Draft: Fetch id only - v1

[nPraml]

Hello @rbygrave ,

I've been trying to find a solution to the problem in https://github.com/ebean-orm/ebean/pull/3644.

I see two ways to approach the problem:

• either recognize when calling fetch that you only need the ID of the OneProperties and convert it immediately to a select (in OrmQueryDetail in the select clause instead of fetchPath)
• or recognize later, when assembling the SqlTree, when a join would be unnecessary.

I've programmed a sketch of the initial idea, which only shows roughly what I envision for the solution. I only provisionally built the extension of the selects in OrmQueryDetail and OrmQueryProperties because we would then need such a mechanism; it's probably not 100% correctly implemented.

I've disabled one test, which I can look at later. In the subsequent tests, the queries have become simpler.

Can you please take a look at this PR to see if it's moving in the right direction? Or should we pursue the second idea that the simplification only happens later in the SqlTree?

[nPraml]

this PR can be closed if https://github.com/ebean-orm/ebean/pull/3644 is found to be better