django model choice option as a multi select box

Question

Assuming that I have such model

COLORS= (
    ('R', 'Red'),
    ('B', 'Yellow'),
    ('G', 'White'),
)
class Car(models.Model):
    name = models.CharField(max_length=20)
    color= models.CharField(max_length=1, choices=COLORS)

It displays as a selectbox in the admin panel however I would like my admin-user to multi select those colors like many-to-many relationship, how can this be achieved without a ('RB', 'Red&Blue'), type of logic

Answer 1

I've constructed a complete working example with meaningful models. It works perfect. I've tested it on Python 3.4.x and Django 1.8.4. At first I run admin panel and create records for each option in Thema model

models.py

from django.db import models

class Author(models.Model):
    fname = models.CharField(max_length=50)
    lname = models.CharField(max_length=80)

    def __str__(self):
        return "{0} {1}".format(self.fname, self.lname)


class Thema(models.Model):
    THEME_CHOICES = (
        ('tech', 'Technical'),
        ('novel', 'Novel'),
        ('physco', 'Phsycological'),
    )
    name = models.CharField(max_length=20,choices=THEME_CHOICES, unique=True)

    def __str__(self):
        return self.name

class Book(models.Model):

    name = models.CharField(max_length=50)
    author = models.ForeignKey(Author)
    themes = models.ManyToManyField(Thema)

    def __str__(self):
        return "{0} by {1}".format(self.name,self.author)

forms.py

from django import forms

from .models import *

class BookForm(forms.ModelForm):
    themes = forms.ModelMultipleChoiceField(queryset=Thema.objects, widget=forms.CheckboxSelectMultiple(), required=False)

admin.py

from django.contrib import admin

from .models import *
from .forms import *

@admin.register(Author)
class AuthorAdmin(admin.ModelAdmin):
    pass


@admin.register(Book)    
class BookAdmin(admin.ModelAdmin):
    form = BookForm


@admin.register(Thema)
class ThemaAdmin(admin.ModelAdmin):
    pass