datascript-dom | uses datascript to query HTML

There is no out-of-the-box schema feature in Datomic for sorting child entities in a to-many relationship, yet this is a very common requirement. Googling has uncovered a few solutions so I wanted to inventory the variations of requirements and solutions here and hope for comments from the community.

Possible Requirements

• R1: small numbers (N) of child entities (not sure what the small/large threshold should be)
• R2: large numbers of child entities
• R3: single-parent children
• R4: multi-parent children
• R5: recursive children i.e. trees stored in Datomic

My particular use case is R1 + R3 + R5 which I suspect is pretty common but I wanted to enumerate as many as possible so that this might become a useful reference for others in future.

Solutions

• S1: nested component entities with a "position" attribute e.g. Datofu
• S2: add a custom "position" attribute to every child as described using transactor fns here and in this post
• S3: linked list using a "next" attribute on every child entity as implemented in datomic-linklist
• S4: separate wrapper entities representing edges as described in this post

Problems

Each solution seems to have challenges. The ones I can think of are:

• P1: maintaining constant time operations for insert, delete or move operations. There have been suggestions to use fractional numbers for "position" values to avoid having to update all children when re-ordering
• P2: supporting multiple parent relationships with ordering
• P3: complexity of maintaining the positions or edges that store the order as ordering or membership changes.
• P4: changes to a "position" attribute affect the implied "last changed" date for a child entity when it was not actually changed
• P5: queries/pull (particularly recursive queries) can become difficult when connecting through wrapper entities

For my tree use-case, I don't care about P2 and P1 is not a big problem because N is generally low

All this research hasn't helped me find clarity on which solution is best for my tree use case but I'm leaning towards S2. Naturally the least complexity is my goal but I suspect that all solutions will be complex.

Question : do you have any experience with this problem and what can you share that will help others in deciding? I'll add more R's, S's and P's above as they are pointed out. I (and many others) will really appreciate any feedback.

A similar question was asked a couple of years ago but not much happened there.