django - How to cross check ModelAdmin and its inlines?

Question

I have two models (ModelParent and ModelChild) with same m2m fields on Subject model. ModelChild has a foreign key on ModelParent and ModelChild is defined as inline for ModelParent on admin page.

### models.py ###
  class Subject(Models.Model):
    pass

  class ModelParent(models.Model):
    subjects_parent = ManyToManyField(Subject)

  class ModelChild(models.Model):
    parent = ForeignKey(ModelParent)
    subjects_child = ManyToManyField(Subject)

### admin.py ###
  class ModelChildInline(admin.TabularInline):
      model = ModelChild

  class ModelParentAdmin(admin.ModelAdmin):
    inlines = [ModelChildInline]

  admin.site.register(ModelParent, ModelParentAdmin)

I have one important restriction though, ModelChild's subjects_child field must not reference any subject that subject_parent does with its subjects_parent.

So, if I select the same Subject (in subject_parent and subject_child) on Admin page for both models, how can I validate this? If only one field changes you validate it against the db, but what if both change (subject_parent and subject_child)? How can I validate both forms together before saving?

Answer 1

I have inherited a new class named ModelAdminWithInline from admin.ModelAdmin and modified methods add_view(...) and change_view(...) to call function is_cross_valid(self, form, formsets), where you can validate all the forms together. Both functions had:

#...
if all_valid(formsets) and form_validated:
#...

changed to:

#...
formsets_validated = all_valid(formsets)
cross_validated = self.is_cross_valid(form, formsets)
if formsets_validated and form_validated and cross_validated:
#...

The new function is_cross_valid(...) is defined like this:

def is_cross_valid(self, form, formsets):
  return True

so the new class should work exactly the same as ModelAdmin if you don't change is_cross_valid(...) function.

Now my admin.py looks like this:

###admin.py###
class ModelAdminWithInline(admin.ModelAdmin):
  def is_cross_valid(self, form, formsets):
    return True

  def add_view(self, request, form_url='', extra_context=None):
    #modified code

  def change_view(self, request, object_id, extra_context=None):
    #modified code

class ModelChildInline(admin.TabularInline):
  model = ModelChild

class ModelParentAdmin(ModelAdminWithInline):
  inlines = [ModelChildInline]

  def is_cross_valid(self, form, formsets):
    #Do some cross validation on forms
    #For example, here is my particular validation:
    valid = True

    if hasattr(form, 'cleaned_data'):   

      subjects_parent = form.cleaned_data.get("subjects_parent")

      #You can access forms from formsets like this:
      for formset in formsets:
        for formset_form in formset.forms:
          if hasattr(formset_form, 'cleaned_data'):

            subjects_child = formset_form.cleaned_data.get("subjects_child")
            delete_form = formset_form.cleaned_data.get("DELETE")

            if subjects_child and (delete_form == False):
              for subject in subjects_child:
                if subject in subjects_parent:
                  valid = False
                  #From here you can still report errors like in regular forms:
                  if "subjects_child" in formset_form.cleaned_data.keys():
                    formset_form._errors["subjects_child"] = ErrorList([u"Subject %s is already selected in parent ModelParent" % subject])
                    del formset_form.cleaned_data["subjects_child"]
                  else:
                    formset_form._errors["subjects_child"] += ErrorList(u"Subject %s is already selected in parent ModelParent" % subject])

      #return True on success or False otherwise.
      return valid

admin.site.register(ModelParent, ModelParentAdmin)

The solution is a little bit hackish but it works :). The errors show up the same as with regular ModelForm and ModelAdmin classes. Django 1.2 (which should be released shortly) should have model validation, so I hope that then this problem could be solved more nicely.