improve relationships to avoid circular loads

[matt-beanland]

Ash doesn't protect against infinite looping due to circular loads. I initiated a support Q on discord about this and learn that i) it probably never will and ii) best practice is to limit or completely avoid loads in actions, iii) loads can be added into the code interface, this is good as they only apply to the immediate resource. I'd like to avoid having the consumer of the code interface need to explicitly load things, but they do have this option.

We'd probably like the relationship not to load the target by default, but of course to still have the href from the target, which will need to be a calculation.

[matt-beanland]

I've tried adding a target_href calculation to the relationship, however it requires the target to be loaded to work. I thought this would happen automatically, but it might be an issue with my ash_neo4j datalayer. Anyway I don't want to load the entire target, it is very expensive.

What I probably need to do is store the href on the relationship when it is created/updated (I may avoid being able to update a relationship and just require it to be destoyed and replaced). This can be done with a custom change, which should go in its own module. See https://elixirforum.com/t/setting-an-attribute-to-a-calculated-value-based-on-a-relation/66102

[matt-beanland]

Wrote a change for relationship to populate target_href and target_type, it needs to load the instance but only does this on relationship create. Updated relationship and instance json, no longer requires loading of relationship targets. Seems to work very well.