feat: add preload_order for has_one

[saveman71]

Hello there,

Stumbling on https://elixirforum.com/t/ecto-has-one-sort-association/62923 earlier this afternoon and after considering what adding support for preload_order to the has_one assoc would entail, I gave it a shot.

I know this is probably a solution to a bad architecture, but it could save us the trouble of converting our assoc to an has_many then adding a limit and/or always considering getting the latest object in business logic.

A brief explanation on why we need this:

• We have Documents

• Documents can be signed and thus some SigningRequests are attached

• Only one request is active at any time, we will never show multiple requests in the UI

• However, some can fail, etc.

• Up until recently we had the following:

  has_one :signing_request, SigningRequest, where: [state: {:in, [Statuses.created(), Statuses.signed()]}]
  

  Which worked but it hid any failed SigningRequest.

  When noticing this, I didn't thing too much about it and removed the filter. However, since the sort is undefined, in some cases if you have a failed signature and a successful one, it returns the first found, the failed one.

• To fix this, with preload_order support we could do:

  has_one :signing_request, SigningRequest, preload_order: [desc: :id]
  

  (or on inserted_at, etc.) which would ensure that we always return the latest one, regardless of it's status (if a user chose to overwrite a successful signature, why not)

This PR is meant as an RFC, and the new case is not pretty (its here to avoid Ecto.Association.BelongsTo which doesn't have preload_order). Documentation would follow if accepted.

Made a quick tests and it seems to work for our use case

Cheers!