Remove "add another" in Django admin screen

N.B. Works for DJango 1.5.2 and possibly older. The can_add_related property appeared around 2 years ago.

The best way I've found is to override your ModelAdmin's get_form function. In my case I wanted to force the author of a post to be the currently logged in user. Code below with copious comments. The really important bit is the setting of widget.can_add_related:

def get_form(self,request, obj=None, **kwargs):
    # get base form object    
    form = super(BlogPostAdmin,self).get_form(request, obj, **kwargs)

    # get the foreign key field I want to restrict
    author = form.base_fields["author"]

    # remove the green + by setting can_add_related to False on the widget
    author.widget.can_add_related = False

    # restrict queryset for field to just the current user
    author.queryset = User.objects.filter(pk=request.user.pk)

    # set the initial value of the field to current user. Redundant as there will
    # only be one option anyway.
    author.initial = request.user.pk

    # set the field's empty_label to None to remove the "------" null 
    # field from the select. 
    author.empty_label = None

    # return our now modified form.
    return form

The interesting part of making the changes here in get_form is that author.widget is an instance of django.contrib.admin.widgets.RelatedFieldWidgetWrapper where as if you try and make changes in one of the formfield_for_xxxxx functions, the widget is an instance of the actual form widget, in this typical ForeignKey case it's a django.forms.widgets.Select.

Though most of the solutions mentioned here work, there is another cleaner way of doing it. Probably it was introduced in a later version of Django, after the other solutions were presented. (I'm presently using Django 1.7)

To remove the "Add another" option,

class ... #(Your inline class)

    def has_add_permission(self, request):
        return False

Similarly if you want to disable "Delete?" option, add the following method in Inline class.

    def has_delete_permission(self, request, obj=None):
        return False

The following answer was my original answer but it is wrong and does not answer OP's question:

Simpler solution, no CSS hack and no editing Django codebase:

Add this to your Inline class:

max_num=0

_{(this is only applicable to inline forms, not foreign key fields as OP asked)}

---

The above answer is only useful to hide the "add related" button for inline forms, and not foreign keys as requested.

When I wrote the answer, IIRC the accepted answer hid both, which is why I got confused.

The following seems to provide a solution (though hiding using CSS seems the most feasible thing to do, especially if the "add another" buttons of FKs are in inline forms):

Django 1.7 removing Add button from inline form