Re: [Django] #10349: ManyToMany field does not take dynamic initial values into account

[Django]

#10349: ManyToMany field does not take dynamic initial values into account
---+
  Reporter:  fas   | Owner:  fas
 
Status:  new   | Milestone:  1.1
 
 Component:  Database layer (models, ORM)  |   Version:  1.0
 
Resolution:|  Keywords:  
ManyToMany, initial, dynamic, lambda
 Stage:  Accepted  | Has_patch:  1  
 
Needs_docs:  0 |   Needs_tests:  1  
 
Needs_better_patch:  0 |  
---+
Comment (by fas):

 Ok, here is an example. We have a many-to-many relation between articles
 and publications.

 {{{
 from django.db import models

 class Publication(models.Model):
 title = models.CharField(max_length=30)
 date = models.DateField()

 class Article(models.Model):
 headline = models.CharField(max_length=100)
 publications = models.ManyToManyField(Publication)
 }}}

 Then we create a model form that has a callback for the form field. Such a
 callback is used in the admin.
 In the following example, the initial value of 'publications' of the
 article form should be the ten latest publications.

 {{{
 from django.forms.models import modelform_factory

 def formfield_for_dbfield(db_field, **kwargs):
 if db_field.name == 'publications':
 kwargs['initial'] = lambda:
 Publication.objects.all().order_by('-date')[:10]
 return db_field.formfield(**kwargs)

 modelform_factory(Article, formfield_callback=formfield_for_dbfield)
 }}}

 A lambda expression is used so that the expression will not be evaluated
 once but every time, preventing the initial selection from getting
 outdated (same reason you would use date.today instead of date.today() as
 an initial value for a date field).

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Re: [Django] #10349: ManyToMany field does not take dynamic initial values into account

[Django]

#10349: ManyToMany field does not take dynamic initial values into account
---+
  Reporter:  fas   | Owner:  fas
 
Status:  new   | Milestone:  1.1
 
 Component:  Database layer (models, ORM)  |   Version:  1.0
 
Resolution:|  Keywords:  
ManyToMany, initial, dynamic, lambda
 Stage:  Accepted  | Has_patch:  1  
 
Needs_docs:  0 |   Needs_tests:  1  
 
Needs_better_patch:  0 |  
---+
Comment (by russellm):

 My suggestion is that you provide a test case, like I asked you to.

 I completely understand what the code is trying to do. I can see the
 analog with what happens with normal form fields. On the face of it, the
 patch looks fine. However, I can't come up with a single code example that
 would actually exercise the patch you have provided.

 If you provide a callable to the initial argument when instantiating a
 form, the callable is evaluated. If you define a form field with a
 callable initial value, it is evaluated.

 Don't just explain your patch with words - you need to show me an example
 of code where you can provide a callable for an m2m field that doesn't
 work with the current Django trunk, but does work when you apply your
 patch. Ideally, that example would be integrated into the existing Django
 test suite, but at this point, I'd take any example, standalone or
 otherwise.

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Re: [Django] #10349: ManyToMany field does not take dynamic initial values into account

[Django]

#10349: ManyToMany field does not take dynamic initial values into account
---+
  Reporter:  fas   | Owner:  fas
 
Status:  new   | Milestone:  1.1
 
 Component:  Database layer (models, ORM)  |   Version:  1.0
 
Resolution:|  Keywords:  
ManyToMany, initial, dynamic, lambda
 Stage:  Accepted  | Has_patch:  1  
 
Needs_docs:  0 |   Needs_tests:  1  
 
Needs_better_patch:  0 |  
---+
Changes (by fas):

 * cc: mben...@gmail.com (added)

Comment:

 When using Field.formfield you can pass a lambda function as the initial
 value which will be passed over to the form field. When rendering the form
 field, the initial value will be evaluated in case it is a function. This
 is done in forms/forms.py, method as_widget.

