Fix computed field pagination

[igniteflow]

Fixes #958

Summary of changes proposed in this Pull Request:

• Fixes bug in ordering of integer fields in pagination
• Adds support for paginating on negative integer fields
• Adds tests for int and char computed fields

PR checklist:

• [x] Updated relevant documentation
• [x] Updated CHANGELOG.md
• [x] Added tests for my change

[adamalton]

Quick couple of thoughts (haven't had time to review in detail):

• elif isinstance(value, float): should just be else, because if the value is a timedelta / boolean / whatever, then we should also blow up.
  • Possibly the previous if statement should also check for long.

[davidwtbuxton]

If we want to be Python 3.x friendly, use isinstance(x, numbers.Real) instead of isinstance(x, (int, long, float)) or isinstance(x, numbers.Integral) instead of isinstance(x, (int, long)).

[igniteflow]

Is Python 3 compatibility a blocker to merging this?

[adamalton]

No, there are lots of other non-python3-compatible things, so it shouldn't be a blocker.

However... when I run the tests on this branch locally, lots of the pagination tests fail with this same error:

ERROR: test_page_jump_updates_count_correctly (djangae.contrib.pagination.tests.DatastorePaginatorTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/aalton/Sites/github/djangae/testapp/djangae/contrib/pagination/tests.py", line 83, in setUp
    self.u1 = TestUser.objects.create(id=1, first_name="A", last_name="A")
  File "/Users/aalton/Sites/github/djangae/testapp/libs/django/db/models/manager.py", line 127, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/Users/aalton/Sites/github/djangae/testapp/libs/django/db/models/query.py", line 348, in create
    obj.save(force_insert=True, using=self.db)
  File "/Users/aalton/Sites/github/djangae/testapp/libs/django/db/models/base.py", line 734, in save
    force_update=force_update, update_fields=update_fields)
  File "/Users/aalton/Sites/github/djangae/testapp/libs/django/db/models/base.py", line 762, in save_base
    updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
  File "/Users/aalton/Sites/github/djangae/testapp/libs/django/db/models/base.py", line 846, in _save_table
    result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)
  File "/Users/aalton/Sites/github/djangae/testapp/libs/django/db/models/base.py", line 885, in _do_insert
    using=using, raw=raw)
  File "/Users/aalton/Sites/github/djangae/testapp/libs/django/db/models/manager.py", line 127, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/Users/aalton/Sites/github/djangae/testapp/libs/django/db/models/query.py", line 920, in _insert
    return query.get_compiler(using=using).execute_sql(return_id)
  File "/Users/aalton/Sites/github/djangae/testapp/libs/django/db/models/sql/compiler.py", line 973, in execute_sql
    for sql, params in self.as_sql():
  File "/Users/aalton/Sites/github/djangae/testapp/djangae/db/backends/appengine/compiler.py", line 68, in as_sql
    self.query.raw), tuple())
  File "/Users/aalton/Sites/github/djangae/testapp/djangae/db/backends/appengine/commands.py", line 643, in __init__
    getattr(obj, field.attname) if raw else field.pre_save(obj, True),
  File "/Users/aalton/Sites/github/djangae/testapp/djangae/fields/computed.py", line 13, in pre_save
    value = self.get_computed_value(model_instance)
  File "/Users/aalton/Sites/github/djangae/testapp/djangae/fields/computed.py", line 31, in get_computed_value
    return self.computer(model_instance)
  File "/Users/aalton/Sites/github/djangae/testapp/djangae/contrib/pagination/decorators.py", line 23, in generator
    value = convert_to_paginatable_value(value, neg)
  File "/Users/aalton/Sites/github/djangae/testapp/djangae/contrib/pagination/decorators.py", line 49, in convert_to_paginatable_value
    value = u''.join([unichr(int(i)) for i in _chunks(str(value), n=5)])
ValueError: unichr() arg not in range(0x10000) (narrow Python build)

I'm guessing that Travis is running the "wide" python unicode build. But on the narrow one it dies.

[igniteflow]

Interesting, like Travis I'm also running a wide build which is why I hadn't come across this. Will check now!

[adamalton]

When running the tests on my Mac, I'm now getting this (different test, same errorr):

ERROR: test_negatives (djangae.contrib.pagination.tests.ComputedIntegerFieldPaginationTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/adamalton/Sites/github/djangae/testapp/djangae/contrib/pagination/tests.py", line 275, in test_negatives
    obj = ComputedIntegerFieldModel.objects.create(count=i)
  File "/Users/adamalton/Sites/github/djangae/testapp/libs/django/db/models/manager.py", line 127, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/Users/adamalton/Sites/github/djangae/testapp/libs/django/db/models/query.py", line 348, in create
    obj.save(force_insert=True, using=self.db)
  File "/Users/adamalton/Sites/github/djangae/testapp/libs/django/db/models/base.py", line 734, in save
    force_update=force_update, update_fields=update_fields)
  File "/Users/adamalton/Sites/github/djangae/testapp/libs/django/db/models/base.py", line 762, in save_base
    updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
  File "/Users/adamalton/Sites/github/djangae/testapp/libs/django/db/models/base.py", line 846, in _save_table
    result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)
  File "/Users/adamalton/Sites/github/djangae/testapp/libs/django/db/models/base.py", line 885, in _do_insert
    using=using, raw=raw)
  File "/Users/adamalton/Sites/github/djangae/testapp/libs/django/db/models/manager.py", line 127, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/Users/adamalton/Sites/github/djangae/testapp/libs/django/db/models/query.py", line 920, in _insert
    return query.get_compiler(using=using).execute_sql(return_id)
  File "/Users/adamalton/Sites/github/djangae/testapp/libs/django/db/models/sql/compiler.py", line 973, in execute_sql
    for sql, params in self.as_sql():
  File "/Users/adamalton/Sites/github/djangae/testapp/djangae/db/backends/appengine/compiler.py", line 68, in as_sql
    self.query.raw), tuple())
  File "/Users/adamalton/Sites/github/djangae/testapp/djangae/db/backends/appengine/commands.py", line 643, in __init__
    getattr(obj, field.attname) if raw else field.pre_save(obj, True),
  File "/Users/adamalton/Sites/github/djangae/testapp/djangae/fields/computed.py", line 13, in pre_save
    value = self.get_computed_value(model_instance)
  File "/Users/adamalton/Sites/github/djangae/testapp/djangae/fields/computed.py", line 31, in get_computed_value
    return self.computer(model_instance)
  File "/Users/adamalton/Sites/github/djangae/testapp/djangae/contrib/pagination/decorators.py", line 24, in generator
    value = convert_to_paginatable_value(value, neg)
  File "/Users/adamalton/Sites/github/djangae/testapp/djangae/contrib/pagination/decorators.py", line 55, in convert_to_paginatable_value
    value = u''.join([unichr(int(i)) for i in _chunks(str(value), n=5)])
ValueError: unichr() arg not in range(0x10000) (narrow Python build)

[adamalton]

I noticed that this has been added:

if value is None:
    return unicode(None)

but that will produce u"None", which is presumably not what we want?

I'm guessing that we should do:

if value is None:
    return u""

?