InvenTree/InvenTree/order/models.py

"""
Order model definitions
"""

# -*- coding: utf-8 -*-

import os
from datetime import datetime
from decimal import Decimal

from django.db import models, transaction
from django.db.models import Q, F, Sum
from django.db.models.functions import Coalesce
from django.core.validators import MinValueValidator
from django.core.exceptions import ValidationError
from django.contrib.auth.models import User
from django.urls import reverse
from django.utils.translation import ugettext_lazy as _

from markdownx.models import MarkdownxField
from mptt.models import TreeForeignKey

from users import models as UserModels
from part import models as PartModels
from stock import models as stock_models
from company.models import Company, SupplierPart

from InvenTree.fields import InvenTreeModelMoneyField, RoundingDecimalField
from InvenTree.helpers import decimal2string, increment, getSetting
from InvenTree.status_codes import PurchaseOrderStatus, SalesOrderStatus, StockStatus, StockHistoryCode
from InvenTree.models import InvenTreeAttachment, ReferenceIndexingMixin


def get_next_po_number():
    """
    Returns the next available PurchaseOrder reference number
    """

    if PurchaseOrder.objects.count() == 0:
        return '0001'

    order = PurchaseOrder.objects.exclude(reference=None).last()

    attempts = set([order.reference])

    reference = order.reference

    while 1:
        reference = increment(reference)

        if reference in attempts:
            # Escape infinite recursion
            return reference

        if PurchaseOrder.objects.filter(reference=reference).exists():
            attempts.add(reference)
        else:
            break

    return reference


def get_next_so_number():
    """
    Returns the next available SalesOrder reference number
    """

    if SalesOrder.objects.count() == 0:
        return '0001'

    order = SalesOrder.objects.exclude(reference=None).last()

    attempts = set([order.reference])

    reference = order.reference

    while 1:
        reference = increment(reference)

        if reference in attempts:
            # Escape infinite recursion
            return reference

        if SalesOrder.objects.filter(reference=reference).exists():
            attempts.add(reference)
        else:
            break

    return reference


class Order(ReferenceIndexingMixin):
    """ Abstract model for an order.

    Instances of this class:

    - PuchaseOrder

    Attributes:
        reference: Unique order number / reference / code
        description: Long form description (required)
        notes: Extra note field (optional)
        creation_date: Automatic date of order creation
        created_by: User who created this order (automatically captured)
        issue_date: Date the order was issued
        complete_date: Date the order was completed
        responsible: User (or group) responsible for managing the order
    """

    def save(self, *args, **kwargs):

        self.rebuild_reference_field()

        if not self.creation_date:
            self.creation_date = datetime.now().date()

        super().save(*args, **kwargs)

    class Meta:
        abstract = True

    description = models.CharField(max_length=250, verbose_name=_('Description'), help_text=_('Order description'))

    link = models.URLField(blank=True, verbose_name=_('Link'), help_text=_('Link to external page'))

    creation_date = models.DateField(blank=True, null=True, verbose_name=_('Creation Date'))

    created_by = models.ForeignKey(User,
                                   on_delete=models.SET_NULL,
                                   blank=True, null=True,
                                   related_name='+',
                                   verbose_name=_('Created By')
                                   )

    responsible = models.ForeignKey(
        UserModels.Owner,
        on_delete=models.SET_NULL,
        blank=True, null=True,
        help_text=_('User or group responsible for this order'),
        verbose_name=_('Responsible'),
        related_name='+',
    )

    notes = MarkdownxField(blank=True, verbose_name=_('Notes'), help_text=_('Order notes'))


class PurchaseOrder(Order):
    """ A PurchaseOrder represents goods shipped inwards from an external supplier.

