Migrating input formats causes errors 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 migrate_plus.migration.osn_custom_d7_node_classified.yml
The code snippets below only show the body configuration, because the other field configurations are generated with the drush command and not altered.

body: body


the default. Content is migrated, input format is set to '2'.

According to this: https://www.drupal.org/docs/8/api/migrate-api/migrate-process-plugins/migrate-process-overview#nested

and this: https://medium.com/drupal-stories-an-insiders-view/migrate-api-custom-drupal-to-drupal-migration-3bab05da4686

and this: https://www.phase2technology.com/blog/drupal-8-migrations

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?
Thanks to @Jdrupal I finally found the solution in this link.

  body:
    plugin: sub_process
    source: body
    process:
      value: value
      format:
        plugin: default_value
        default_value: basic_html


(sorry I can't seem to get the mention appropriately formatted)