class MyAdmin(admin.ModelAdmin):
    def get_form(self, request, obj=None, **kwargs):
        form = super(MyAdmin,self).get_form(self,request, obj,**kwargs)
        # form class is created per request by modelform_factory function
        # so it's safe to modify
        #we modify the the queryset
        form.base_fields['foreign_key_field].queryset = form.base_fields['foreign_key_field].queryset.filter(user=request.user)
        return form