    Attributes:
        supplier: Reference to the company supplying the goods in the order
        supplier_reference: Optional field for supplier order reference code
        received_by: User that received the goods
        target_date: Expected delivery target date for PurchaseOrder completion (optional)
    """

    @staticmethod
    def get_api_url():
        return reverse('api-po-list')

    OVERDUE_FILTER = Q(status__in=PurchaseOrderStatus.OPEN) & ~Q(target_date=None) & Q(target_date__lte=datetime.now().date())

    @staticmethod
    def filterByDate(queryset, min_date, max_date):
        """
        Filter by 'minimum and maximum date range'

        - Specified as min_date, max_date
        - Both must be specified for filter to be applied
        - Determine which "interesting" orders exist bewteen these dates

        To be "interesting":
        - A "received" order where the received date lies within the date range
        - A "pending" order where the target date lies within the date range
        - TODO: An "overdue" order where the target date is in the past
        """

        date_fmt = '%Y-%m-%d'  # ISO format date string

        # Ensure that both dates are valid
        try:
            min_date = datetime.strptime(str(min_date), date_fmt).date()
            max_date = datetime.strptime(str(max_date), date_fmt).date()
        except (ValueError, TypeError):
            # Date processing error, return queryset unchanged
            return queryset

        # Construct a queryset for "received" orders within the range
        received = Q(status=PurchaseOrderStatus.COMPLETE) & Q(complete_date__gte=min_date) & Q(complete_date__lte=max_date)

        # Construct a queryset for "pending" orders within the range
        pending = Q(status__in=PurchaseOrderStatus.OPEN) & ~Q(target_date=None) & Q(target_date__gte=min_date) & Q(target_date__lte=max_date)

        # TODO - Construct a queryset for "overdue" orders within the range

        queryset = queryset.filter(received | pending)

        return queryset

    def __str__(self):

        prefix = getSetting('PURCHASEORDER_REFERENCE_PREFIX')

        return f"{prefix}{self.reference} - {self.supplier.name}"

    reference = models.CharField(
        unique=True,
        max_length=64,
        blank=False,
        verbose_name=_('Reference'),
        help_text=_('Order reference'),
        default=get_next_po_number,
    )

    status = models.PositiveIntegerField(default=PurchaseOrderStatus.PENDING, choices=PurchaseOrderStatus.items(),
                                         help_text=_('Purchase order status'))

    supplier = models.ForeignKey(
        Company, on_delete=models.CASCADE,
        limit_choices_to={
            'is_supplier': True,
        },
        related_name='purchase_orders',
        verbose_name=_('Supplier'),
        help_text=_('Company from which the items are being ordered')
    )

    supplier_reference = models.CharField(max_length=64, blank=True, verbose_name=_('Supplier Reference'), help_text=_("Supplier order reference code"))

    received_by = models.ForeignKey(
        User,
        on_delete=models.SET_NULL,
        blank=True, null=True,
        related_name='+',
        verbose_name=_('received by')
    )

    issue_date = models.DateField(
        blank=True, null=True,
        verbose_name=_('Issue Date'),
        help_text=_('Date order was issued')
    )

    target_date = models.DateField(
        blank=True, null=True,
        verbose_name=_('Target Delivery Date'),
        help_text=_('Expected date for order delivery. Order will be overdue after this date.'),
    )

    complete_date = models.DateField(
        blank=True, null=True,
        verbose_name=_('Completion Date'),
        help_text=_('Date order was completed')
    )

    def get_absolute_url(self):
        return reverse('po-detail', kwargs={'pk': self.id})

    @transaction.atomic
    def add_line_item(self, supplier_part, quantity, group=True, reference='', purchase_price=None):
        """ Add a new line item to this purchase order.
        This function will check that:

        * The supplier part matches the supplier specified for this purchase order
        * The quantity is greater than zero

        Args:
            supplier_part - The supplier_part to add
            quantity - The number of items to add
            group - If True, this new quantity will be added to an existing line item for the same supplier_part (if it exists)
        """

        try:
            quantity = int(quantity)
            if quantity <= 0:
                raise ValidationError({
                    'quantity': _("Quantity must be greater than zero")})
        except ValueError:
            raise ValidationError({'quantity': _("Invalid quantity provided")})

        if not supplier_part.supplier == self.supplier:
            raise ValidationError({'supplier': _("Part supplier must match PO supplier")})

        if group:
            # Check if there is already a matching line item (for this PO)
            matches = self.lines.filter(part=supplier_part)

            if matches.count() > 0:
                line = matches.first()

