Fixed #9321 -- Deprecated hard-coding of help text in model ManyToManyField fields.

This is backward incompatible for custom form field/widgets that rely
on the hard-coded 'Hold down "Control", or "Command" on a Mac, to select
more than one.' sentence.

Application that use standard model form fields and widgets aren't
affected but need to start handling these help texts by themselves
before Django 1.8.

For more details, see the related release notes and deprecation timeline
sections added with this commit.

2 files changed, 5 insertions, 5 deletions

diff --git a/django/db/models/fields/related.py b/django/db/models/fields/related.py
index 256c3e0f61..fd0b24433f 100644
--- a/django/db/models/fields/related.py
+++ b/django/db/models/fields/related.py
@@ -11,7 +11,7 @@ from django.db.models.deletion import CASCADE
 from django.utils.encoding import smart_text
 from django.utils import six
 from django.utils.deprecation import RenameMethodsBase
-from django.utils.translation import ugettext_lazy as _, string_concat
+from django.utils.translation import ugettext_lazy as _
 from django.utils.functional import curry, cached_property
 from django.core import exceptions
 from django import forms
@@ -1348,9 +1348,6 @@ class ManyToManyField(RelatedField):
 
         super(ManyToManyField, self).__init__(**kwargs)
 
-        msg = _('Hold down "Control", or "Command" on a Mac, to select more than one.')
-        self.help_text = string_concat(self.help_text, ' ', msg)
-
     def _get_path_info(self, direct=False):
         """
         Called by both direct an indirect m2m traversal.
diff --git a/django/forms/models.py b/django/forms/models.py
index 93c8b89efe..68c1341cf7 100644
--- a/django/forms/models.py
+++ b/django/forms/models.py
@@ -18,7 +18,7 @@ from django.utils.encoding import smart_text, force_text
 from django.utils.datastructures import SortedDict
 from django.utils import six
 from django.utils.text import get_text_list, capfirst
-from django.utils.translation import ugettext_lazy as _, ugettext
+from django.utils.translation import ugettext_lazy as _, ugettext, string_concat
 
 
 __all__ = (
@@ -1104,6 +1104,9 @@ class ModelMultipleChoiceField(ModelChoiceField):
         super(ModelMultipleChoiceField, self).__init__(queryset, None,
             cache_choices, required, widget, label, initial, help_text,
             *args, **kwargs)
+        if isinstance(self.widget, SelectMultiple):
+            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
 
     def clean(self, value):
         if self.required and not value: