Drop concept of “quantities”

[dboehmer]

When we invented the data structure for Coocook we tried to group units in order to convert values between units. User reports have shown that nobody ever understands what a quantity is and why it should be created.

In theory we could hardcode basic quantities like mass and volume but this has disadvantages:

• needs translation to resemble current state (customized quantities)
• difficult migration from existing data (probably not a real issue because only 1 real production instance)
• I am still not 100% sure that count, length, area, volume, mass are all quantities ever needed
• still a little confusion about quantities necessary to be selected

My goal is now is to entirely drop quantities to get these benefits:

• easily import ingredients from JSON schema for recipes https://schema.org/Recipe
• easily add ingredients to recipes by typing just 1kg flour→unit kg and article flour automatically created and connected

[dboehmer]

Notes from video conference on 14 January 2021:

Database table unit_conversions

How to implement conversion:

• store unit_id1, unit_id2 — always symmetric (works both ways)
• by default transitive — can convert g→kg→t in 1 step

How to handle special units:

• idea: conversion can be marked non-transitive, e.g. tablespoon→g doesn't allow bathtub→tablespoon→g→t in 1 step
• multiple different conversions possible for the same pair of units?? e.g. 1tbsp=3g, 1tbsp=5g
• comments for conversions??