                # update quantity and price
                quantity_new = line.quantity + quantity
                line.quantity = quantity_new
                supplier_price = supplier_part.get_price(quantity_new)
                if line.purchase_price and supplier_price:
                    line.purchase_price = supplier_price / quantity_new
                line.save()

                return

        line = PurchaseOrderLineItem(
            order=self,
            part=supplier_part,
            quantity=quantity,
            reference=reference,
            purchase_price=purchase_price,
        )

        line.save()

    @transaction.atomic
    def place_order(self):
        """ Marks the PurchaseOrder as PLACED. Order must be currently PENDING. """

        if self.status == PurchaseOrderStatus.PENDING:
            self.status = PurchaseOrderStatus.PLACED
            self.issue_date = datetime.now().date()
            self.save()

    @transaction.atomic
    def complete_order(self):
        """ Marks the PurchaseOrder as COMPLETE. Order must be currently PLACED. """

        if self.status == PurchaseOrderStatus.PLACED:
            self.status = PurchaseOrderStatus.COMPLETE
            self.complete_date = datetime.now().date()
            self.save()

    @property
    def is_overdue(self):
        """
        Returns True if this PurchaseOrder is "overdue"

        Makes use of the OVERDUE_FILTER to avoid code duplication.
        """

        query = PurchaseOrder.objects.filter(pk=self.pk)
        query = query.filter(PurchaseOrder.OVERDUE_FILTER)

        return query.exists()

    def can_cancel(self):
        """
        A PurchaseOrder can only be cancelled under the following circumstances:
        """

        return self.status in [
            PurchaseOrderStatus.PLACED,
            PurchaseOrderStatus.PENDING
        ]

    def cancel_order(self):
        """ Marks the PurchaseOrder as CANCELLED. """

        if self.can_cancel():
            self.status = PurchaseOrderStatus.CANCELLED
            self.save()

    def pending_line_items(self):
        """ Return a list of pending line items for this order.
        Any line item where 'received' < 'quantity' will be returned.
        """

        return self.lines.filter(quantity__gt=F('received'))

    def completed_line_items(self):
        """
        Return a list of completed line items against this order
        """
        return self.lines.filter(quantity__lte=F('received'))

    @property
    def line_count(self):
        return self.lines.count()

    @property
    def completed_line_count(self):

        return self.completed_line_items().count()

    @property
    def pending_line_count(self):
        return self.pending_line_items().count()

    @property
    def is_complete(self):
        """ Return True if all line items have been received """

        return self.lines.count() > 0 and self.pending_line_items().count() == 0

    @transaction.atomic
    def receive_line_item(self, line, location, quantity, user, status=StockStatus.OK, purchase_price=None, **kwargs):
        """ Receive a line item (or partial line item) against this PO
        """

        notes = kwargs.get('notes', '')
        barcode = kwargs.get('barcode', '')

        # Prevent null values for barcode
        if barcode is None:
            barcode = ''

        if not self.status == PurchaseOrderStatus.PLACED:
            raise ValidationError(
                "Lines can only be received against an order marked as 'PLACED'"
            )

        try:
            if not (quantity % 1 == 0):
                raise ValidationError({
                    "quantity": _("Quantity must be an integer")
                })
            if quantity < 0:
                raise ValidationError({
                    "quantity": _("Quantity must be a positive number")
                })
            quantity = int(quantity)
        except (ValueError, TypeError):
            raise ValidationError({
                "quantity": _("Invalid quantity provided")
            })

        # Create a new stock item
        if line.part and quantity > 0:
            stock = stock_models.StockItem(
                part=line.part.part,
                supplier_part=line.part,
                location=location,
                quantity=quantity,
                purchase_order=self,
                status=status,
                purchase_price=line.purchase_price,
                uid=barcode
            )

            stock.save(add_note=False)

            tracking_info = {
                'status': status,
                'purchaseorder': self.pk,
            }

            stock.add_tracking_entry(
                StockHistoryCode.RECEIVED_AGAINST_PURCHASE_ORDER,
                user,
                notes=notes,
                deltas=tracking_info,
                location=location,
                purchaseorder=self,
                quantity=quantity
            )

        # Update the number of parts received against the particular line item
        line.received += quantity
        line.save()

        # Has this order been completed?
        if len(self.pending_line_items()) == 0:

            self.received_by = user
            self.complete_order()  # This will save the model


class SalesOrder(Order):
    """
    A SalesOrder represents a list of goods shipped outwards to a customer.