 The ManyToManyField however overwrites the formfield method in order to
 set its initial value to be a list of IDs. It assumes that the given
 initial value is a list of related objects. It also assumes that the form
 class to be used is forms.ModelMultipleChoiceField or that the form field
 to be used takes a list of IDs. Both assumptions are incorrect.
 {{{
 def formfield(self, **kwargs):
 defaults = {'form_class': forms.ModelMultipleChoiceField,
 'queryset':
 self.rel.to._default_manager.complex_filter(self.rel.limit_choices_to)}
 defaults.update(kwargs)
 # If initial is passed in, it's a list of related objects, but the
 # MultipleChoiceField takes a list of IDs.
 if defaults.get('initial') is not None:
 defaults['initial'] = [i._get_pk_val() for i in
 defaults['initial']]
 return super(ManyToManyField, self).formfield(**defaults)
 }}}

 This makes it impossible to pass a function for the initial value and also
 makes it impossible to create a custom form field that needs the related
 object and not only its IDs. I revoke my initial patch and propose that we
 should find a solution that works without the two assumptions made above.
 I think ModelChoiceField (forms/models.py) might the the right place.
 Applying the same transformation (object->ID) there before passing the
 initial to the super (ChoiceField) should solve the problem.

 Any suggestions?

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Re: [Django] #10349: ManyToMany field does not take dynamic initial values into account

[Django]

#10349: ManyToMany field does not take dynamic initial values into account
---+
  Reporter:  fas   | Owner:  fas
 
Status:  new   | Milestone:  1.1
 
 Component:  Database layer (models, ORM)  |   Version:  1.0
 
Resolution:|  Keywords:  
ManyToMany, initial, dynamic, lambda
 Stage:  Accepted  | Has_patch:  1  
 
Needs_docs:  0 |   Needs_tests:  1  
 
Needs_better_patch:  0 |  
---+
Changes (by russellm):

  * needs_tests:  0 => 1

Comment:

 It would also be helpful if you provided a testcase as part of your patch
 - or, at the very least, an example of the usage that is corrected by
 applying the patch. It isn't immediately obvious to me what problem this
 patch is trying to solve.

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Re: [Django] #10349: ManyToMany field does not take dynamic initial values into account

[Django]

#10349: ManyToMany field does not take dynamic initial values into account
---+
  Reporter:  fas   | Owner:  fas
 
Status:  new   | Milestone:  1.1
 
 Component:  Database layer (models, ORM)  |   Version:  1.0
 
Resolution:|  Keywords:  
ManyToMany, initial, dynamic, lambda
 Stage:  Accepted  | Has_patch:  1  
 
Needs_docs:  0 |   Needs_tests:  0  
 
Needs_better_patch:  0 |  
---+
Changes (by jacob):

  * needs_better_patch:  => 0
  * needs_docs:  => 0
  * stage:  Unreviewed => Accepted
  * needs_tests:  => 0
  * milestone:  => 1.1

Comment:

 In the future, please provide patches against trunk if possible. We
 backport patches from trunk to the release branches.

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[Django] #10349: ManyToMany field does not take dynamic initial values into account

[Django]

#10349: ManyToMany field does not take dynamic initial values into account
--+-
 Reporter:  fas   |   Owner:  fas   
   Status:  new   |   Milestone:  post-1.0  
Component:  Database layer (models, ORM)  | Version:  1.0   
 Keywords:  ManyToMany, initial, dynamic, lambda  |   Stage:  Unreviewed
Has_patch:  1 |  
--+-
 For dbfields it is possible to provide the initial value in form of a
 anonymous function, e.g. models.CharField(..., initial=lambda: "I am
 dynamic!").
 The formfield method of the ManyToMany field does not take this into
 account:
 {{{
 defaults['initial'] = [i._get_pk_val() for i in defaults['initial']]
 }}}

 The fix is simple and provided in a patch, diffed against release 1.0.2.

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