Move BomItem into the Part app

- Does not make sense to keep these separate
- Removed bom app entirely

InvenTree/part/models.py

@@ -118,3 +118,32 @@ class Part(models.Model):
     """
 
 
+class BomItem(models.Model):
+    """ A BomItem links a part to its component items.
+    A part can have a BOM (bill of materials) which defines
+    which parts are required (and in what quatity) to make it
+    """
+
+    # A link to the parent part
+    # Each part will get a reverse lookup field 'bom_items'
+    part = models.ForeignKey(Part, on_delete=models.CASCADE, related_name='bom_items')
+
+    # A link to the child item (sub-part)
+    # Each part will get a reverse lookup field 'used_in'
+    sub_part = models.ForeignKey(Part, on_delete=models.CASCADE, related_name='used_in')
+
+    # Quantity required
+    quantity = models.PositiveIntegerField(default=1, validators=[MinValueValidator(0)])
+
+
+    class Meta:
+        verbose_name = "BOM Item"
+
+        # Prevent duplication of parent/child rows
+        unique_together = ('part', 'sub_part')
+
+    def __str__(self):
+        return "{par} -> {child} ({n})".format(
+            par=self.part.name,
+            child=self.sub_part.name,
+            n=self.quantity)