Move SupplierPart and SupplierPriceBreak to the 'Company' app

- https://docs.djangoproject.com/en/2.2/ref/models/options/#django.db.models.Options.db_table
- https://stackoverflow.com/questions/3519143/django-how-to-specify-a-database-for-a-model
- And others, presumably

InvenTree/company/models.py

@@ -7,6 +7,8 @@ from __future__ import unicode_literals
 
 import os
 
+from django.core.validators import MinValueValidator
+
 from django.apps import apps
 from django.db import models
 from django.urls import reverse
@@ -150,3 +152,174 @@ class Contact(models.Model):
 
     company = models.ForeignKey(Company, related_name='contacts',
                                 on_delete=models.CASCADE)
+
+
+class SupplierPart(models.Model):
+    """ Represents a unique part as provided by a Supplier
+    Each SupplierPart is identified by a MPN (Manufacturer Part Number)
+    Each SupplierPart is also linked to a Part object.
+    A Part may be available from multiple suppliers
+
+    Attributes:
+        part: Link to the master Part
+        supplier: Company that supplies this SupplierPart object
+        SKU: Stock keeping unit (supplier part number)
+        manufacturer: Manufacturer name
+        MPN: Manufacture part number
+        URL: Link to external website for this part
+        description: Descriptive notes field
+        note: Longer form note field
+        base_cost: Base charge added to order independent of quantity e.g. "Reeling Fee"
+        multiple: Multiple that the part is provided in
+        minimum: MOQ (minimum order quantity) required for purchase
+        lead_time: Supplier lead time
+        packaging: packaging that the part is supplied in, e.g. "Reel"
+    """
+
+    def get_absolute_url(self):
+        return reverse('supplier-part-detail', kwargs={'pk': self.id})
+
+    class Meta:
+        unique_together = ('part', 'supplier', 'SKU')
+
+        # This model was moved from the 'Part' app
+        db_table = 'part_supplierpart'
+
+    part = models.ForeignKey('part.Part', on_delete=models.CASCADE,
+                             related_name='supplier_parts',
+                             limit_choices_to={'purchaseable': True},
+                             help_text='Select part',
+                             )
+
+    supplier = models.ForeignKey(Company, on_delete=models.CASCADE,
+                                 related_name='parts',
+                                 limit_choices_to={'is_supplier': True},
+                                 help_text='Select supplier',
+                                 )
+
+    SKU = models.CharField(max_length=100, help_text='Supplier stock keeping unit')
+
+    manufacturer = models.CharField(max_length=100, blank=True, help_text='Manufacturer')
+
+    MPN = models.CharField(max_length=100, blank=True, help_text='Manufacturer part number')
+
+    URL = models.URLField(blank=True, help_text='URL for external supplier part link')
+
+    description = models.CharField(max_length=250, blank=True, help_text='Supplier part description')
+
+    note = models.CharField(max_length=100, blank=True, help_text='Notes')
+
+    base_cost = models.DecimalField(max_digits=10, decimal_places=3, default=0, validators=[MinValueValidator(0)], help_text='Minimum charge (e.g. stocking fee)')
+
+    packaging = models.CharField(max_length=50, blank=True, help_text='Part packaging')
+    
+    multiple = models.PositiveIntegerField(default=1, validators=[MinValueValidator(1)], help_text='Order multiple')
+
+    minimum = models.PositiveIntegerField(default=1, validators=[MinValueValidator(1)], help_text='Minimum order quantity (MOQ)')
+
+    lead_time = models.DurationField(blank=True, null=True)
+
+    @property
+    def manufacturer_string(self):
+
+        items = []
+
+        if self.manufacturer:
+            items.append(self.manufacturer)
+        if self.MPN:
+            items.append(self.MPN)
+
+        return ' | '.join(items)
+
+    @property
+    def has_price_breaks(self):
+        return self.price_breaks.count() > 0
+
+    @property
+    def price_breaks(self):
+        """ Return the associated price breaks in the correct order """
+        return self.pricebreaks.order_by('quantity').all()
+
+    def get_price(self, quantity, moq=True, multiples=True):
+        """ Calculate the supplier price based on quantity price breaks.
+
+        - Don't forget to add in flat-fee cost (base_cost field)
+        - If MOQ (minimum order quantity) is required, bump quantity
+        - If order multiples are to be observed, then we need to calculate based on that, too
+        """
+
+        price_breaks = self.price_breaks.all()
+
+        # No price break information available?
+        if len(price_breaks) == 0:
+            return None
+
+        # Minimum ordering requirement
+        if moq and self.minimum > quantity:
+            quantity = self.minimum
+
+        # Order multiples
+        if multiples:
+            quantity = int(math.ceil(quantity / self.multipe) * self.multiple)
+
+        pb_found = False
+        pb_quantity = -1
+        pb_cost = 0.0
+
+        for pb in self.price_breaks.all():
+            # Ignore this pricebreak (quantity is too high)
+            if pb.quantity > quantity:
+                continue
+
+            pb_found = True
+
+            # If this price-break quantity is the largest so far, use it!
+            if pb.quantity > pb_quantity:
+                pb_quantity = pb.quantity
+                pb_cost = pb.cost
+
+        if pb_found:
+            cost = pb_cost * quantity
+            return cost + self.base_cost
+        else:
+            return None
+
+    def __str__(self):
+        s = "{supplier} ({sku})".format(
+            sku=self.SKU,
+            supplier=self.supplier.name)
+
+        if self.manufacturer_string:
+            s = s + ' - ' + self.manufacturer_string
+        
+        return s
+
+
+class SupplierPriceBreak(models.Model):
+    """ Represents a quantity price break for a SupplierPart.
+    - Suppliers can offer discounts at larger quantities
+    - SupplierPart(s) may have zero-or-more associated SupplierPriceBreak(s)
+
+    Attributes:
+        part: Link to a SupplierPart object that this price break applies to
+        quantity: Quantity required for price break
+        cost: Cost at specified quantity
+    """
+
+    part = models.ForeignKey(SupplierPart, on_delete=models.CASCADE, related_name='pricebreaks')
+
+    quantity = models.PositiveIntegerField(default=1, validators=[MinValueValidator(1)])
+
+    cost = models.DecimalField(max_digits=10, decimal_places=3, validators=[MinValueValidator(0)])
+
+    class Meta:
+        unique_together = ("part", "quantity")
+
+        # This model was moved from the 'Part' app
+        db_table = 'part_supplierpricebreak'
+
+    def __str__(self):
+        return "{mpn} - {cost} @ {quan}".format(
+            mpn=self.part.MPN,
+            cost=self.cost,
+            quan=self.quantity)