Document inventory management in the data model docs.

docs/model/index.md

@@ -27,6 +27,7 @@ keeping data types are **Assets** and **Logs**. Other types include
 
 - [Location](/model/logic/location)
 - [Group membership](/model/logic/group)
+- [Inventory](/model/logic/inventory)
 
 ## Conventions
 

docs/model/logic/inventory.md

@@ -0,0 +1,20 @@
+# Inventory
+
+farmOS can track the inventory of Assets over time.
+
+Inventory is tracked via [Logs](/model/type/log) with *inventory adjustment*
+[Quantities](/model/type/quantity). The Inventory module adds two fields to
+Quantity records: "inventory asset" and "inventory adjustment". Each Quantity
+can reference a single Asset, and either "reset", "increment", or "decrement"
+that Asset's inventory. The Quantity's "measure", "value", and "units" fields
+are used in the inventory calculations.
+
+Asset inventory is determined by querying all Quantities that reference the
+Asset and define an adjustment type of "reset", "increment", or "decrement".
+Inventory is calculated by adding all "increment" adjustments and subtracting
+all "decrement" adjustments, starting from the most recent "reset" adjustment
+(or zero if no "reset" adjustment exists).
+
+A separate inventory is tracked for each measure+unit pair, so Assets can have
+a single simple inventory (without a specified measure or unit), or they can
+have multiple inventories of different measures and units.

docs/model/type/asset.md

@@ -140,6 +140,23 @@ created or managed by software outside of farmOS, it can be identified easily.
 It can also be used to store additional structured metadata that does not fit
 into the standard Asset attributes.
 
+### Additional attributes
+
+Assets *may* contain additional attributes:
+
+- Inventory
+
+#### Inventory
+
+The inventory attribute summarizes current Asset inventory levels. This field
+is not editable itself, but is rather computed based on "inventory adjustment"
+logs. Each inventory can have a "measure", "value", and "units".
+
+For more information, see [farmOS Inventory Logic](/model/logic/inventory).
+
+This field is added to all Asset types by default only if the Inventory module
+is enabled.
+
 ## Relationships
 
 Assets can be related to other records in farmOS These relationships are

docs/model/type/quantity.md

@@ -74,6 +74,19 @@ integers (numerator and denominator).
 A Quantity may have a label assigned to it. This helps to distinguish multiple
 Quantities of the same measure within a Log.
 
+### Additional attributes
+
+Quantities *may* contain additional attributes:
+
+- Inventory adjustment
+
+#### Inventory adjustment
+
+A Quantity can be designated as an "inventory adjustment" to reset, increment,
+or decrement the inventory of Assets referenced (see "Inventory asset" below).
+
+For more information, see [farmOS Inventory Logic](/model/logic/inventory).
+
 ## Relationships
 
 All Quantities have the same standard set of relationships. Modules can add
@@ -90,6 +103,22 @@ Relationships that are common to all Quantity types include:
 The Unit of measurement is stored as a [Term](/model/type/term) in the Units
 vocabulary.
 
+### Additional relationships
+
+Quantities *may* contain additional relationships:
+
+- Inventory asset
+
+#### Inventory asset
+
+Quantities can reference Assets, along with the "Inventory adjustment"
+attribute (above) to record adjustments to the Asset's inventory.
+
+This field is added to all Quantity types by default only if the Inventory
+module is enabled.
+
+For more information, see [farmOS Inventory Logic](/model/logic/inventory).
+
 ## Type-specific fields
 
 In addition to the fields that are common to all Quantity types described

mkdocs.yml

@@ -12,6 +12,7 @@ nav:
     - Logic:
       - Location: model/logic/location.md
       - Group membership: model/logic/group.md
+      - Inventory: model/logic/inventory.md
     - Conventions: model/convention/index.md
   - Development:
     - API: