feat: add capability to use existing content of OpenAPI as input to allow for renaming/editing scenarios

[ThomasRooney]

Closes #121

[lornajane]

After talking about with in the meeting this week, we discussed that instead of doing new destinations, we would aim to handle the rename case by adding support for new source information - a use case we already discussed and that has applications beyond the rename.

The idea is that instead of only doing:

actions:
  - target: $.paths['/foo'].get
    update:
      description: This is the new description

We would also support using something like the existing value of the target, so you could do:

actions:
  - target: $.paths['/foo'].get
    update:
      description: $target.summary

To re-use the content of another field in this one. Also $env and $doc (meaning the API description the overlay is being applied to. For renames, you would add a followup remove action.

Consideration will be needed for values where the value itself might reasonably contain a $, such as where the value is $ref.

[mikeschinkel]

Today @lornajane and I discussed this on call with both of us finding the exact syntax proposed not idea.

@lornajane took the position that update should be enhanced to support this used case, but that would necessitate a 2.0.0 release. She wanted to enhance update to make it more powerful so that it could address this use-case as well as others (@lornajane please correct my characterization if I got it wrong.)

My perspective is that we could instead offer a rename property in addition to update that could allow for keeping same major version — i.e. no backward compatible breakage — and having a rename would make it more obvious to those looking for such a feature. It would also decouple the use-cases so that update would not have to be compatible with a rename use-case, existing uses-cases, and future use-cases; rename could instead be bespoke for its needs and only its needs.

A quick strawman example might look like this (and since I am doing this in a rush before the next meeting I will most likely not get this right, but hopefully I will.):

overlay: 1.0.0
info:
  title: Rename path items
  version: 1.0.0
actions:
  - target: $.paths
    update: 
      "/newitem": {}
  - target: $.paths
    rename:
      existing: /items
      new: /newitems
    remove: true

Please comment and bikeshed away.

[lornajane]

Perhaps I did not explain myself well although I did try in the discussion. I suggested that we could support the use case that this pull request addresses by adding support for input sources. This does not require a major version bump, unless we need to add extra options for handling content such as $ signs within OpenAPI project (since the proposed new syntax would support $target and $doc) - the additional options probably want to go inside update and that change would cause a major version bump. Mike suggested adding additional operations with their own data types for each supported use case, so I did not proceed with the work to propose the additional input sources.

However we have both thoroughly invaded @ThomasRooney 's original pull request - I'll add some comments in line with the changes.

[lornajane]

I'm very keen to find a way to enable this use case, but I'm not in favour of the proposed implementation.

[lornajane]

Why change this behaviour? Remove will still cause the other fields to have no impact?

[lornajane]

I support solving the use case, but I am not in favour of this syntax. In 1.0, the changes are applied to the nodes that match the target expression. In this instance, the target isn't the target, but is more like the source which has potential to be confusing.

Is there another way we could indicate which field to update, and where to get the data from to put there?

[lornajane]

I'd prefer to see copy and remove as two different steps - it's likely that we'll do action as update or remove (or whatever else) in a future version and I think it's clearer.

[ThomasRooney]

Closing this in preference for #150 which I consider more succinct and addresses PR comments