[Django] #30564: Cannot create custom field that returns a queryset AND uses pre_save()
Django
-------------------------------------+-------------------------------------
Reporter: Dan J | Owner: nobody
Strohl |
Type: Bug | Status: new
Component: Database | Version: 2.2
layer (models, ORM) |
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
I tried creating a custom field that would store a string that was a list
of pks, then return that as a queryset. (yes, I could probably have done
it using a many2many field, but I was (am) trying to reduce the number of
db queries for this.
In any case, all seems to work OK until I implemented a def pre_save(self,
model_instance, add) method on my custom field.
What seems to be happening is that when the queryset comes back from the
pre-save, it is run through the SQLInsertCompiler.prepare_value, which
checks the value to see if it has a "resolve_expression" attrubute, which
it assumes is then a SQL expression and then tries checking for
"contains_column_references"... The QuerySet object DOES have the
"resolve_expression" attribute, but does NOT have the others that are in
the SQL expression objects.
I suspect this doesn't come up much.
My trace back
Error
Traceback (most recent call last):
{{{
File
"C:\Users\strohl\Documents\Project\Who\who_db\who_db_tests\test_model_methods.py",
line 101, in setUp
self.M1 = MixinTest.objects.create(name='M1')
File "C:\Users\strohl\Documents\VirtualEnv\Who\lib\site-
packages\django\db\models\manager.py", line 82, in manager_method
return getattr(self.get_queryset(), name)(*args, **kwargs)
File "C:\Users\strohl\Documents\VirtualEnv\Who\lib\site-
packages\django\db\models\query.py", line 422, in create
obj.save(force_insert=True, using=self.db)
File "C:\Users\strohl\Documents\Project\Who\who_db\models.py", line 132,
in save
super(MixinTest, self).save(*args, **kwargs)
File "C:\Users\strohl\Documents\VirtualEnv\Who\lib\site-
packages\django\db\models\base.py", line 741, in save
force_update=force_update, update_fields=update_fields)
File "C:\Users\strohl\Documents\VirtualEnv\Who\lib\site-
packages\django\db\models\base.py", line 779, in save_base
force_update, using, update_fields,
File "C:\Users\strohl\Documents\VirtualEnv\Who\lib\site-
packages\django\db\models\base.py", line 870, in _save_table
result = self._do_insert(cls._base_manager, using, fields, update_pk,
raw)
File "C:\Users\strohl\Documents\VirtualEnv\Who\lib\site-
packages\django\db\models\base.py", line 908, in _do_insert
using=using, raw=raw)
File "C:\Users\strohl\Documents\VirtualEnv\Who\lib\site-
packages\django\db\models\manager.py", line 82, in manager_method
return getattr(self.get_queryset(), name)(*args, **kwargs)
File "C:\Users\strohl\Documents\VirtualEnv\Who\lib\site-
packages\django\db\models\query.py", line 1186, in _insert
return query.get_compiler(using=using).execute_sql(return_id)
File "C:\Users\strohl\Documents\VirtualEnv\Who\lib\site-
packages\django\db\models\sql\compiler.py", line 1331, in execute_sql
for sql, params in self.as_sql():
File "C:\Users\strohl\Documents\VirtualEnv\Who\lib\site-
packages\django\db\models\sql\compiler.py", line 1275, in as_sql
for obj in self.query.objs
File "C:\Users\strohl\Documents\VirtualEnv\Who\lib\site-
packages\django\db\models\sql\compiler.py", line 1275, in <listcomp>
for obj in self.query.objs
File "C:\Users\strohl\Documents\VirtualEnv\Who\lib\site-
packages\django\db\models\sql\compiler.py", line 1274, in <listcomp>
[self.prepare_value(field, self.pre_save_val(field, obj)) for field in
fields]
File "C:\Users\strohl\Documents\VirtualEnv\Who\lib\site-
packages\django\db\models\sql\compiler.py", line 1205, in prepare_value
if value.contains_column_references:
AttributeError: 'Query' object has no attribute
'contains_column_references'
}}}
My custom field:
{{{
#!python
class PkListField(Field):
empty_strings_allowed = False
description = "PK List"
default_error_messages = {}
def __init__(self, *args, max_recs=100, max_pk_size=5, sep=',',
ordered=None, as_pks=True, model=None, manager='objects',
pre_save_func=None, **kwargs):
self.pk_list_model_manager = manager
self.max_recs = max_recs
self.max_pk_size = max_pk_size
self.sep = sep
self.as_pks = as_pks
self.ordered = ordered
self.pre_save_func = pre_save_func
self._pk_list_model = model
if not as_pks and model is None:
raise AttributeError('Model must be specified if not returning
as_pks')
if as_pks and ordered is not None and ordered != 'pk':
raise AttributeError('Data can only be ordered by PKs when
returning as pks')
kwargs['max_length'] = ((max_pk_size + 2) * max_recs) + 1
kwargs['blank'] = True
kwargs['default'] = sep + sep
super().__init__(*args, **kwargs)
@property
def pk_list_model(self):
if isinstance(self._pk_list_model, str):
self._pk_list_model = apps.get_model(self._pk_list_model)
return self._pk_list_model
def deconstruct(self):
name, path, args, kwargs = super().deconstruct()
if self.pre_save_func is not None:
kwargs['pre_save_func'] = self.pre_save_func
if self.pk_list_model is not None:
kwargs['model'] = self.pk_list_model
if self.pk_list_model_manager is not 'objects':
kwargs['manager'] = self.pk_list_model_manager
if not self.as_pks:
kwargs['as_pks'] = self.as_pks
if self.max_recs != 100:
kwargs['max_recs'] = self.max_recs
if self.max_pk_size != 5:
kwargs['max_pk_size'] = self.max_pk_size
if self.sep != ',':
kwargs['sep'] = self.sep
if self.ordered is not None:
kwargs['ordered'] = self.ordered
del kwargs['max_length']
del kwargs['blank']
del kwargs['default']
return name, path, args, kwargs
def get_internal_type(self):
return "CharField"
def conv_str_to_python(self, value):
def conv_obj(obj_in):
if obj_in is None:
return []
elif isinstance(obj_in, int):
return [obj_in]
elif isinstance(obj_in, str):
if self.sep in obj_in:
return
conv_obj(obj_in.strip().strip(self.sep).split(self.sep))
if obj_in:
return [int(obj_in.strip())]
else:
return []
elif issubclass(obj_in.__class__, models.Model):
return [obj_in.pk]
else:
try:
tmp_ret = []
for item in obj_in:
tmp_ret.extend(conv_obj(item))
return tmp_ret
except TypeError:
raise ValidationError('Invalid object type: %r' %
obj_in)
if issubclass(value.__class__, models.QuerySet):
if self.as_pks:
return list(value.values_list('pk', flat=True))
else:
return value
if not value:
value = []
else:
value = conv_obj(value)
if self.as_pks:
if self.ordered == 'pk':
value.sort()
return value
tmp_mgr = getattr(self.pk_list_model, self.pk_list_model_manager)
print('in conv_to_python: value= %r' % value)
# if value:
tmp_ret = tmp_mgr.filter(pk__in=value)
if self.ordered:
tmp_ret = tmp_ret.order_by(*make_list(self.ordered))
print('in conv_to_python: returning filtered = %r' % tmp_ret)
return tmp_ret
# else:
# tmp_ret = tmp_mgr.none()
# print('in conv_to_python: returning none = %r' % tmp_ret)
# return tmp_ret
def to_python(self, value):
print(f'IN ({self.attname}) to_python({repr(value)})')
tmp_ret = self.conv_str_to_python(value)
print(f'OUT ({self.attname}) to_python({repr(tmp_ret)})')
return tmp_ret
def from_db_value(self, value, expression, connection):
print(f'IN ({self.attname}) from_db_value({repr(value)})')
tmp_ret = self.conv_str_to_python(value)
print(f'OUT ({self.attname}) from_db_value({repr(tmp_ret)})')
return tmp_ret
def conv_to_str(self, value, wrapped=True):
def conv_obj(obj_in):
if obj_in is None:
return []
elif isinstance(obj_in, int):
return [str(obj_in)]
elif isinstance(obj_in, str):
if self.sep in obj_in:
return
conv_obj(obj_in.strip().strip(self.sep).split(self.sep))
if obj_in:
return [obj_in.strip()]
else:
return []
elif issubclass(obj_in.__class__, models.Model):
return [str(obj_in.pk)]
else:
try:
tmp_ret = []
for item in obj_in:
tmp_ret.extend(conv_obj(item))
return tmp_ret
except TypeError:
raise ValidationError('Invalid object type: %r' %
obj_in)
if issubclass(value.__class__, models.QuerySet):
value = list(value.values_list('pk', flat=True))
value = self.sep.join(conv_obj(value))
if wrapped:
return self.sep + value + self.sep
else:
return value
def get_db_prep_value(self, value, connection, prepared=False):
print(f'IN ({self.attname}) get_db_prep_value({repr(value)})')
if not prepared:
value = self.get_prep_value(value)
value = super(PkListField, self).get_db_prep_value(value,
connection, prepared=True)
print(f'OUT ({self.attname}) get_db_prep_value({repr(value)})')
return value
def get_prep_value(self, value):
print(f'IN ({self.attname}) get_prep_value({repr(value)})')
value = super().get_prep_value(value)
value = self.conv_to_str(value)
print(f'OUT ({self.attname}) get_prep_value({repr(value)})')
return value
def pre_save(self, model_instance, add):
if self.pre_save_func is not None:
value = super(PkListField, self).pre_save(model_instance, add)
value = self.pre_save_func(value=value, field=self.attname,
model=model_instance, add=add)
setattr(model_instance, self.attname, value)
print(f'({self.attname}) OUT pre_Save({repr(value)})')
return value
else:
value = super().pre_save(model_instance, add)
print(f'OUT ({self.attname}) pre_save({repr(value)})')
return value
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/30564>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
Django
-------------------------------------+-------------------------------------
Type: Bug | Status: new
Component: Database layer | Version: 2.2
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by Dan J Strohl):
Perhaps instead of using "hasattr(value, 'resolve_expression')", a
"issubclass(value, <sql class>):" could be used?
