Limit foreign key choices in select in an inline f

Question

The logic is of the model is:

• A Building has many Rooms
• A Room may be inside another Room (a closet, for instance--ForeignKey on 'self')
• A Room can only be inside another Room in the same building (this is the tricky part)

Here's the code I have:

#spaces/models.py
from django.db import models    

class Building(models.Model):
    name=models.CharField(max_length=32)
    def __unicode__(self):
        return self.name

class Room(models.Model):
    number=models.CharField(max_length=8)
    building=models.ForeignKey(Building)
    inside_room=models.ForeignKey('self',blank=True,null=True)
    def __unicode__(self):
        return self.number

and:

#spaces/admin.py
from ex.spaces.models import Building, Room
from django.contrib import admin

class RoomAdmin(admin.ModelAdmin):
    pass

class RoomInline(admin.TabularInline):
    model = Room
    extra = 2

class BuildingAdmin(admin.ModelAdmin):
    inlines=[RoomInline]

admin.site.register(Building, BuildingAdmin)
admin.site.register(Room)

The inline will display only rooms in the current building (which is what I want). The problem, though, is that for the inside_room drop down, it displays all of the rooms in the Rooms table (including those in other buildings).

In the inline of rooms, I need to limit the inside_room choices to only rooms which are in the current building (the building record currently being altered by the main BuildingAdmin form).

I can't figure out a way to do it with either a limit_choices_to in the model, nor can I figure out how exactly to override the admin's inline formset properly (I feel like I should be somehow create a custom inline form, pass the building_id of the main form to the custom inline, then limit the queryset for the field's choices based on that--but I just can't wrap my head around how to do it).

Maybe this is too complex for the admin site, but it seems like something that would be generally useful...

Answer 1

I found a fairly elegant solution that works well for inline forms.

Applied to my model, where I'm filtering the inside_room field to only return rooms that are in the same building:

#spaces/admin.py
class RoomInlineForm(ModelForm):
  def __init__(self, *args, **kwargs):
    super(RoomInlineForm, self).__init__(*args, **kwargs)  #On init...
  if 'instance' in kwargs:
    building = kwargs['instance'].building
  else:
    building_id = tuple(i[0] for i in self.fields['building'].widget.choices)[1]
    building = Building.objects.get(id=building_id)
  self.fields['inside_room'].queryset = Room.objects.filter(building__exact=building)

Basically, if an 'instance' keyword is passed to the form, it's an existing record showing in the inline, and so I can just grab the building from the instance. If not an instance, it's one of the blank "extra" rows in the inline, and so it goes through the hidden form fields of the inline that store the implicit relation back to the main page, and grabs the id value from that. Then, it grabs the building object based on that building_id. Finally, now having the building, we can set the queryset of the drop downs to only display the relevant items.

More elegant than my original solution, which crashed and burned as inline (but worked--well, if you don't mind saving the form partway to make the drop downs fill in-- for the individual forms):

class RoomForm(forms.ModelForm): # For the individual rooms
  class Meta:
mode = Room
  def __init__(self, *args, **kwargs):  # Limits inside_room choices to same building only
    super(RoomForm, self).__init__(*args, **kwargs)  #On init...
try:
  self.fields['inside_room'].queryset = Room.objects.filter( 
    building__exact=self.instance.building)   # rooms with the same building as this room
    except:                  #and hide this field (why can't I exclude?)
    self.fields['inside_room']=forms.CharField( #Add room throws DoesNotExist error
        widget=forms.HiddenInput,   
        required=False,
        label='Inside Room (save room first)')

For non-inlines, it worked if the room already existed. If not, it would throw an error (DoesNotExist), so I'd catch it and then hide the field (since there was no way, from the Admin, to limit it to the right building, since the whole room record was new, and no building was yet set!)...once you hit save, it saves the building and on reload it could limit the choices...

I just need to find a way to cascade the foreign key filters from one field to another in a new record--i.e., new record, select a building, and it automatically limits the choices in the inside_room select box--before the record gets saved. But that's for another day...

Answer 2

After reading through this post and experimenting a lot I think I have found a rather definitive answer to this question. As this is a design pattern that is ofter used I have written a Mixin for the Django admin to make use of it.

(Dynamically) limiting the queryset for ForeignKey fields is now as simple as subclassing LimitedAdminMixin and defining a get_filters(obj) method to return the relevant filters. Alternateively, a filters property can be set on the admin if dynamic filtering is not required.

Example usage:

class MyInline(LimitedAdminInlineMixin, admin.TabularInline):
    def get_filters(self, obj):
        return (('<field_name>', dict(<filters>)),)

Here, <field_name> is the name of the FK field to be filtered and <filters> is a list of parameters as you would normally specify them in the filter() method of querysets.

Answer 3

This question and answer is very similar, and works for a regular admin form

Inside of an inline--and that's where it falls apart... I just can't get at the main form's data to get the foreign key value I need in my limit (or to one of the inline's records to grab the value).

Here's my admin.py. I guess I'm looking for the magic to replace the ???? with--if I plug in a hardcoded value (say, 1), it works fine and properly limits the the available choices in the inline...

#spaces/admin.py
from demo.spaces.models import Building, Room
from django.contrib import admin
from django.forms import ModelForm


class RoomInlineForm(ModelForm):
  def __init__(self, *args, **kwargs):
    super(RoomInlineForm, self).__init__(*args, **kwargs)
    self.fields['inside_room'].queryset = Room.objects.filter(
                               building__exact=????)                       # <------

class RoomInline(admin.TabularInline):
  form = RoomInlineForm
  model=Room

class BuildingAdmin(admin.ModelAdmin):
  inlines=[RoomInline]

admin.site.register(Building, BuildingAdmin)
admin.site.register(Room)

Answer 4

You can create a couple of custom classes that will then pass along a reference to the parent instance to the form.

from django.forms.models import BaseInlineFormSet
from django.forms import ModelForm

class ParentInstInlineFormSet(BaseInlineFormSet):
    def _construct_forms(self):
        # instantiate all the forms and put them in self.forms
        self.forms = []
        for i in xrange(self.total_form_count()):
            self.forms.append(self._construct_form(i, parent_instance=self.instance))

    def _get_empty_form(self, **kwargs):
        return super(ParentInstInlineFormSet, self)._get_empty_form(parent_instance=self.instance)
    empty_form = property(_get_empty_form)


class ParentInlineModelForm(ModelForm):
    def __init__(self, *args, **kwargs):
        self.parent_instance = kwargs.pop('parent_instance', None)
        super(ParentInlineModelForm, self).__init__(*args, **kwargs)

in class RoomInline just add:

class RoomInline(admin.TabularInline):
      formset = ParentInstInlineFormset
      form = RoomInlineForm #(or something)

In your form you now have access in the init method to self.parent_instance! parent_instance can now be used to filter choices and whatnot

something like:

class RoomInlineForm(ParentInlineModelForm):
    def __init__(self, *args, **kwargs):
        super(RoomInlineForm, self).__init__(*args, **kwargs)
        building = self.parent_instance
        #Filtering and stuff