Filtering the drop down list of ReferenceModels created by appengine using django

Question

I have some classes in my app engine project

class First(db.Model):
  count = db.IntegerProperty()

class Second(db.Model):
  first = db.ReferenceProperty(First)

class SecondForm(djangoforms.ModelForm)
  class Meta:
    model = Second

The SecondForm model generates a really nice drop down menu in the template when rendered but it currently displays all the Models of type first. I was wandering if anyone had an elegant strategy allow conditions to be placed on the objects returned (such as to first.count > 10) to reduce the number of objects that will be rendered in the drop down list.

Thanks,

Richard

Answer 1

Add the following init method to the SecondForm class:

def __init__(self, *args, **kwargs):
    super(SecondForm, self).__init__(*args, **kwargs)
    self.fields['first'].query = db.Query(First).fetch(10)

Add filters etc to the query to control the dropdown list contents!!

Answer 2

I don't have experience using App Engine, but this recipe might help you out:

http://appengine-cookbook.appspot.com/recipe/django-modelchoicefield-filter-input-select-by-foreign-key/

They are passing in a filter value, but I'm sure you can get what you need from reading through that post.

Answer 3

For those interested in the complete solution of reducing the scope of the dropdown menu to only ancestor objects I've pasted it below. If you then set up a Model with (parent=...) and use a form similar to below, only the ancestors appear in the drop down. Enjoy.

class WombatForm(djangoforms.ModelForm):
  def __init__(self, *args, **kwargs):
    super(WombatForm, self).__init__(*args, **kwargs)
    for field_name in self.fields:
      field_model = self.fields[field_name] 
      if isinstance(field_model,djangoforms.ModelChoiceField):
        root = WombatForm.get_root_node(self.instance)
        self.fields[field_name].query.ancestor(root)

@staticmethod
def  get_root_node(entity):
  '''
    returns the final parent ancestor of the given entity
  '''
  parent_ent = entity.parent()
  if parent_ent == None:
    return entity
  return WombatForm.get_root_node(parent_ent)

class SecondForm(WombatForm)
  class Meta:
    model = Second