Migrations can now opt into validation for content entities

Summary

The Drupal 8 content entities are using typed data that can be validated (see \Drupal\Core\Entity\ContentEntityBase::validate()).

The good news is that core's migrate module now supports entity validation that may help to sift out undesired content from being imported. Let's take a look at the configuration:

id: my_migration
source:
  # ...
process:
  # ...
destination:
  plugin: 'entity:node'
  default_bundle: page
  # Run entity and fields validation before saving an entity.
  # @see \Drupal\Core\Entity\FieldableEntityInterface::validate()
  validate: true

The only thing added is validate: true. This simple addition can easily be brought to either new or existing migration yamls to avoid the aftermath of importing broken data. This is not turned on by default as it could lead to failures in migrations that were previously functional.

Detailed description

The \Drupal\Core\Entity\FieldableEntityInterface::isValidationRequired() returns FALSE by default for the most entity types. (see \Drupal\Core\Entity\ContentEntityBase::$validationRequired) which makes valid the execution of the code like \Drupal::entityTypeManager()->getStorage('node')->create(['type' => 'page'])->save(), despite the fact that title is mandatory. That's also how the import to content entity behaves; by default migrate module - literally creates an object from source and fires the save() method, without calling validate(). In some cases missing validation may result in broken entities that would be hard to find after a migration.