Made fix for #9321 less buggy and more effective.

Don't try to be smart about building a good-looking help string
because it evaluates translations too early, simply use the same old
strategy as before. Thanks Donald Stufft for the report.

Also, actually fix the case reported by the OP by special-casing
CheckboxSelectMultiple.

Added tests.

Refs #9321.

1 files changed, 4 insertions, 3 deletions

diff --git a/django/forms/models.py b/django/forms/models.py
index 68c1341cf7..9d5aa5d4d8 100644
--- a/django/forms/models.py
+++ b/django/forms/models.py
@@ -13,7 +13,7 @@ from django.forms.forms import BaseForm, get_declared_fields
 from django.forms.formsets import BaseFormSet, formset_factory
 from django.forms.util import ErrorList
 from django.forms.widgets import (SelectMultiple, HiddenInput,
-    MultipleHiddenInput, media_property)
+    MultipleHiddenInput, media_property, CheckboxSelectMultiple)
 from django.utils.encoding import smart_text, force_text
 from django.utils.datastructures import SortedDict
 from django.utils import six
@@ -1104,9 +1104,10 @@ class ModelMultipleChoiceField(ModelChoiceField):
         super(ModelMultipleChoiceField, self).__init__(queryset, None,
             cache_choices, required, widget, label, initial, help_text,
             *args, **kwargs)
-        if isinstance(self.widget, SelectMultiple):
+        # Remove this in Django 1.8
+        if isinstance(self.widget, SelectMultiple) and not isinstance(self.widget, CheckboxSelectMultiple):
             msg = _('Hold down "Control", or "Command" on a Mac, to select more than one.')
-            self.help_text = string_concat(self.help_text, ' ', msg) if self.help_text else msg
+            self.help_text = string_concat(self.help_text, ' ', msg)
 
     def clean(self, value):
         if self.required and not value: