I am trying to write custom migrations to migrate my Drupal 7 website to Drupal 8. I am currently using Drupal 8.6.7 and Drupal 7.61 for a copy of the production website.
So far I have set up the D8 website with composer (docroot is path/to/web) (https://www.drupal.org/docs/develop/using-composer/using-composer-to-manage-drupal-site-dependencies )
I have always used Drush for maintenance actions on the D7 site and I am using Drush 9 for most of the actions including the migrations on the D8 site.
I have installed the most relevant modules in the D8 site so that I can start the migration, most notably all the D8 equivalents of the modules I used in the text formats/filters as well as migrate_plus, migrate_tools and migrate_upgrade.
The migration files are generated with the Drush command:
drush @dev migrate-upgrade "$LEGACYDB" "$LEGACYURL" --configure-only --migration-prefix=osn_custom_
So far I have successfully migrated the users and their roles so I have some knowledge on how migrations work.
The problem I am currently working on is the migration of a custom content type called 'classified'. This type has a default Drupal 7 body field and a field 'field_image'. So quite ordinary.
When I use the default generated migration file, it looks like everything is migrated, but the text is not visible. After research I found this is caused by the fact that the input format is set to the D7 code (in my case 2 aka 'full html') while there is no filter code in D8 that is called '2'. (src: https://www.drupal.org/docs/8/upgrade/known-issues-when-upgrading-from-drupal-6-or-7-to-drupal-8#d7_to_d8 ).
I decided that I might as well convert 2 to 'basic html' and use the D8 configuration and not migrate the D7 text filters at all.
I created a migration lookup plugin with a static map that converts all D7 codes to the preferred D8 code. It might work well, but I am running into the problem that I can't get the migration plugin to work with the migration of the 'classified' migration configuration. So I decided to simplify it and set the format to a default value 'basic_html'.
Here is what I tried in the
The code snippets below only show the body configuration, because the other field configurations are generated with the drush command and not altered.
the default. Content is migrated, input format is set to '2'.
it should be done like this:
'body/format': plugin: default_value default_value: basic_html 'body/value': body
However, this results in the following error:
[error] InvalidArgumentException: Placeholders must have a trailing  if they are to be expanded with an array of values. in /path/to/dev/web/core/lib/Drupal/Core/Database/Connection.php).
[error] Placeholders must have a trailing  if they are to be expanded with an array of values. (/path/to/dev/web/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php:783)
I have no clue what is wrong and what I should do instead. I also don't know how to debug this to find out what it's expecting.
Can someone please give me some pointers?