    Attributes:
        customer: Reference to the company receiving the goods in the order
        customer_reference: Optional field for customer order reference code
        target_date: Target date for SalesOrder completion (optional)
    """

    @staticmethod
    def get_api_url():
        return reverse('api-so-list')

    OVERDUE_FILTER = Q(status__in=SalesOrderStatus.OPEN) & ~Q(target_date=None) & Q(target_date__lte=datetime.now().date())

    @staticmethod
    def filterByDate(queryset, min_date, max_date):
        """
        Filter by "minimum and maximum date range"

        - Specified as min_date, max_date
        - Both must be specified for filter to be applied
        - Determine which "interesting" orders exist between these dates

        To be "interesting":
        - A "completed" order where the completion date lies within the date range
        - A "pending" order where the target date lies within the date range
        - TODO: An "overdue" order where the target date is in the past
        """

        date_fmt = '%Y-%m-%d'  # ISO format date string

        # Ensure that both dates are valid
        try:
            min_date = datetime.strptime(str(min_date), date_fmt).date()
            max_date = datetime.strptime(str(max_date), date_fmt).date()
        except (ValueError, TypeError):
            # Date processing error, return queryset unchanged
            return queryset

        # Construct a queryset for "completed" orders within the range
        completed = Q(status__in=SalesOrderStatus.COMPLETE) & Q(shipment_date__gte=min_date) & Q(shipment_date__lte=max_date)

        # Construct a queryset for "pending" orders within the range
        pending = Q(status__in=SalesOrderStatus.OPEN) & ~Q(target_date=None) & Q(target_date__gte=min_date) & Q(target_date__lte=max_date)

        # TODO: Construct a queryset for "overdue" orders within the range

        queryset = queryset.filter(completed | pending)

        return queryset

    def save(self, *args, **kwargs):

        self.rebuild_reference_field()

        super().save(*args, **kwargs)

    def __str__(self):

        prefix = getSetting('SALESORDER_REFERENCE_PREFIX')

        return f"{prefix}{self.reference} - {self.customer.name}"

    def get_absolute_url(self):
        return reverse('so-detail', kwargs={'pk': self.id})

    reference = models.CharField(
        unique=True,
        max_length=64,
        blank=False,
        verbose_name=_('Reference'),
        help_text=_('Order reference'),
        default=get_next_so_number,
    )

    customer = models.ForeignKey(
        Company,
        on_delete=models.SET_NULL,
        null=True,
        limit_choices_to={'is_customer': True},
        related_name='sales_orders',
        verbose_name=_('Customer'),
        help_text=_("Company to which the items are being sold"),
    )

    status = models.PositiveIntegerField(default=SalesOrderStatus.PENDING, choices=SalesOrderStatus.items(),
                                         verbose_name=_('Status'), help_text=_('Purchase order status'))

    customer_reference = models.CharField(max_length=64, blank=True, verbose_name=_('Customer Reference '), help_text=_("Customer order reference code"))

    target_date = models.DateField(
        null=True, blank=True,
        verbose_name=_('Target completion date'),
        help_text=_('Target date for order completion. Order will be overdue after this date.')
    )

    shipment_date = models.DateField(blank=True, null=True, verbose_name=_('Shipment Date'))

    shipped_by = models.ForeignKey(
        User,
        on_delete=models.SET_NULL,
        blank=True, null=True,
        related_name='+',
        verbose_name=_('shipped by')
    )

    @property
    def is_overdue(self):
        """
        Returns true if this SalesOrder is "overdue":

