Migrate media with alternative text I'm trying to import image with new media module on core 8.5.

I've a json source and on my .yml I've:

  field_media_image:
    target_id:
      plugin: migration
      migration: immagini
      source: fid


This works, immagini is another importer that imports files.

In the new media there is also an "alt" (mandatory) and "title" attribute on the media entity.
But if I try to add something like this, it doesn't work (alt_text is the correct source for alternative text). The media is created but is not referenced to the correct id:

  field_media_image:
    alt: alt_text
    target_id:
      plugin: migration
      migration: immagini
      source: fid


How should I pass the alt attribute to this field?
I also tried to use a syntax like:

  field_media_image:
    target_id:
      plugin: migration
      migration: immagini
      source: fid
  'field_media_image/alt': alt_text


But it doesn't work, the media is created but there is no reference to the correct id.
field_media_image/alt belongs in the immagini migration, not in the node migration.

In migration that creates the media item (destination plugin entity:media) you can map the media item's fields:

  # Image field see media_entity.bundle.image.yml.
  field_media_image/target_id: fid
  field_media_image/alt: field_file_image_alt_text/0/value
  field_media_image/title: field_file_image_title_text/0/value

  # Description field.
  field_description: field_image_description
  # Caption field.
  field_caption: field_caption

destination:
  plugin: entity:media


See https://www.previousnext.com.au/blog/migrating-drupal-7-file-entities-drupal-8-media-entities