Migrate Process Entity

This module provides an additional process plugin to get field values from Drupal 8 entities within a migration.

Use

In your migration, you may want to retrieve a key field value from an entity which already exists. This could be one manually created, or one created via an upstream migration.

In your migration, use like any other process plugin:

field_title:
  source: 'migrated_entity_id'
  plugin: entity_value
  entity_type: my_custom_entity
  field: field_name

Typically, this plugin is combined with an entity_lookup to get the target entity ID from an upstream migration:

field_title:
  -
    plugin: migration_lookup
    migration: the_upstream_migration
    entity_type: my_custom_entity
    no_stub: true
  -
    plugin: entity_value
    entity_type: my_custom_entity
    field: field_name

Often, you may need the upstream entity ID in addition to its value. In that cause, you can use a 'psudo-pipeline' to only lookup the entity once:

_migrated_entity_id
    plugin: migration_lookup
    migration: the_upstream_migration
    entity_type: my_custom_entity
    no_stub: true
field_entity_ref: '@_migrated_entity_id'
field_title:
    source: '@_migrated_entity_id'
    plugin: entity_value
    entity_type: my_custom_entity
    field: field_name

You may also use constants and the concat module with multiple psudo-pipelines to create more complex values.

Comparison with entity_exists and entity_lookup

Both core's entity_exists and Migrate Plus's entity_lookup can look up an existing Drupal entity and return its ID. entity_value can also do this, but that's not it's primary purpose.

The primary purpose is to retrieve field values from the entity instead of just the ID.

Best practices

The entity_value plugin duplicates data, and as such, is really only intended for niche use. If possible, you should always rely on an entity reference field instead.

One use case is in a complex content model, an entity may require a title for the sake of the admin UI. That title may need to be generated based on which entities it references. In that case, this module is appropriate.