Clarify value vs price

[bhaugen]

I think we use value to mean cost, and price only for items for sale. But it is not explained clearly, and may not be used consistently. And value=cost does not communicate well.

[bhaugen]

Some observations:

• Both Resource and ResourceType have the field value_per_unit_of_use, but the one on ResourceType is never used, only the one on Resource.
  • It is used now in two ways:
  • to compute the value of a use event for passing on to contributors to the Resource that was used,
  • and now in equipment logging as the price per unit for 3D printer use.
  • The second one seems clearly to be a price. The first one probably is, too, but is it more ambiguous?
• Both Resource and ResourceType have the field value_per_unit, and again,, only the one on Resource is ever used. AgentResourceType has this same field, but also never used.
  • Resource.value_per_unit is computed in a rollup by adding up the values of all the contributions to making the resource. In other words, it is computed like cost of production.
  • AgentResourceType.value_per_unit could be used for supplier prices or differential values of work ResourceTypes for different agents.
• The field price_per_unit only occurs on ResourceTypes and now is used only (optionally) in value equation claim computations.

[bhaugen]

Tentative proposal:

• General principles:
  • All fields named "price" mean sale price.
  • Prices are decided by people, not computed by the system.
  • The field now named "value_per_unit_of_use" should be renamed to "price_per_unit_of_use".
  • All fields named "value_per_unit" are computed by the system and mean (roughly) cost.
  • For purchased resources, that means the purchase cost. If more than one purchase, it means the weighted average cost.
  • For created resources, it means the cost of the inputs of all the processes that went into its creation.
• Changes:
  • Remove "value_per_unit" from ResourceType.
  • Rename "value_per_unit_of_use" to "price_per_unit_of_use".
  • When a resource is purchased or created, recompute its value_per_unit as the weighted average of its purchase, production and other costs, using the Resource.roll_up_value() method.
  • This should use an appropriate value equation if one exists.