[PYTHON] How to use ManyToManyField with Django's Admin

When I specified the ManyToManyField column in list_display of admin.py, I got the following error.

django.core.management.base.SystemCheckError: SystemCheckError: System check identified some issues:

ERRORS:
<class 'proj.admin.ArticleAdmin'>: (admin.E109) The value of 'list_display[2]' must not be a ManyToManyField.

I thought it would be convenient to be able to list the data of another table linked by ManyToManyField for a certain record on the management site, but it seems that the specification cannot be specified according to the error message.

However, it was possible by doing the following, so I will record the method.

What you want to do

Imagine an example where metadata is added to a document, such as Hatena Bookmark.

イメージ図

How to respond

Suppose you have three authors, an Article, and a Tag.

Article.meta is ManyToManyField, and Tag is specified as the relation destination.

models.py

from django.db import models

class Author(models.Model):
    name = models.CharField(max_length=30)

    def __unicode__(self):
        return self.name

class Article(models.Model):
    title = models.CharField(max_length=100)
    author = models.ForeignKey('Author', blank=True, null=True)
    meta = models.ManyToManyField('Tag')

    def __unicode__(self):
        return self.title

class Tag(models.Model):
    name = models.CharField(max_length=30)

    def __unicode__(self):
        return self.name

admin.py

If you define a function with an arbitrary name such as _meta and specify the corresponding function name in list_display, you can list the data linked by commas as shown in the figure above.

class ArticleAdmin(admin.ModelAdmin):
    list_display = ('title', 'author', '_meta')

    def	_meta(self, row):
        return ','.join([x.name for x in row.meta.all()])

admin.site.register(Article, ArticleAdmin)

Reference material

http://stackoverflow.com/questions/18108521/many-to-many-in-list-display-django