        Makes use of the OVERDUE_FILTER to avoid code duplication.
        """

        query = SalesOrder.objects.filter(pk=self.pk)
        query = query.filter(SalesOrder.OVERDUE_FILTER)

        return query.exists()

    @property
    def is_pending(self):
        return self.status == SalesOrderStatus.PENDING

    @property
    def stock_allocations(self):
        """
        Return a queryset containing all allocations for this order
        """

        return SalesOrderAllocation.objects.filter(
            line__in=[line.pk for line in self.lines.all()]
        )

    def is_fully_allocated(self):
        """ Return True if all line items are fully allocated """

        for line in self.lines.all():
            if not line.is_fully_allocated():
                return False

        return True

    def is_over_allocated(self):
        """ Return true if any lines in the order are over-allocated """

        for line in self.lines.all():
            if line.is_over_allocated():
                return True

        return False

    def is_completed(self):
        """
        Check if this order is "shipped" (all line items delivered),
        """

        return self.lines.count() > 0 and all([line.is_completed() for line in self.lines.all()])

    def complete_order(self, user):
        """
        Mark this order as "complete"
        """

        if self.lines.count() == 0:
            # Order without line items cannot be completed
            raise ValidationError(_('Order cannot be completed as no parts have been assigned'))

        if self.status != SalesOrderStatus.PENDING:
            # Only a PENDING order can be marked as SHIPPED
            raise ValidationError(_('Only a pending order can be marked as complete'))

        # Check if there are any incomplete shipments
        for shipment in self.shipments.all():
            if not shipment.shipment_date:
                raise ValidationError(_('Order cannot be completed as there are pending shipments'))

        self.status = SalesOrderStatus.SHIPPED
        self.shipped_by = user
        self.shipment_date = datetime.now()

        self.save()

    def can_cancel(self):
        """
        Return True if this order can be cancelled
        """

        if not self.status == SalesOrderStatus.PENDING:
            return False

        return True

    @transaction.atomic
    def cancel_order(self):
        """
        Cancel this order (only if it is "pending")

        - Mark the order as 'cancelled'
        - Delete any StockItems which have been allocated
        """

        if not self.can_cancel():
            return False

        self.status = SalesOrderStatus.CANCELLED
        self.save()

        for line in self.lines.all():
            for allocation in line.allocations.all():
                allocation.delete()

        return True

    @property
    def line_count(self):
        return self.lines.count()

    def completed_line_items(self):
        """
        Return a queryset of the completed line items for this order
        """
        return self.lines.filter(shipped__gte=F('quantity'))

    def pending_line_items(self):
        """
        Return a queryset of the pending line items for this order
        """
        return self.lines.filter(shipped__lt=F('quantity'))

    @property
    def completed_line_count(self):
        return self.completed_line_items().count()

    @property
    def pending_line_count(self):
        return self.pending_line_items().count()


class PurchaseOrderAttachment(InvenTreeAttachment):
    """
    Model for storing file attachments against a PurchaseOrder object
    """

    @staticmethod
    def get_api_url():
        return reverse('api-po-attachment-list')

    def getSubdir(self):
        return os.path.join("po_files", str(self.order.id))

    order = models.ForeignKey(PurchaseOrder, on_delete=models.CASCADE, related_name="attachments")


class SalesOrderAttachment(InvenTreeAttachment):
    """
    Model for storing file attachments against a SalesOrder object
    """

    @staticmethod
    def get_api_url():
        return reverse('api-so-attachment-list')

    def getSubdir(self):
        return os.path.join("so_files", str(self.order.id))

    order = models.ForeignKey(SalesOrder, on_delete=models.CASCADE, related_name='attachments')


class OrderLineItem(models.Model):
    """ Abstract model for an order line item

    Attributes:
        quantity: Number of items
        note: Annotation for the item

    """

    class Meta:
        abstract = True

    quantity = RoundingDecimalField(
        verbose_name=_('Quantity'),
        help_text=_('Item quantity'),
        default=1,
        max_digits=15, decimal_places=5,
        validators=[MinValueValidator(0)],
    )

    reference = models.CharField(max_length=100, blank=True, verbose_name=_('Reference'), help_text=_('Line item reference'))

    notes = models.CharField(max_length=500, blank=True, verbose_name=_('Notes'), help_text=_('Line item notes'))


class PurchaseOrderLineItem(OrderLineItem):
    """ Model for a purchase order line item.

