Upgrade path from D7 entityreference

[mikemccaffrey]

• We should provide an automatic upgrade of entityreference fields when upgrading from D7 to backdrop, since the reference module is the direct successor of that project.

• For existing Backdrop sites running entityreference module, we'll need to provide them with an opt-in to convert their entityreference fields to reference fields.

@moonray started work on an update function in pull request #25, but it also included other unrelated upgrades that we are not ready to do yet, so we need a new pull request that just provides the upgrade from entityreference.

[laryn]

FWIW I plucked these functions out of @moonray's PR as is (just replacing function names) into a custom module's install file, and it worked to convert an entityreference field to a reference field -- the only caveat being I needed to clear caches with drush to bring the site back from a WSOD afterwards...

• https://github.com/backdrop-contrib/reference/pull/25/files#diff-8580ad6bb7f26f9bde2bcb3391731a3254d45619ded1db9e30fc02bbba48cb90R35-R125

[jenlampton]

Today at BackdropLIVE we discussed other ways of changing field types, and I mentioned that @davereid has a helper Drupal module that allows a developer to change field types. Linking here for future reference: https://www.drupal.org/project/helper

[jenlampton]

I've pushed up a new PR that will upgrade a Drupal 7 site using entityreference to reference automatically, but if you are using a Backdrop site with entityreference you need to opt-in to the conversion by adding $settings['reference_convert_entityreference'] = TRUE; to your settings.php file.

I don't think the settings.php value is the best place for the opt-in, but it was the fastest thing to do to get a PR up that would do the trick :)

It might be better to also have a UI, so that people who don't have access to settings.php (like in production) would still have a way to opt-in and convert their fields.

[jenlampton]

It might be better to also have a UI, so that people who don't have access to settings.php (like in production) would still have a way to opt-in and convert their fields.

I've started work on an upgrade path UI. I've added a submodule named Reference upgrade that will allow anyone running reference to convert fields of other reference types to reference fields. I've tested so far only with entity reference fields, but the code for the other conversions is in there too.