How to add capability to author role to assign custom taxonomy terms to a custom post type?

Question

my question title says it all.

I am not sure if I should approach this by adding a capability to the author role or adding a capabilities array when I register the custom post type or even when I register the custom taxonomy.

If the correct approach is to add a capabilities array to the arguments to register_post_type it is still not clear to me from the documentation how this works. (I don't even understand the concept of adding a capability to a post because a post is not a role; there seems to be some suggestion that this array is a mapping e.g. assign_terms =>edit_posts will allow roles who have edit posts capabilities to assign terms - but that did not work for me).

The documentation definitely does not explain this.

Edit1: I don't need the ability for authors to manage taxonomy terms - just see the existing terms and assign them to the (custom) post.

Thanks

Answer 1

Yes. It is some kind of mapping. So now, when I register the custom taxonomy I pass it:

 $caps = array(
            'manage_terms'  => 'activate_plugins',
            'edit_terms'    => 'activate_plugins',
            'delete_terms'  => 'activate_plugins',
            'assign_terms'  => 'edit_posts'
        );

With this result authors can now assign terms. I am just using 'activate_plugins' because, by default, authors do not have this capability and so by the mapping they cannot manage terms. If I put edit_posts here then they can manage the terms and I don't want that.

So, what this seems to be saying, is "for this custom taxonomy any role which has the edit_posts capability will be able to assign terms from this taxonomy to any post which the taxonomy is registered to"

While this actually seems to work it is still opaque to me what is going on. I would be interested in any further explanations.