How do I add a Foreign Key Field to a ModelForm in Django?

What I would like to do is to display a single form that lets the user:

• Enter a document title (from Document model)
• Select one of their user_defined_code choices from a drop down list (populated by the UserDefinedCode model)
• Type in a unique_code (stored in the Code model)

I'm not sure how to go about displaying the fields for the foreign key relationships in a form. I know in a view you can use document.code_set (for example) to access the related objects for the current document object, but I'm not sure how to apply this to a ModelForm.

My model:

class UserDefinedCode(models.Model):
    name = models.CharField(max_length=8)
    owner = models.ForeignKey(User)

class Code(models.Model):
    user_defined_code = models.ForeignKey(UserDefinedCode)
    unique_code = models.CharField(max_length=15)

class Document(models.Model):
    title = models.CharField(blank=True, null=True, max_length=200)
    code = models.ForeignKey(Code)
    active = models.BooleanField(default=True)

My ModelForm

class DocumentForm(ModelForm):
    class Meta:
        model = Document