Limiting choices from an intermediary ManyToMany junction table in Django

Question

Background

I've created three Django models—Inventory, SalesOrder, and Invoice—to model items in inventory, sales orders for those items, and invoices for a particular sales order. Each sales order can have multiple items, so I've used an intermediary junction table—SalesOrderItems—using the through argument for the ManyToManyField. Also, partial billing of a sales orders is allowed, so I've created a ForeignKey in the Invoice model related to the SalesOrder model, so that a particular sales order can have multiple invoices.

Here's where I deviate from what I've normally seen. Instead of relating the Invoice model to the Item model via a ManyToManyField, I've related the Invoice model to the SalesOrderItem intermediary junction table through the intermediary junction table InvoiceItem. I've done this because it better models reality—our invoices are tied to sales orders and can only include items that are tied to that sales order as opposed to any item in inventory. I will admit that it does seem strange having the intermediary junction table of a ManyToManyField related to the intermediary junction table of another ManyToManyField.

Questions

• Is there a better way to model the relationship of only allowing items on an invoice to be those items that are associated with the invoice's sales order?
• Is the model the correct place to include the logic to limit the invoice items to only those items that are associated with the invoice's sales order? IMO, the answer is "yes" given that the [Django documentation][dj-model-doc] describes a model as "the single, definitive source of data about your data."
• How can I limit the choices available for the invoice_items in the Invoice model to just the sales_order_items of the SalesOrder model for that particular Invoice regardless of accessing the model via the admin pages or a form?

Code

class Item(models.Model):
    item_num = models.SlugField(unique=True)
    default_price = models.DecimalField(max_digits=10, decimal_places=2,
        blank=True, null=True)

class SalesOrderItem(models.Model):
    item = models.ForeignKey(Item)
    sales_order = models.ForeignKey('SalesOrder')
    unit_price = models.DecimalField(max_digits=10, decimal_places=2)
    quantity = models.DecimalField(max_digits=10, decimal_places=4)

class SalesOrder(models.Model):
    customer = models.ForeignKey(Party)
    so_num = models.SlugField(max_length=40, unique=True)
    sales_order_items = models.ManyToManyField(Item, 
        through=SalesOrderItem)

class InvoiceItem(models.Model):
    item = models.ForeignKey(SalesOrderItem)
    invoice = models.ForeignKey('Invoice')
    unit_price = models.DecimalField(max_digits=10, decimal_places=2)
    quantity = models.DecimalField(max_digits=10, decimal_places=4)

class Invoice(models.Model):
    invoice_num = models.SlugField(max_length=25)
    sales_order = models.ForeignKey(SalesOrder)
    invoice_items = models.ManyToManyField(SalesOrderItem,
        through='InvoiceItem')

Answer 1

limit_choices_to only affects the admin pages. If you want to change the available choices in a normal ModelForm then you'll need to modify the queryset attribute of the form field.