The migrate_lookup migration process plugin can take a list of migrations, in which case, it returns the lookup value from the first migration that produced a result.
eg.
field_destination:
-
plugin: migration_lookup
no_stub: true
migration:
- migration_alpha
- migration_beta
source_ids:
migration_alpha:
- id
- language
migration_beta:
- id
- language
If migration_alpha has a map row for the values, but that row has NULLs in the destination IDs, then these are returned instead of advancing to try migration_beta.
This means that if migration_alpha needs to skip rows, those rows are saved in the map with a source_row_status of STATUS_IGNORED, and rows from migration_beta won't be used at all.