    Attributes:
        order: Reference to a PurchaseOrder object

    """

    @staticmethod
    def get_api_url():
        return reverse('api-po-line-list')

    class Meta:
        unique_together = (
            ('order', 'part', 'quantity', 'purchase_price')
        )

    def __str__(self):
        return "{n} x {part} from {supplier} (for {po})".format(
            n=decimal2string(self.quantity),
            part=self.part.SKU if self.part else 'unknown part',
            supplier=self.order.supplier.name,
            po=self.order)

    order = models.ForeignKey(
        PurchaseOrder, on_delete=models.CASCADE,
        related_name='lines',
        verbose_name=_('Order'),
        help_text=_('Purchase Order')
    )

    def get_base_part(self):
        """
        Return the base part.Part object for the line item

        Note: Returns None if the SupplierPart is not set!
        """
        if self.part is None:
            return None
        else:
            return self.part.part

    # TODO - Function callback for when the SupplierPart is deleted?

    part = models.ForeignKey(
        SupplierPart, on_delete=models.SET_NULL,
        blank=True, null=True,
        related_name='purchase_order_line_items',
        verbose_name=_('Part'),
        help_text=_("Supplier part"),
    )

    received = models.DecimalField(
        decimal_places=5,
        max_digits=15,
        default=0,
        verbose_name=_('Received'),
        help_text=_('Number of items received')
    )

    purchase_price = InvenTreeModelMoneyField(
        max_digits=19,
        decimal_places=4,
        null=True, blank=True,
        verbose_name=_('Purchase Price'),
        help_text=_('Unit purchase price'),
    )

    destination = TreeForeignKey(
        'stock.StockLocation', on_delete=models.SET_NULL,
        verbose_name=_('Destination'),
        related_name='po_lines',
        blank=True, null=True,
        help_text=_('Where does the Purchaser want this item to be stored?')
    )

    def get_destination(self):
        """
        Show where the line item is or should be placed

        NOTE: If a line item gets split when recieved, only an arbitrary
              stock items location will be reported as the location for the
              entire line.
        """

        for stock in stock_models.StockItem.objects.filter(supplier_part=self.part, purchase_order=self.order):
            if stock.location:
                return stock.location
        if self.destination:
            return self.destination
        if self.part and self.part.part and self.part.part.default_location:
            return self.part.part.default_location

    def remaining(self):
        """ Calculate the number of items remaining to be received """
        r = self.quantity - self.received
        return max(r, 0)


class SalesOrderLineItem(OrderLineItem):
    """
    Model for a single LineItem in a SalesOrder

    Attributes:
        order: Link to the SalesOrder that this line item belongs to
        part: Link to a Part object (may be null)
        sale_price: The unit sale price for this OrderLineItem
        shipped: The number of items which have actually shipped against this line item
    """

    @staticmethod
    def get_api_url():
        return reverse('api-so-line-list')

    order = models.ForeignKey(
        SalesOrder,
        on_delete=models.CASCADE,
        related_name='lines',
        verbose_name=_('Order'),
        help_text=_('Sales Order')
    )

    part = models.ForeignKey('part.Part', on_delete=models.SET_NULL, related_name='sales_order_line_items', null=True, verbose_name=_('Part'), help_text=_('Part'), limit_choices_to={'salable': True})

    sale_price = InvenTreeModelMoneyField(
        max_digits=19,
        decimal_places=4,
        null=True, blank=True,
        verbose_name=_('Sale Price'),
        help_text=_('Unit sale price'),
    )

    shipped = RoundingDecimalField(
        verbose_name=_('Shipped'),
        help_text=_('Shipped quantity'),
        default=0,
        max_digits=15, decimal_places=5,
        validators=[MinValueValidator(0)]
    )

    class Meta:
        unique_together = [
        ]

    def fulfilled_quantity(self):
        """
        Return the total stock quantity fulfilled against this line item.
        """

        query = self.order.stock_items.filter(part=self.part).aggregate(fulfilled=Coalesce(Sum('quantity'), Decimal(0)))

        return query['fulfilled']

    def allocated_quantity(self):
        """ Return the total stock quantity allocated to this LineItem.

