There is no explicit documentation for the Drupal 8 version of the flag module, and now that the Drupal 8 version has hit alpha, it's a good time to share some information on it.
In Drupal 8, migration destinations are easy to set up for entities. Since flags are entities in Drupal 8, all you need to do is set up a standard entity migration configuration with the fields for creating a flag.
The fields are:
flag_id - The flag's bundle machine name in Drupal
entity_type - The type of entity the flag is referencing
entity_id - The entity ID of the entity the flag is referencing
global - Globalization
uid - The user who made the flag
In an ideal situation, you have the data needed for these, with just a normal field mapping you can set up the configuration like this:
In most cases when coming from a custom CMS, the data will have to be manipulated in some way. In that case you can use your migrate source class in the same way done for any other entity migration.
For more information on the Flag module or the Migrate API, check out their pages on drupal.org. If you have any questions, feel free to comment below.
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