speaker profile. If it's None, that functionality is disabled. '''

        return None

    attendee = models.OneToOneField(Attendee, on_delete=models.CASCADE)

# Inventory Models

@python_2_unicode_compatible

class Category(models.Model):

    ''' Registration product categories '''

    class Meta:

        verbose_name_plural = _("categories")

    def __str__(self):

        return self.name

    RENDER_TYPE_RADIO = 1

    RENDER_TYPE_QUANTITY = 2

    CATEGORY_RENDER_TYPES = [

        (RENDER_TYPE_RADIO, _("Radio button")),

        (RENDER_TYPE_QUANTITY, _("Quantity boxes")),

    ]

    name = models.CharField(

        max_length=65,

        verbose_name=_("Name"),

    )

    description = models.CharField(

        max_length=255,

        verbose_name=_("Description"),

    )

    limit_per_user = models.PositiveIntegerField(

        null=True,

        blank=True,

        verbose_name=_("Limit per user"),

        help_text=_("The total number of items from this category one "

                    "attendee may purchase."),

    )

    required = models.BooleanField(

        blank=True,

        help_text=_("If enabled, a user must select an "

                    "item from this category."),

    )

    order = models.PositiveIntegerField(

        verbose_name=("Display order"),

    )

    render_type = models.IntegerField(

        choices=CATEGORY_RENDER_TYPES,

        verbose_name=_("Render type"),

        help_text=_("The registration form will render this category in this "

                    "style."),

    )

@python_2_unicode_compatible

class Product(models.Model):

    ''' Registration products '''

    def __str__(self):

        return "%s - %s" % (self.category.name, self.name)

    name = models.CharField(

        max_length=65,

        verbose_name=_("Name"),

    )

    description = models.CharField(

        max_length=255,

        verbose_name=_("Description"),

        null=True,

        blank=True,

    )

    category = models.ForeignKey(

        Category,

        verbose_name=_("Product category")

    )

    price = models.DecimalField(

        max_digits=8,

        decimal_places=2,

        verbose_name=_("Price"),

    )

    limit_per_user = models.PositiveIntegerField(

        null=True,

        blank=True,

        verbose_name=_("Limit per user"),

    )

    reservation_duration = models.DurationField(

        default=datetime.timedelta(hours=1),

        verbose_name=_("Reservation duration"),

        help_text=_("The length of time this product will be reserved before "

                    "it is released for someone else to purchase."),

    )

    order = models.PositiveIntegerField(

        verbose_name=("Display order"),

    )

@python_2_unicode_compatible

class Voucher(models.Model):

    ''' Registration vouchers '''

    # Vouchers reserve a cart for a fixed amount of time, so that

    # items may be added without the voucher being swiped by someone else

    RESERVATION_DURATION = datetime.timedelta(hours=1)

    def __str__(self):

        return "Voucher for %s" % self.recipient

    @classmethod

    def normalise_code(cls, code):

        return code.upper()

    def save(self, *a, **k):

        ''' Normalise the voucher code to be uppercase '''

        self.code = self.normalise_code(self.code)

        super(Voucher, self).save(*a, **k)

    recipient = models.CharField(max_length=64, verbose_name=_("Recipient"))

    code = models.CharField(max_length=16,

                            unique=True,

                            verbose_name=_("Voucher code"))

    limit = models.PositiveIntegerField(verbose_name=_("Voucher use limit"))

# Product Modifiers

@python_2_unicode_compatible

class DiscountBase(models.Model):

    ''' Base class for discounts. Each subclass has controller code that

    determines whether or not the given discount is available to be added to

    the current cart. '''

    objects = InheritanceManager()

    def __str__(self):

        return "Discount: " + self.description

    def effects(self):

        ''' Returns all of the effects of this discount. '''

        products = self.discountforproduct_set.all()

        categories = self.discountforcategory_set.all()

        return itertools.chain(products, categories)

                    "a product or its category"))

        if len(cats) > 1:

            raise ValidationError(

                _("You may only have one discount line per category"))

    discount = models.ForeignKey(DiscountBase, on_delete=models.CASCADE)

    category = models.ForeignKey(Category, on_delete=models.CASCADE)

    percentage = models.DecimalField(

        max_digits=4,

        decimal_places=1)

    quantity = models.PositiveIntegerField()

class TimeOrStockLimitDiscount(DiscountBase):

    ''' Discounts that are generally available, but are limited by timespan or

    usage count. This is for e.g. Early Bird discounts. '''

    class Meta:

        verbose_name = _("Promotional discount")

    start_time = models.DateTimeField(

        null=True,

        blank=True,

        verbose_name=_("Start time"),

        help_text=_("This discount will only be available after this time."),

    )

    end_time = models.DateTimeField(

        null=True,

        blank=True,

        verbose_name=_("End time"),

        help_text=_("This discount will only be available before this time."),

    )

    limit = models.PositiveIntegerField(

        null=True,

        blank=True,

        verbose_name=_("Limit"),

        help_text=_("This discount may only be applied this many times."),

    )

class VoucherDiscount(DiscountBase):

    ''' Discounts that are enabled when a voucher code is in the current

    cart. '''

    voucher = models.OneToOneField(

        Voucher,

        on_delete=models.CASCADE,

        verbose_name=_("Voucher"),

    )

class IncludedProductDiscount(DiscountBase):