        This is a summation of the quantity of each attached StockItem
        """

        query = self.allocations.aggregate(allocated=Coalesce(Sum('quantity'), Decimal(0)))

        return query['allocated']

    def is_fully_allocated(self):
        """ Return True if this line item is fully allocated """

        if self.order.status == SalesOrderStatus.SHIPPED:
            return self.fulfilled_quantity() >= self.quantity

        return self.allocated_quantity() >= self.quantity

    def is_over_allocated(self):
        """ Return True if this line item is over allocated """
        return self.allocated_quantity() > self.quantity

    def is_completed(self):
        """
        Return True if this line item is completed (has been fully shipped)
        """

        return self.shipped >= self.quantity


class SalesOrderShipment(models.Model):
    """
    The SalesOrderShipment model represents a physical shipment made against a SalesOrder.

    - Points to a single SalesOrder object
    - Multiple SalesOrderAllocation objects point to a particular SalesOrderShipment
    - When a given SalesOrderShipment is "shipped", stock items are removed from stock

    Attributes:
        order: SalesOrder reference
        shipment_date: Date this shipment was "shipped" (or null)
        checked_by: User reference field indicating who checked this order
        reference: Custom reference text for this shipment (e.g. consignment number?)
        notes: Custom notes field for this shipment
    """

    class Meta:
        # Shipment reference must be unique for a given sales order
        unique_together = [
            'order', 'reference',
        ]

    @staticmethod
    def get_api_url():
        return reverse('api-so-shipment-list')

    order = models.ForeignKey(
        SalesOrder,
        on_delete=models.CASCADE,
        blank=False, null=False,
        related_name='shipments',
        verbose_name=_('Order'),
        help_text=_('Sales Order'),
    )

    shipment_date = models.DateField(
        null=True, blank=True,
        verbose_name=_('Shipment Date'),
        help_text=_('Date of shipment'),
    )

    checked_by = models.ForeignKey(
        User,
        on_delete=models.SET_NULL,
        blank=True, null=True,
        verbose_name=_('Checked By'),
        help_text=_('User who checked this shipment'),
        related_name='+',
    )

    reference = models.CharField(
        max_length=100,
        blank=False,
        verbose_name=('Shipment'),
        help_text=_('Shipment number'),
        default='1',
    )

    notes = MarkdownxField(
        blank=True,
        verbose_name=_('Notes'),
        help_text=_('Shipment notes'),
    )

    tracking_number = models.CharField(
        max_length=100,
        blank=True,
        unique=False,
        verbose_name=_('Tracking Number'),
        help_text=_('Shipment tracking information'),
    )

    def is_complete(self):
        return self.shipment_date is not None

    def check_can_complete(self):

        if self.shipment_date:
            # Shipment has already been sent!
            raise ValidationError(_("Shipment has already been sent"))

        if self.allocations.count() == 0:
            raise ValidationError(_("Shipment has no allocated stock items"))

    @transaction.atomic
    def complete_shipment(self, user, **kwargs):
        """
        Complete this particular shipment:

        1. Update any stock items associated with this shipment
        2. Update the "shipped" quantity of all associated line items
        3. Set the "shipment_date" to now
        """

        # Check if the shipment can be completed (throw error if not)
        self.check_can_complete()

        allocations = self.allocations.all()

        # Iterate through each stock item assigned to this shipment
        for allocation in allocations:
            # Mark the allocation as "complete"
            allocation.complete_allocation(user)

        # Update the "shipment" date
        self.shipment_date = datetime.now()
        self.shipped_by = user

        # Was a tracking number provided?
        tracking_number = kwargs.get('tracking_number', None)

        if tracking_number is not None:
            self.tracking_number = tracking_number

        self.save()


class SalesOrderAllocation(models.Model):
    """
    This model is used to 'allocate' stock items to a SalesOrder.
    Items that are "allocated" to a SalesOrder are not yet "attached" to the order,
    but they will be once the order is fulfilled.