--
Ticket URL: <https://code.djangoproject.com/ticket/30564#comment:1>
Django
-------------------------------------+-------------------------------------
Reporter: Dan J Strohl | Owner: nobody
Component: Database layer | Version: master
(models, ORM) |
Severity: Normal | Resolution: invalid
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
* status: new => closed
* version: 2.2 => master
* resolution: => invalid
Comment:
Thanks for the report, however it is not an issue in Django. It seems that
you want to get help with your custom implementation.
If you're on PostgreSQL you can use
[https://docs.djangoproject.com/en/2.2/ref/contrib/postgres/fields/#arrayfield
ArrayField] without implementing a custom field. If you're on a different
database and you need to implement a custom field for that, then please
use one of
[https://code.djangoproject.com/wiki/TicketClosingReasons/UseSupportChannels
support channels].
`if hasattr(value, 'resolve_expression')` is the way that we prefer to
check if something is an expression, it is used in many places.
Closing per TicketClosingReasons/UseSupportChannels.
--
Ticket URL: <https://code.djangoproject.com/ticket/30564#comment:2>
Django
-------------------------------------+-------------------------------------
Reporter: Dan J Strohl | Owner: nobody
Component: Database layer | Version: master
(models, ORM) |
Severity: Normal | Resolution:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
* status: closed => new
* resolution: invalid =>
Comment:
Re-opening this, I think you mis-understood my point. I understand that
there are other approaches I can take, and I am looking at them (and am
happy for the suggestions). however, the issue as I reported it actually
IS with Django. The issue is that by using ```if hasattr(value,
'resolve_expression')```, you cannot deterministically tell the difference
between a SQL expression and a Django QuerySet. perhaps there is a way
around this that I haven't seen, in which case, I apologize for missing
it. however by following the docs, when I return a QuerySet object, and
when a pre_save method is used, the system raises an error since the
QuerySet object matches your check, but is not an expression object. (or,
if you ARE considering it an expression object for some reason, then the
bug is that it does not have all of the OTHER required methods that the
expected one does.).
I understand that using ```if hasattr``` is the preferred approach, and I
am not saying that the only fix is to change that, you could so a second
check after it, change the method name in the queryset, have an
alternative path that doesn't check for the method... etc... (all of which
seem like much more work than changing the "if" check), but there may be
other issues that I don't see (I haven't looked deeply enough at the code
to see what other issues that would cause). I don't know the flow or code
well enough to say either way. I simply brought it up as a potential
approach.
Either way though, the bug I am reporting is that when returning a django
queryset, via a custom field with a pre_save, it raises an AttributeError.
Since these are all normal Django objects, and I'm using documented django
approaches, I see this as a bug. If the returned object was something I
controlled, I would simply re-name the "resolve_expression" method so it
didn't' get caught, but I can't (easily) do that for a django QuerySet.
I won't argue this again if you want to close this again, I'm not trying
to get into a war here or anything. I think it's pretty much a corner
case, but I do see it as a bug and wanted to make sure I was at least
being clear.
--
Ticket URL: <https://code.djangoproject.com/ticket/30564#comment:3>
Django
-------------------------------------+-------------------------------------
Reporter: Dan J Strohl | Owner: nobody
Component: Database layer | Version: master
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage:
| Unreviewed
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by Simon Charette):
> The issue is that by using if hasattr(value, 'resolve_expression'), you
cannot deterministically tell the difference between a SQL expression and
a Django QuerySet.
FWIW even if the `QuerySet` class doesn't completely implement the
expression API it partially behaves like one; that's what makes usage of
`QuerySet` as subqueries work (e.g.
`.filter(foo__in=Foo.objects.filter(bar=1))`. In that sense `QuerySet` is
an SQL expression.
--
Ticket URL: <https://code.djangoproject.com/ticket/30564#comment:4>
Django
-------------------------------------+-------------------------------------
Reporter: Dan J Strohl | Owner: nobody
Component: Database layer | Version: master
(models, ORM) |
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
* status: new => closed
* resolution: => invalid
Comment:
> Either way though, the bug I am reporting is that when returning a
django queryset, via a custom field with a pre_save, it raises an
AttributeError. Since these are all normal Django objects, and I'm using
documented django approaches, I see this as a bug.
Your custom field implementation is quite complicated. Custom fields with
`pre_save()` works properly because you can treat any Django's field as a
custom field e.g. `DateField`. It seems (but it is hard to tell without
`pre_save_func()`) that in your case `pre_save()` returns `Query` which is
not expected by `SQLInsertCompiler.prepare_value()`, that's why IMO it is
an issue in your implementation/approach.
> Since these are all normal Django objects.
You cannot assume that Django internals will handle everything, even if
these objects are defined by Django.
I think that `return self.get_db_prep_value(value)` in `pre_save()` in
branch with `pre_save_func()` should fix your issue.
--
Ticket URL: <https://code.djangoproject.com/ticket/30564#comment:5>