matchAuthors(model, options) {
            let ownerUser;
            const ops = [];

            ops.push(() => {
                return ghostBookshelf
                    .model('User')
                    .getOwnerUser(Object.assign({}, _.pick(options, 'transacting')))
                    .then((_ownerUser) => {
                        ownerUser = _ownerUser;
                    });
            });

            ops.push(() => {
                const authors = model.get('authors');
                const authorsToSet = [];

                return Promise.all(authors.map((author, index) => {
                    const query = {};

                    if (author.id) {
                        query.id = author.id;
                    } else if (author.slug) {
                        query.slug = author.slug;
                    } else if (author.email) {
                        query.email = author.email;
                    }

                    return ghostBookshelf
                        .model('User')
                        .where(query)
                        .fetch(Object.assign({columns: ['id']}, _.pick(options, 'transacting')))
                        .then((user) => {
                            let userId = user ? user.id : ownerUser.id;

                            // CASE: avoid attaching duplicate authors relation
                            const userExists = _.find(authorsToSet, {id: userId.id});

                            if (!userExists) {
                                authorsToSet[index] = {};
                                authorsToSet[index].id = userId;
                            }
                        });
                })).then(() => {
                    model.set('authors', authorsToSet);
                });
            });

            return sequence(ops);
        }
Authors relation is special. You cannot add new authors via relations.
But you can for the tags relation. That's why we have to sort this out before
we trigger bookshelf-relations.

@TODO: Add a feature to bookshelf-relations to configure if relations can be added or should be matched only.