Part units (#4854)

* Add validation to part units field

* Add "pack_units" field to the SupplierPart model

* Migrate old units to new units, and remove old field

* Table fix

* Fixture fix

* Update migration

* Improve "hook" for loading custom unit database

* Display part units column in part table

- Also allow ordering by part units
- Allow filtering to show parts which have defined units

* Adds data migration for converting units to valid values

* Add "pack_units_native" field to company.SupplierPart model

* Clean pack units when saving a SupplierPart

- Convert to native part units
- Handle empty units value
- Add unit tests

* Add background function to rebuild supplier parts when a part is saved

- Required to ensure that the "pack_size_native" is up to date

* Template updates

* Sort by native units first

* Bump API version

* Rename "pack_units" to "pack_quantity"

* Update migration file

- Allow reverse migration

* Fix for currency migration

- Handle case where no currencies are provided
- Handle case where base currency is not in provided options

* Adds unit test for data migration

* Add unit test for part.units data migration

- Check that units fields are updated correctly

* Add some extra "default units"

- each / piece
- dozen / hundred / thousand
- Add unit testing also

* Update references to "pack_size"

- Replace with "pack_quantity" or "pack_quantity_native" as appropriate

* Improvements based on unit testing

* catch error

* Docs updates

* Fixes for pricing tests

* Update unit tests for part migrations · 1b6b6d9d

* Bug fix for conversion code

* javascript updates

* JS formatting fix

docs/docs/part/part.md

@@ -55,7 +55,19 @@ Trackable parts can be assigned batch numbers or serial numbers which uniquely i
 
 ### Purchaseable
 
-If a part is designated as *Purchaseable* it can be purchased from external suppliers. Setting this flag allows parts to be added to [purchase orders](../order/purchase_order.md).
+If a part is designated as *Purchaseable* it can be purchased from external suppliers. Setting this flag allows parts be linked to supplier parts and procured via purchase orders.
+
+#### Suppliers
+
+A [Supplier](../order/company.md#suppliers) is an external vendor who provides goods or services.
+
+#### Supplier Parts
+
+Purchaseable parts can be linked to [Supplier Parts](../order/company.md#supplier-parts). A supplier part represents an individual piece or unit that is procured from an external vendor.
+
+#### Purchase Orders
+
+A [Purchase Order](../order/purchase_order.md) allows parts to be ordered from an external supplier.
 
 ### Salable
 
@@ -65,6 +77,31 @@ If a part is designated as *Salable* it can be sold to external customers. Setti
 
 By default, all parts are *Active*. Marking a part as inactive means it is not available for many actions, but the part remains in the database. If a part becomes obsolete, it is recommended that it is marked as inactive, rather than deleting it from the database.
 
+## Units of Measure
+
+Each type of part can define a custom "unit of measure" which is a standardized unit which is used to track quantities for a particular part. By default, the "unit of measure" for each part is blank, which means that each part is tracked in dimensionless quantities of "pieces".
+
+### Physical Units
+
+It is possible to track parts using physical quantity values, such as *metres* or *litres*. For example, it would make sense to track a "wire" in units of "metres":
+
+{% with id="part_units", url="part/part_units.png", description="Parts units" %}
+{% include 'img.html' %}
+{% endwith %}
+
+### Supplier Part Units
+
+By default, units of measure for [supplier parts](../order/company.md#supplier-parts) are specified in the same unit as their base part. However, supplier part units can be changed to any unit *which is compatible with the base unit*.
+
+!!! info "Example: Supplier Part Units"
+    If the base part has a unit of `metres` then valid units for any supplier parts would include `feet`, `cm`, `inches` (etc)
+
+If an incompatible unit type is specified, an error will be displayed:
+
+{% with id="part_units_invalid", url="part/part_units_invalid.png", description="Invalid supplier part units" %}
+{% include 'img.html' %}
+{% endwith %}
+
 ## Part Images
 
 Each part can have an associated image, which is used for display purposes throughout the InvenTree interface. A prominent example is on the part detail page itself: