The goal is to be able to set a status for post type for user.
Example: User can mark the post type (in this case 'book') as one of - 'want to read', 'currently reading', 'already read'.
I see I can add a relation between post type and user, but I want the relation to be an array.
I'm afraid that it is not possible to add a status custom field for a relationship. You can create a cloneable group, inside it, create two fields. One is field type user, one is field type select (3 status options). Then each cloneable group is a user with a status.
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