How to override default display values for a select field for foreign keys in the Django Admin?

You need to create custom Form for this and set form attribute in your ModelAdmin.

In that Form you will need to override form field type of user field on model to custom ModelChoiceField.

ModelChoiceField has a method called label_from_instance, you need to override that to get full name.

Example code

######################################
## models.py ##
######################################

from django.db import models
from django.contrib.auth.models import User

class Item(models.Model):
    user = models.ForeignKey(User)
    name = models.CharField("name", max_length=60)

    def __unicode__(self):
        return self.name

######################################
## forms.py ##
######################################

from django import forms
from django.contrib.auth.models import User

from .models import Item


class CustomUserChoiceField(forms.ModelChoiceField):
    def label_from_instance(self, obj):
         return obj.get_full_name()


class ItemForm(forms.ModelForm):
    user = CustomUserChoiceField(queryset=User.objects.all())

    class Meta:
        model = Item

######################################
## admin.py ##
######################################

from django.contrib import admin

from .models import Item
from .forms import ItemForm


class ItemAdmin(admin.ModelAdmin):
    form = ItemForm


admin.site.register(Item, ItemAdmin)

Source Code Reference

https://github.com/django/django/blob/1.4.5/django/forms/models.py#L948

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9. Use method other than unicode in ModelChoiceField Django

Django uses unicode(obj) (or the related function, str(obj)) in a number of places. Most notably, to display an object in the Django admin site and as the value inserted into a template when it displays an object.

Please see __unicode__ from https://docs.djangoproject.com/en/dev/ref/models/instances/

You can change __unicode__ method of User class. See below example codes.

from django.db import models
from django.contrib.auth.models import User

def myunicode(self):
    return self.get_full_name()

# Create your models here.
class Item(models.Model):
    User.__unicode__ = myunicode
    user = models.ForeignKey(User)
    name = models.CharField("name", max_length=60)

    def __unicode__(self):
        return self.name