ENH: Allow adding link(s) to unsaved DataObjects (closes #387)

[lozcalver]

Description

This seems surprisingly trivial in terms of code changes! There’s some minor refactoring in that the jsonError() calls are moved out of most of the getters, but these are all private methods anyway so there aren’t any BC concerns.

Manual testing steps

Add a DataObject class which has_one and/or has_many links. Create a new, unsaved DataObject (basically anything that isn’t a page or elemental block) and you should now be able to add links instead of seeing the old “Cannot create links” message.

Issues

• Remove requirement to save record before adding link(s) #387

Pull request checklist

• The target branch is correct
  • See picking the right version
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• The commit messages follow our commit message guidelines
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• Code changes follow our coding conventions
• This change is covered with tests (or tests aren't necessary for this change)
• Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
• CI is green

[lozcalver]

Marked as draft because I still need to look at PHP/Behat coverage

Edit: actually it’s not working properly for MultiLinkField, but is working for a single has_one link. I’ll investigate, but even if that can’t be fixed it’s probably still a worthwhile change for just has_one links.

[lozcalver]

I’ve pushed up a working solution and test coverage, but I’m struggling to get Behat running at the moment

[GuySartorelli]

We aren't releasing any new minor releases for the CMS 5 compatible branches. Please retarget this to the 5 branch (you may need to reset your commits).

This also needs a behat test - if you're using DDEV my solution lately has been to install chromedriver and chromium into the web container since the various DDEV selenium addons stopped working for me a few months ago.
here is my docker file and here is my behat command

Alternatively if you're still having trouble let me know and I can make a behat feature for this separately.

Please also update docs and make this new behaviour configurable per #387 (comment)

[lozcalver]

I’ve rebased onto the 5 branch and pushed up a Behat scenario, I don’t know what’s going on with CI though – seems like it’s trying to install the CMS5 installer. I guess it’s a side-effect of switching the base branch?

[lozcalver]

Even after re-running all the CI actions, the base ref is still wrong (it says 4 when it should be 5): https://github.com/silverstripe/silverstripe-linkfield/actions/runs/20235733741/job/58090381662?pr=388#step:2:317. I’ll try closing this and opening a new PR

[GuySartorelli]

Replaced by #425