[Fixed]-Model limit_choices_to={'user': user}

This limiting of choices to current user is a kind of validation that needs to happen dynamically in the request cycle, not in the static Model definition.

In other words: at the point where you are creating an instance of this model you will be in a View and at that point you will have access to the current user and can limit the choices.

Then you just need a custom ModelForm to pass in the request.user to, see the example here:
http://collingrady.wordpress.com/2008/07/24/useful-form-tricks-in-django/

from datetime import datetime, timedelta
from django import forms
from mysite.models import Project, TimeWorked

class TimeWorkedForm(forms.ModelForm):
    def __init__(self, user, *args, **kwargs):
        super(ProjectForm, self).__init__(*args, **kwargs)
        self.fields['project'].queryset = Project.objects.filter(user=user)

    class Meta:
        model = TimeWorked

then in your view:

def time_worked(request):
    form = TimeWorkedForm(request.user, request.POST or None)
    if form.is_valid():
        obj = form.save()
        # redirect somewhere
    return render_to_response('time_worked.html', {'form': form})

Model itself doesn’t know anything about current user but you can give this user in a view to the form which operates models objects (and in form reset choices for necessary field).

If you need this on admin site – you can try raw_id_admin along with django-granular-permissions (http://code.google.com/p/django-granular-permissions/ but I couldn’t rapidly get it working on my django but it seems to be fresh enough for 1.0 so…).

At last, if you heavily need a selectbox in admin – then you’ll need to hack django.contrib.admin itself.

Using class-based generic Views in Django 1.8.x / Python 2.7.x, here is what my colleagues and I came up with:

In models.py:

# ...

class Proposal(models.Model):
    # ...

    # Soft foreign key reference to customer
    customer_id = models.PositiveIntegerField()

    # ...

In forms.py:

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.forms import ModelForm, ChoiceField, Select
from django import forms
from django.forms.utils import ErrorList
from django.core.exceptions import ValidationError
from django.utils.translation import ugettext as _
from .models import Proposal
from account.models import User
from customers.models import customer



def get_customers_by_user(curUser=None):
    customerSet = None

    # Users with userType '1' or '2' are superusers; they should be able to see
    # all the customers regardless. Users with userType '3' or '4' are limited
    # users; they should only be able to see the customers associated with them
    # in the customized user admin.
    # 
    # (I know, that's probably a terrible system, but it's one that I
    # inherited, and am keeping for now.)
    if curUser and (curUser.userType in ['1', '2']):
        customerSet = customer.objects.all().order_by('company_name')
    elif curUser:
        customerSet = curUser.customers.all().order_by('company_name')
    else:
        customerSet = customer.objects.all().order_by('company_name')

    return customerSet


def get_customer_choices(customerSet):
    retVal = []

    for customer in customerSet:
        retVal.append((customer.customer_number, '%d: %s' % (customer.customer_number, customer.company_name)))

    return tuple(retVal)


class CustomerFilterTestForm(ModelForm):

    class Meta:
        model = Proposal
        fields = ['customer_id']

    def __init__(self, user=None, *args, **kwargs):
        super(CustomerFilterTestForm, self).__init__(*args, **kwargs)
        self.fields['customer_id'].widget = Select(choices=get_customer_choices(get_customers_by_user(user)))

# ...

In views.py:

# ...

class CustomerFilterTestView(generic.UpdateView):
    model = Proposal
    form_class = CustomerFilterTestForm
    template_name = 'proposals/customer_filter_test.html'
    context_object_name = 'my_context'
    success_url = "/proposals/"

    def get_form_kwargs(self):
        kwargs = super(CustomerFilterTestView, self).get_form_kwargs()
        kwargs.update({
            'user': self.request.user,
        })
        return kwargs

In templates/proposals/customer_filter_test.html:

{% extends "base/base.html" %}

{% block title_block %}
<title>Customer Filter Test</title>
{% endblock title_block %}

{% block header_add %}
<style>
    label {
        min-width: 300px;
    }
</style>
{% endblock header_add %}

{% block content_body %}
<form action="" method="POST">
    {% csrf_token %}
    <table>
        {{ form.as_table }}
    </table>
    <input type="submit" value="Save" class="btn btn-default" />
</form>
{% endblock content_body %}

I’m not sure that I fully understand exactly what you want to do, but I think that there’s a good chance that you’ll get at least part the way there using a custom Manager. In particular, don’t try to define your models with restrictions to the current user, but create a manager that only returns objects that match the current user.

Hmmm, I don’t fully understand your question. But if you can’t do it when you declare the model maybe you can achieve the same thing with overriding methods of the class of objects where you “send” the user object, maybe start with the constructor.