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copied!<p>Be careful to realize that there are some differences between <code>OneToOneField(SomeModel)</code> and <code>ForeignKey(SomeModel, unique=True)</code>. As stated in <a href="http://djangobook.com/model-definition-reference/#leanpub-auto-onetoonefield" rel="noreferrer"><em>The Definitive Guide to Django</em></a>:</p> <blockquote> <p><strong>OneToOneField</strong></p> <p>A one-to-one relationship. Conceptually, this is similar to a <code>ForeignKey</code> with <code>unique=True</code>, but the "reverse" side of the relation will directly return a single object.</p> </blockquote> <p>In contrast to the <code>OneToOneField</code> "reverse" relation, a <code>ForeignKey</code> "reverse" relation returns a <code>QuerySet</code>.</p> <h2>Example</h2> <p>For example, if we have the following two models (full model code below):</p> <ol> <li><code>Car</code> model uses <code>OneToOneField(Engine)</code></li> <li><code>Car2</code> model uses <code>ForeignKey(Engine2, unique=True)</code></li> </ol> <p>From within <code>python manage.py shell</code> execute the following:</p> <h3><code>OneToOneField</code> Example</h3> <pre><code>>>> from testapp.models import Car, Engine >>> c = Car.objects.get(name='Audi') >>> e = Engine.objects.get(name='Diesel') >>> e.car <Car: Audi> </code></pre> <h3><code>ForeignKey</code> with <code>unique=True</code> Example</h3> <pre><code>>>> from testapp.models import Car2, Engine2 >>> c2 = Car2.objects.get(name='Mazda') >>> e2 = Engine2.objects.get(name='Wankel') >>> e2.car2_set.all() [<Car2: Mazda>] </code></pre> <h2>Model Code</h2> <pre><code>from django.db import models class Engine(models.Model): name = models.CharField(max_length=25) def __unicode__(self): return self.name class Car(models.Model): name = models.CharField(max_length=25) engine = models.OneToOneField(Engine) def __unicode__(self): return self.name class Engine2(models.Model): name = models.CharField(max_length=25) def __unicode__(self): return self.name class Car2(models.Model): name = models.CharField(max_length=25) engine = models.ForeignKey(Engine2, unique=True, on_delete=models.CASCADE) def __unicode__(self): return self.name </code></pre>

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