So can you tell us what's your easieast way to implement something like:

    books: [
        {id: 1, tags: ['a', 'b'], author: ['c', 'd'], count_read: 123, count_bought: 456},
    ]

where count_read and count_bought is atomic increment, tags are arbitrary string array and could be indexed for searching.

Yeah I knew Postgre could do that. But MongoDB is the most simple and direct way on the market. Stuff like tags makes MySQL m2m joins very inefficient.