models.Model):
process = models.TextField()
unit = models.ForeignKey(Unit)

class Exchange(models.Model):
input = models.ForeignKey(Process)
output = models.ForeignKey(Process)
amount = models.FloatField()

Is there a way for me to get a queryset for Process outputs which have
a sum of inputs greater than one kilogram?

You can do this (painfully) now by looping:

foo = []
for p in Process.objects.all():
if sum(Exchange.objects.filter(ouput=p,
input__unit__unit="kilogram").values_list('amount', flat=True)) > 1:
foo.append(p)

Sorry if this is a basic question, but I have been trying to figure it
out for a while, and without success.

-Chris

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Does using the IPython shell add "LIMIT" to Django ORM discussion (using SQLite)

[Christopher Mutel]

Hello all-

I recently filed a bug about incorrect SQL generation, and Malcom
Tredinnick said that the example SQL I provided couldn't be correct,
because there was an extra LIMIT clause that shouldn't be there. After
poking around for a bit, I realized that everytime I was executing my
query in the IPython shell, it was appending a LIMIT 21 to the SQL,
but this limit clause wasn't being appended if the query was part of a
regular python process. Both queries are run against SQLite 3.

I uploaded a small test case here that demonstrates this behaviour:

http://www.bitbucket.org/cmutel/extra_limit_21/

It is possible that I have poor python-fu, or is this known behaviour?
Is it possible that Django is truncating the result list because
IPython won't show all results anyway?

I have tried searching the Django codebase, and the mailing list, but
to no avail.

-Chris

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A twist on cross importing models

[Christopher Mutel]

Hello all-

I have tried a couple of tips found on the list to solve my problem
with circular model imports, but with no lucks. Here is my basic
schema:

#foo/models.py

from bar.models import Formula

class Variable(models.Model):
  amount = models.FloatField(null=True)
  is_formula = models.BooleanField(default=False)
  formula = models.OneToOneField(Formula, null=True,
related_name="variable_formula")

  def get_amount(self):
if self.is_formula:
  return self.formula.evaluate()
else:
  return self.amount

#bar/models.py

from foo.models import Variable

class Formula(models.Model):
  formula = models.TextField() # Where formula is a text string like
'var:1337*var:42', and the numbers are id numbers.

  def calculate(self):
# re stuff
for each_found_variable in groups(): # Simplified to show process
- this obviously doesn't work.
  raw_string.append(Variable.objects.get(id=this_id).get_amount())
return eval(raw_string, stuff_to_make_eval_safe)

In this case, the circular reference occurs only in a method of the
class - I don't know if this makes a difference. How can I structure
my import statements so that this is possible? I should note that
there are more than one variable, so that combining everything into
one models file is not really possible (and doesn't really address the
chicken/egg problem either).

I thought I saw a recent svn commit that addressed this problem, but
can't find it now.

Any help would be greatly appreciated?


-- 

Chris Mutel
Ökologisches Systemdesign - Ecological Systems Design
Institut f.Umweltingenieurwissenschaften - Institute for Environmental
Engineering
ETH Zürich - HIF C 42 - Schafmattstr. 6
8093 Zürich

Telefon: +41 44 633 71 45 - Fax: +41 44 633 10 61


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