    Attributes:
        line: SalesOrderLineItem reference
        shipment: SalesOrderShipment reference
        item: StockItem reference
        quantity: Quantity to take from the StockItem

    """

    @staticmethod
    def get_api_url():
        return reverse('api-so-allocation-list')

    class Meta:
        unique_together = [
            # Cannot allocate any given StockItem to the same line more than once
            ('line', 'item'),
        ]

    def clean(self):
        """
        Validate the SalesOrderAllocation object:

        - Cannot allocate stock to a line item without a part reference
        - The referenced part must match the part associated with the line item
        - Allocated quantity cannot exceed the quantity of the stock item
        - Allocation quantity must be "1" if the StockItem is serialized
        - Allocation quantity cannot be zero
        """

        super().clean()

        errors = {}

        try:
            if not self.item:
                raise ValidationError({'item': _('Stock item has not been assigned')})
        except stock_models.StockItem.DoesNotExist:
            raise ValidationError({'item': _('Stock item has not been assigned')})

        try:
            if not self.line.part == self.item.part:
                errors['item'] = _('Cannot allocate stock item to a line with a different part')
        except PartModels.Part.DoesNotExist:
            errors['line'] = _('Cannot allocate stock to a line without a part')

        if self.quantity > self.item.quantity:
            errors['quantity'] = _('Allocation quantity cannot exceed stock quantity')

        # TODO: The logic here needs improving. Do we need to subtract our own amount, or something?
        if self.item.quantity - self.item.allocation_count() + self.quantity < self.quantity:
            errors['quantity'] = _('StockItem is over-allocated')

        if self.quantity <= 0:
            errors['quantity'] = _('Allocation quantity must be greater than zero')

        if self.item.serial and not self.quantity == 1:
            errors['quantity'] = _('Quantity must be 1 for serialized stock item')

        if self.line.order != self.shipment.order:
            errors['line'] = _('Sales order does not match shipment')
            errors['shipment'] = _('Shipment does not match sales order')

        if len(errors) > 0:
            raise ValidationError(errors)

    line = models.ForeignKey(
        SalesOrderLineItem,
        on_delete=models.CASCADE,
        verbose_name=_('Line'),
        related_name='allocations'
    )

    shipment = models.ForeignKey(
        SalesOrderShipment,
        on_delete=models.CASCADE,
        related_name='allocations',
        verbose_name=_('Shipment'),
        help_text=_('Sales order shipment reference'),
    )

    item = models.ForeignKey(
        'stock.StockItem',
        on_delete=models.CASCADE,
        related_name='sales_order_allocations',
        limit_choices_to={
            'part__salable': True,
            'belongs_to': None,
            'sales_order': None,
        },
        verbose_name=_('Item'),
        help_text=_('Select stock item to allocate')
    )

    quantity = RoundingDecimalField(max_digits=15, decimal_places=5, validators=[MinValueValidator(0)], default=1, verbose_name=_('Quantity'), help_text=_('Enter stock allocation quantity'))

    def get_serial(self):
        return self.item.serial

    def get_location(self):
        return self.item.location.id if self.item.location else None

    def get_location_path(self):
        if self.item.location:
            return self.item.location.pathstring
        else:
            return ""

    def get_po(self):
        return self.item.purchase_order

    def complete_allocation(self, user):
        """
        Complete this allocation (called when the parent SalesOrder is marked as "shipped"):

        - Determine if the referenced StockItem needs to be "split" (if allocated quantity != stock quantity)
        - Mark the StockItem as belonging to the Customer (this will remove it from stock)
        """

        order = self.line.order

        item = self.item.allocateToCustomer(
            order.customer,
            quantity=self.quantity,
            order=order,
            user=user
        )

        # Update the 'shipped' quantity
        self.line.shipped += self.quantity
        self.line.save()

        # Update our own reference to the StockItem
        # (It may have changed if the stock was split)
        self.item = item
        self.save()