add a process plugin for extracting an entity value

Closing #2926200: Add process plugin for loading values from existing entities as duplicate to here..

1. +++ b/src/Plugin/migrate/process/EntityValue.php
   @@ -0,0 +1,113 @@
   + *     field_property: uri
   

   Can we remove this option and just return the whole field? If we need to cherry-pick specific values, we can use extract process plugin.

2. +++ b/src/Plugin/migrate/process/EntityValue.php
   @@ -0,0 +1,113 @@
   +    if (empty($this->configuration['entity_type'])) {
   +      throw new MigrateException("'entity_type' configuration must be specified for migrate_plus_entity_value process plugin.");
   +    }
   +    if (empty($this->configuration['field_name'])) {
   +      throw new MigrateException("'field_name' configuration must be specified for migrate_plus_entity_value process plugin.");
   +    }
   

   Can we move all this to the constructor and return a BadPluginDefinitionException instead of migration exception?

3. +++ b/src/Plugin/migrate/process/EntityValue.php
   @@ -0,0 +1,113 @@
   +    $entity_type = $this->configuration['entity_type'];
   +
   +    $entity = $this->entityTypeManager->getStorage($entity_type)->load($value);
   +
   +    if (empty($entity)) {
   +      throw new MigrateException("Unable to load an {$entity_type} entity with ID {$value} in migrate_plus_entity_value process plugin.");
   

   And check the entity type is a valid entity type in the constructor and return an InvalidArgumentException if it isn't? This also in constructor.

4. +++ b/src/Plugin/migrate/process/EntityValue.php
   @@ -0,0 +1,113 @@
   +    try {
   +      $field_value = $entity->get($this->configuration['field_name'])->{$field_property};
   ...
   +    catch (\Exception $e) {
   +      // Re-throw any exception thrown by the entity system.
   +      throw new MigrateException("Got exception reading field value {$this->configuration['field_name']} on 'entity_type' {$entity_type} entity with ID {$value} in migrate_plus_entity_value process plugin:" . $e->getMessage());
   

   I like this! I forgot to ask for that in the other issue. This is great.

Also, needs tests.

Should this maybe be based on the entity lookup plugin? I mean, the difference could be very small and we could share most of the code I believe.

Following the unix phliosophy of doing just one thing well, i'd propose that related issue.

Much improved work in the other issue.

I have updated the patch with code from "#3018849: Add a LoadEntity process plugin" but with loading concrete field.

tests are now in the patch

I'm actively working with the patch from the other issue and think it obsoletes this one.

I can see the other issue has the status "Closed (won't fix)" now so I've used the patch in this issue. I can confirm I can extract the field value correctly from the loaded entity.
In my case I combined it with a 'migration_lookup' plugin where I had to extract the 'fid' (file id) of a previously migrated Media entity and worked great:

  field_avatar:
    - plugin: migration_lookup
      source: picture
      migration: images_to_media
      no_stub: true
    - plugin: entity_value
      entity_type: media
      field_name: field_media_image

Really helpful, that's what I needed.

Thanks for your contributions.

Thanks to all people involved here!

For reference, I closed a related gitlab issue #262: Retrieve a field from an existing entity, with a similar plugin.