stackingSubjectsWSelector.js

const bookListSelector = state => ({
    subjects: Object.assign({}, state.bookList.subjects, {list: stackAndGetTopLevelSubjects(state.bookList.subjects.list)}),
    books: Object.assign({}, state.bookList.books, {list: booksWithSubjectsSelector(state.bookList)})
});

function stackAndGetTopLevelSubjects(subjectsHash){
    let subjects = Object.keys(subjectsHash).map(_id => subjectsHash[_id]);
    subjects.forEach(s => {
        s.children = [];
        s.children.push(...subjects.filter(sc => new RegExp(`,${s._id},$`).test(sc.path)));
    });
    return subjects.filter(s => s.path == null);
}