Preserve Value Order of Multi-Valued Elements

[alexanderkiel]

In JSON multi-values elements are represented by lists. That lists have an order. In Datomic multi-values elements are sets. We have to create our own list implementation here to revere order. Also keep an eye on performance.

For most data elements the order of its values has no meaning. Only in case ElementDefinition.orderMeaning is present, the order has a meaning. So we might mostly ok with not preserving the order. But for example HumanName.given has an orderMeaning.

[dspiteself]

Currently for HumanName.given we make it cardinality 1 and use a ascii null as a separator. Then we split and join as it gets to the edge of the database.
We were planning to use homogeneous-tuples for some cases and a tuple with order and value for other cases.

[alexanderkiel]

In R4 there are only 5 data elements with an orderMeaning:

Element	Data Type	orderMeaning
Address.line	string	The order in which lines should appear in an address label
HumanName.given	string	Given Names appear in the correct order for presenting the name
HumanName.prefix	string	Prefixes appear in the correct order for presenting the name
HumanName.suffix	string	Suffixes appear in the correct order for presenting the name
Bundle.entry	BackboneElement	For bundles of type 'document' and 'message', the first resource is special (must be Composition or MessageHeader respectively). For all bundles, the meaning of the order of entries depends on the bundle type

I expected more. Do I miss some? So we have only two cases, string typed elements and the Bundle.entry element.

FHIR discourages one from using the null char in strings:

Strings SHOULD not contain Unicode character points below 32, except for u0009 (horizontal tab), u0010 (carriage return) and u0013 (line feed).

So the use of the null char as separator is a valid idea.

Using tuples with order and value might be a good fit for Bundle.entry. But we can't use tuples in the Free edition. I don't really need to store bundles for my use cases.

Independent of orderMeaning there is the value set List Order Codes which also gives order meaning to List.entry, Composition.section.entry and others. What should be do here? I'll ask in the FHIR chat whether orderMeaning should be present for that elements.

[alexanderkiel]

Thread in #implementers is here.

[dspiteself]

you can do an :order attribute if the type was a :db.type/ref like bundle

[alexanderkiel]

Is solved by switching to RocksDB.