Effectively adds a "ForeignKey" to a 3rd-party application's model (Post in this example) without monkey-patching the code. This is accomplished by adding a ManyToMany field instead on the other side of the desired relationship on the Item model (in this example), then, removing it from Item's admin and adding a ModelChoice field and Select widg…

admin.py

# -*- coding: utf-8 -*-
# Django 1.6
from django.contrib import admin
from django.forms import ModelChoiceField
from django.forms.models import ModelForm
 
from blog.models import Post
 
from .models import Item
 
class MyPostAdminForm(ModelForm):
    '''
    We're forcing Admin to display the opposite side of the M2M:
    items.Item.posts as an editable [single] ModelChoice field. Remember,
    Post.items is already present as the related_name.
    '''
 
    items = ModelChoiceField(
        queryset=Item.objects.all(),
        # The default widget for a ModelChoiceField is Select. Let it ride.
        required=False,
    )
 
    # Let's re-label this. Leaving the plural form would be rather confusing.
    items.label = 'item'
 
    def __init__(self, *args, **kwargs):
        '''
        If this post exists yet, set the initial item. Remember, we're
        using an M2M to fake a ModelChoice field, so we only need to choose a
        single item, if any. We'll just grab the first one we find.
        '''
 
        super(MyPostAdminForm, self).__init__(*args, **kwargs)
 
        if self.instance.pk and self.instance.items.count():
            self.fields['items'].initial = self.instance.items.all()[0]
 
    def save(self, *args, **kwargs):
        '''
        Save the selected item on this post in the Item model's M2M.
        '''
 
        # Saves the form and create the post object.
        post = super(MyPostAdminForm, self).save(*args, **kwargs)
 
        #
        # If this is a new post, save it so that Django can setup the virtual
        # relationship: items.
        #
        if not post.pk:
            post.save()
 
        # Get the (single) Item the user selected from the cleaned form.
        item = self.cleaned_data['items']
 
        #
        # Clear out any old entries (that aren't the desired one), otherwise,
        # they'll just accumulate, since this is really a M2M field rather
        # than a ModelChoiceField.
        #
        for old in post.items.all():
            if old.id != item:
                post.items.remove(old)
 
        #
        # Add this new one, if any, no harm if it already existed, we
        # won't end up with duplicates.
        #
        if item:
            post.items.add(item)
 
        return post
 
class MyPostAdmin(PostAdmin):
    form = MyPostAdminForm
 
    def get_fieldsets(self, request, obj=None):
        '''
        Alter the existing fieldsets to include the field: items
        '''
 
        fieldsets = super(MyPostAdmin, self).get_fieldsets(request, obj)
        fieldsets[1][1]['fields'] = ['items', ... ]
 
        return fieldsets

models.py

# -*- coding: utf-8 -*-
# Django 1.6
from django.db import models

class Item(models.Model):
    label = models.CharField('label', blank=False, default='')
    ...
    posts = models.ManyToMany('blog_app.Post',
        blank=True,
        null=True,
        related_name='items',  # This is the default, but let's be explicit here.
    )