Now max_length is not required, the Multiselect field calculate it. And max_choices could be a param in the model

example/example/app/models.py

@@ -41,9 +41,10 @@ TAGS_CHOICES = (
 
 class Book(models.Model):
     title = models.CharField(max_length=200)
-    categories = MultiSelectField(choices=CATEGORY_CHOICES, max_length=10,
-                                  null=True, blank=True)
-    tags = MultiSelectField(choices=TAGS_CHOICES, max_length=10)
+    categories = MultiSelectField(choices=CATEGORY_CHOICES,
+                                  max_choices=3)
+    tags = MultiSelectField(choices=TAGS_CHOICES,
+                            null=True, blank=True)
 
     def __str__(self):
         return self.title

src/multiselectfield/db/fields.py

@@ -21,7 +21,9 @@ from django.db import models
 from django.utils.text import capfirst
 from django.core import exceptions
 
-from ..forms.fields import MultiSelectFormField
+from ..forms.fields import MultiSelectFormField, MaxChoicesValidator
+from ..utils import get_max_length
+from ..validators import MaxValueMultiFieldValidator
 
 if sys.version_info[0] == 2:
     string = basestring
@@ -48,11 +50,19 @@ def add_metaclass(metaclass):
 class MultiSelectField(models.CharField):
     """ Choice values can not contain commas. """
 
+    def __init__(self, *args, **kwargs):
+        self.max_choices = kwargs.pop('max_choices', None)
+        super(MultiSelectField, self).__init__(*args, **kwargs)
+        self.max_length = get_max_length(self.choices, self.max_length)
+        self.validators[0] = MaxValueMultiFieldValidator(self.max_length)
+        if self.max_choices is not None:
+            self.validators.append(MaxChoicesValidator(self.max_choices))
+
     def get_choices_default(self):
         return self.get_choices(include_blank=False)
 
-    def get_choices_selected(self, arr_choices=''):
-        if not arr_choices:
+    def get_choices_selected(self, arr_choices=None):
+        if arr_choices is None:
             return False
         list = []
         for choice_selected in arr_choices:
@@ -75,8 +85,12 @@ class MultiSelectField(models.CharField):
         return
 
     def formfield(self, **kwargs):
-        defaults = {'required': not self.blank, 'label': capfirst(self.verbose_name),
-                    'help_text': self.help_text, 'choices': self.choices}
+        defaults = {'required': not self.blank,
+                    'label': capfirst(self.verbose_name),
+                    'help_text': self.help_text,
+                    'choices': self.choices,
+                    'max_length': self.max_length,
+                    'max_choices': self.max_choices}
         if self.has_default():
             defaults['initial'] = self.get_default()
         defaults.update(kwargs)

src/multiselectfield/forms/fields.py

@@ -15,22 +15,19 @@
 # along with this programe.  If not, see <http://www.gnu.org/licenses/>.
 
 from django import forms
-from django.contrib.humanize.templatetags.humanize import apnumber
-from django.template.defaultfilters import pluralize
+
+from ..utils import get_max_length
+from ..validators import MaxValueMultiFieldValidator, MaxChoicesValidator
 
 
 class MultiSelectFormField(forms.MultipleChoiceField):
     widget = forms.CheckboxSelectMultiple
 
     def __init__(self, *args, **kwargs):
-        self.max_choices = kwargs.pop('max_choices', 0)
+        self.max_choices = kwargs.pop('max_choices', None)
+        self.max_length = kwargs.pop('max_length', None)
         super(MultiSelectFormField, self).__init__(*args, **kwargs)
-
-    def clean(self, value):
-        if not value and self.required:
-            raise forms.ValidationError(self.error_messages['required'])
-        if value and self.max_choices and len(value) > self.max_choices:
-            raise forms.ValidationError('You must select a maximum of %s choice%s.'
-                                        % (apnumber(self.max_choices),
-                                           pluralize(self.max_choices)))
-        return value
+        self.max_length = get_max_length(self.choices, self.max_length)
+        self.validators.append(MaxValueMultiFieldValidator(self.max_length))
+        if self.max_choices is not None:
+            self.validators.append(MaxChoicesValidator(self.max_choices))

src/multiselectfield/utils.py

@@ -0,0 +1,34 @@
+# -*- coding: utf-8 -*-
+# Copyright (c) 2013 by Pablo Martín <goinnn@gmail.com>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this programe.  If not, see <http://www.gnu.org/licenses/>.
+
+import sys
+
+
+if sys.version_info[0] == 2:
+    string = basestring
+    string_type = unicode
+else:
+    string = str
+    string_type = string
+
+
+def get_max_length(choices, max_length, default=200):
+    if max_length is None:
+        if choices:
+            return len(','.join([string_type(key) for key, label in choices]))
+        else:
+            return default
+    return max_length

src/multiselectfield/validators.py

@@ -0,0 +1,29 @@
+# -*- coding: utf-8 -*-
+# Copyright (c) 2013 by Pablo Martín <goinnn@gmail.com>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this programe.  If not, see <http://www.gnu.org/licenses/>.
+
+
+from django.core import validators
+from django.utils.translation import ugettext_lazy as _
+
+
+class MaxValueMultiFieldValidator(validators.MaxLengthValidator):
+    clean = lambda self, x: len(','.join(x))
+    code = 'max_multifield_value'
+
+
+class MaxChoicesValidator(validators.MaxLengthValidator):
+    message = _(u'You must select a maximum of  %(limit_value)d choices.')
+    code = 'max_choices'