CODEWITHSUNDEEP
pythondjangoforeign-keys
Terrence Brannon
Terrence Brannon

Reputation: 4968

Accessing a full object through a relation in Django

I am attempting to model study resources (books, DVDs, etc) and what prerequisites those resources have.

A prerequisite is a resource also. So my question is, what Django modelling technique best captures this relation? The study resource is straightforward:

from django.db import models
from django.utils import timezone

class Resource(models.Model):
    title = models.CharField(max_length=300)
    shortcode = models.CharField(max_length=20, null=True, blank=True)
    img = models.URLField(null=True, blank=True)
    summary = models.TextField(null=True, blank=True)
    url = models.URLField('Link to Resource', null=True, blank=True)
    pub_date = models.DateTimeField('date published')

    def was_published_recently(self):
        return self.pub_date >= timezone.now() - datetime.timedelta(days=1)

    def __unicode__(self):
        return self.title

However, I am not sure what the "best" way to model the prerequisite, given that it is a Resource also and most importantly that there are a variable number of prequisites (thus eliminating the possibility of simply having a self-referential parent ). Here is my current approach:

class Prereq(models.Model):
    prereq = models.ForeignKey(Resource, null=True, related_name='prereq_backlink')
    resource = models.ForeignKey(Resource, null=True, related_name='prereq_resource')

    def __unicode__(self):
        return self.resource.title

The problem with this approach is that accessing a prerequisite does not give me a full resource object to work with, so I cannot access its various fields (title, shortcode, etc):

schemelab@schemelab2:~/domains/org/metaperl/tmp/idhhb/django/mysite$ ./manage.py shell
>>> from idhhb.models import Resource, Prereq
from idhhb.models import Resource, Prereq
>>> p = Resource.objects.get(id=2)
>>> p.title
u'American Book of the Dead'
>>> r = Resource.objects.get(id=2)
r = Resource.objects.get(id=2)
>>> p1 = r.prereq_backlink
>>> p2 = r.prereq_resource
>>> p1
<django.db.models.fields.related.RelatedManager object at 0x2e279d0>
>>> p2
<django.db.models.fields.related.RelatedManager object at 0x2e27a90>
>>> p1.title
p1.title
Traceback (most recent call last):
  File "<console>", line 1, in <module>
AttributeError: 'RelatedManager' object has no attribute 'title'

>>> p2.title
p2.title
Traceback (most recent call last):
  File "<console>", line 1, in <module>
AttributeError: 'RelatedManager' object has no attribute 'title'
>>>

Upvotes: 0

Views: 89

Answers (2)

Terrence Brannon
Terrence Brannon

Reputation: 4968

Thanks to this blog post I was able to implement this using ManyToManyField as suggested. I must say this blog post is better than any docs I found on the Django site itself.

MODEL:

class Resource(models.Model):
    title = models.CharField(max_length=300)
    shortcode = models.CharField(max_length=20, null=True, blank=True)
    img = models.URLField(null=True, blank=True)
    summary = models.TextField(null=True, blank=True)
    url = models.URLField('Link to Resource', null=True, blank=True)
    pub_date = models.DateTimeField('date published')
    prereqs = models.ManyToManyField(
        'self', through='Prereq', symmetrical=False, related_name='prerequired')

    def was_published_recently(self):
        return self.pub_date >= timezone.now() - datetime.timedelta(days=1)

    def __unicode__(self):
        return self.title

class Prereq(models.Model):
    resource = models.ForeignKey(Resource, null=True, related_name='main_resource')
    prereq = models.ForeignKey(Resource, null=True, related_name='prereq_resource')

ADMIN:

from django.contrib import admin
from idhhb.models import Resource, Prereq

class PrereqInline(admin.TabularInline):
    model = Prereq
    fk_name = 'resource'
    extra = 5

class ResourceAdmin(admin.ModelAdmin):
    fieldsets = [
        (None, {'fields': 'title shortcode img summary url pub_date'.split() }),
    ]
    inlines = [PrereqInline,]


admin.site.register(Resource, ResourceAdmin)

TEMPLATE:

{% for prereq in resource.prereqs.all %}
    <li>{{ prereq.id }}</li>
{% endfor %}

Upvotes: 1

catavaran
catavaran

Reputation: 45575

prereq_backlink and prereq_resource are managers so access to the Resource will look like:

r = Resource.objects.get(id=2)
for prereq in r.prereq_backlink.all():
    print prereq.resource.title

But I join to @daniel-roseman's question: why you didn't use the ManyToManyField for this task?

class Resource(models.Model):
    ...
    prereqs = models.ManyToManyField('self')

Upvotes: 0

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