Provide sane migration-steps to Volto for existing sites

[pbauer]

We should:

• [x] Make existing content folderish (using plone.app.contenttypes.utils.migrate_base_class_to_new_class)
• [x] Transform existing Richtext to blocks (using plone.volto.browser.migrate_richtext.migrate_richtext_to_slate)
• [x] Migrate Folders with a defaultPage to FolderishDocument with the content of the default_page as blocks
• [x] Migrate Collections to Documents with a listing block
• [x] Discuss what should be done automatically and what should be optional
• [x] Document all of that in the upgrade guide

[pbauer]

Proposal on Migrating from Classic to Volto

General

• This only applies to existing sites

• Migration to Volto is only supported in Plone 6 (not backported)

• Migration to Volto is not done automatically as a Plone Upgrade but a special step similar to migration from AT to DX.

• A migrated site will basically still work in Plone Classic but behave different. Editors can only work with the Data using Volto.

• We Provide one form (in plone.volto) that handles all migration-steps and shows documentation.

• We disable Folders and only offer Document, News Item, Event and Link.

• Maybe later also disable Links when https://github.com/plone/volto/issues/3303 is merged

Implementation

• Move Folderish classes from plone.volto to plone.app.contenttypes

• Hide plone.volto from installable addons to prevent unintendet breaking changes. (We need to check if the interfaces in addons-controlpanel and createplonesite forms are different)

• Show link to migration page after the last upgrade-step to Plone 6: After a Plone-Upgrade the success view will check if plone.volto is not installed and Plone-version is >=6.0. If yes then show additonal text and link to the Migration form.

  • Alternative option: Don't change anything when installing plone.volto (by moving all actuall changes out of the default profile) and offer the migration and installation first then. Would be more work.

• Combine all Migration-Steps in one view/form (e.g. @@migrate_to_volto) in Plone Classic

• Automatic Steps (non-optional)

  • Install plone.volto to enable behaviors and restapi
  • Migrate existing content to folderish (using plone.app.contenttypes.utils.migrate_base_class_to_new_class)
  • Convert Richtext to Slate (using plone.volto.browser.migrate_richtext.migrate_richtext_to_slate)
  • Migrate Collections to Documents with listing blocks (during migrate_to_folderish)
  • Add preview-image-block for News Items

• Recommended options that can be disabled

  • Migrate all Folders to Documents (during migrate_to_folderish)
    • Migrate Default Pages into the Document if possible (during migrate_to_folderish)
  • Purge old Richtext-Fields when migrating to Slate (during migrate_richtext)

• Show Information:

  • Show Requirements (blocks-conversion-tool)
  • Document all above steps that will happen
  • Link to installation docs for Volto
  • Section with Info on the Differences to Classic:
    • Mentions Portlets are not supported by Volto
    • You have x Add-ons installed. Please check if there is a migration path for them to Volto: [Show list of non default installed profiles]
    • Custom Layouts are gone, please check those layouts
      • Optional: Link to a generated list of content with layouts that are not default (additional because we don't want to wake up all objects)

Technical details

• When Migrating Folders with a defaultPage to FolderishDocument with the content of the default_page as blocks

  • Recreate relations that pointed to the dropped default page
  • Add redirect from dropped default-page to container
  • Move (or merge?) field-data: Subjects, Creator, Dates, Constributor, Rights
  • Keep local roles of folder (or merge)

• Keep default page as content of the FolderishDocument and add a listing-block if...

  • ... the default page is in [Event, News Item, Link]
  • ... the workflow state of default page and container is not the same

Documentation

• Document all of that in the upgrade guide

Optional work on @@migrate_richtext

• Allow migrating to draftjs instead of slate (done in https://github.com/plone/plone.volto/pull/56)
• Migrate draftjs-blocks to slate-blocks

[pbauer]

The first version works fine with migrating existing sites to Volto. A funny issue we found is:

• A fresh site with Volto by default has Folders and Collections.
• A fresh site with Classic has Folders and Collections. When migrating that to volto using @@migrate_to_volto is has FolderishDocuments with listing blocks instead.

I'll change the default-content created in p.a.c to FolderishDocuments with listing blocks if plone.volto is installed (i.e. a Volto site is created).

[pbauer]

Documentation: https://github.com/plone/documentation/pull/1254