    ''' Discounts that are enabled because another product has been purchased.

    e.g. A conference ticket includes a free t-shirt. '''

    class Meta:

        verbose_name = _("Product inclusion")

    enabling_products = models.ManyToManyField(

        Product,

        verbose_name=_("Including product"),

        help_text=_("If one of these products are purchased, the discounts "

                    "below will be enabled."),

    )

class RoleDiscount(object):

    ''' Discounts that are enabled because the active user has a specific

    role. This is for e.g. volunteers who can get a discount ticket. '''

    # TODO: implement RoleDiscount

    pass

@python_2_unicode_compatible

class EnablingConditionBase(models.Model):

    ''' This defines a condition which allows products or categories to

    be made visible. If there is at least one mandatory enabling condition

    defined on a Product or Category, it will only be enabled if *all*

    mandatory conditions are met, otherwise, if there is at least one enabling

    condition defined on a Product or Category, it will only be enabled if at

    least one condition is met. '''

    objects = InheritanceManager()

    def __str__(self):

        return self.description

    def effects(self):

        ''' Returns all of the items enabled by this condition. '''

        return itertools.chain(self.products.all(), self.categories.all())

    description = models.CharField(max_length=255)

    mandatory = models.BooleanField(

        default=False,

        help_text=_("If there is at least one mandatory condition defined on "

                    "a product or category, all such conditions must be met. "

    enabling_products = models.ManyToManyField(

        Product,

        help_text=_("If one of these products are purchased, this condition "

                    "is met."),

    )

@python_2_unicode_compatible

class CategoryEnablingCondition(EnablingConditionBase):

    ''' The condition is met because a product in a particular product is

    purchased. '''

    def __str__(self):

        return "Enabled by product in category: " + str(self.enabling_category)

    enabling_category = models.ForeignKey(

        Category,

        help_text=_("If a product from this category is purchased, this "

                    "condition is met."),

    )

@python_2_unicode_compatible

class VoucherEnablingCondition(EnablingConditionBase):

    ''' The condition is met because a Voucher is present. This is for e.g.

    enabling sponsor tickets. '''

    def __str__(self):

        return "Enabled by voucher: %s" % self.voucher

    voucher = models.OneToOneField(Voucher)

# @python_2_unicode_compatible

class RoleEnablingCondition(object):

    ''' The condition is met because the active user has a particular Role.

    This is for e.g. enabling Team tickets. '''

    # TODO: implement RoleEnablingCondition

    pass

# Commerce Models

@python_2_unicode_compatible

class Cart(models.Model):

    ''' Represents a set of product items that have been purchased, or are

    pending purchase. '''

    def __str__(self):

        return "%d rev #%d" % (self.id, self.revision)

    user = models.ForeignKey(User)

    # ProductItems (foreign key)

    vouchers = models.ManyToManyField(Voucher, blank=True)

    reservation_duration = models.DurationField()

    revision = models.PositiveIntegerField(default=1)

    @classmethod

    def reserved_carts(cls):

        ''' Gets all carts that are 'reserved' '''

        return Cart.objects.filter(

            (Q(active=True) &

                Q(time_last_updated__gt=(

                    timezone.now()-F('reservation_duration')

                                        ))) |

            (Q(active=False) & Q(released=False))

        )

@python_2_unicode_compatible

class ProductItem(models.Model):

    ''' Represents a product-quantity pair in a Cart. '''

    def __str__(self):

        return "product: %s * %d in Cart: %s" % (

            self.product, self.quantity, self.cart)

    cart = models.ForeignKey(Cart)

    product = models.ForeignKey(Product)

@python_2_unicode_compatible

class DiscountItem(models.Model):

    ''' Represents a discount-product-quantity relation in a Cart. '''

    def __str__(self):

        return "%s: %s * %d in Cart: %s" % (

            self.discount, self.product, self.quantity, self.cart)

    cart = models.ForeignKey(Cart)

    product = models.ForeignKey(Product)

    discount = models.ForeignKey(DiscountBase)

    quantity = models.PositiveIntegerField()

@python_2_unicode_compatible

class Invoice(models.Model):

    ''' An invoice. Invoices can be automatically generated when checking out

    a Cart, in which case, it is attached to a given revision of a Cart. '''

    def __str__(self):

        return "Invoice #%d" % self.id

    def clean(self):

        if self.cart is not None and self.cart_revision is None:

            raise ValidationError(

                "If this is a cart invoice, it must have a revision")

    # Invoice Number

    user = models.ForeignKey(User)

    cart = models.ForeignKey(Cart, null=True)

    cart_revision = models.IntegerField(null=True)

    # Line Items (foreign key)

    void = models.BooleanField(default=False)

    paid = models.BooleanField(default=False)

    value = models.DecimalField(max_digits=8, decimal_places=2)

@python_2_unicode_compatible

class LineItem(models.Model):

    ''' Line items for an invoice. These are denormalised from the ProductItems

    and DiscountItems that belong to a cart (for consistency), but also allow

    for arbitrary line items when required. '''

    def __str__(self):

        return "Line: %s * %d @ %s" % (

            self.description, self.quantity, self.price)