Reverse on :db/isComponent attributes has a special case for reading.

[scott-abernethy]

Fixes a 'bug' where RichEntity.read can not be used for reversed attributes that are components, due to an undocumented special case in datomic.Entity.get. Datomisca is expecting a collection but receives a single entity response, and throws an exception.
We've run in to this issue a few times, and had to drop down to using Entity.getAs.

Refer:

• https://groups.google.com/forum/#!searchin/datomic/reverse$20get/datomic/Eu7l__rA-xk/MWqTTECbJI0J

[dwhjames]

Sorry for the slow response.

So, if we take this approach, then I’d also like to apply the same code in the type class instance that returns just the entity id Long.

However, another approach could be to add an alternative to reverse, for example,

def reverseComponent(implicit ev: =:=[DatomicRef.type, DD]): Attribute[DatomicRef.type, Cardinality.one.type] = {
    require(isComponent.getOrElse(false))
    copy(
      ident       = clojure.lang.Keyword.intern(ident.getNamespace, "_" + ident.getName),
      valueType   = SchemaType.ref,
      cardinality = Cardinality.one
    )
}

This would give just the parent entity when used, rather than the entity in a singleton set.

[scott-abernethy]

No problem. :)
Having the single entity returned would be better, as it removes the redundant singleton set. I'm happy to go with the reverseComponent solution. I made the change, added a test for the read[Long] case, and rebased.

[ghost]

In hindsight, I wish I’d gone with reversed, rather than reverse. When combined to make reverseComponent, it sits even more uncomfortably… as it sounds like an action on a component.

Unfortunately, it’s not possible to combine the two methods into one as the component flag is not present at the type level, so the choice between one and many can’t be computed with a dependent type.

Any suggestions for something better than reverseComponent? Or do you think it is livable?

[scott-abernethy]

Naming, the hardest problem in comp sci...
I'm fine with it.
I did consider adding the component type to Attribute originally, but it seemed like overkill so I ditched the idea.

[ghost]

Yeah, that extra type information would come at a pretty high cost in terms of noise everywhere else, just to solve that problem.

[scott-abernethy]

Ooops, used tabs.

[ghost]

Thanks for catching that.

I spent a little while with @rjsvaljean brainstorming alternative names. We didn’t really come up with anything demonstrably better, so we can keep it as is.