[{"data":{"books":[{"id":"631664448cb20310bc25c89d","tags":["database","middle-layer"]},{"id":"6316945f8995f05ac71d3b22","tags":["relational","database"]},]}}]

db.collection.aggregate([{"$unwind":"$data.books"},{"$unwind":"$data.books.tags"},{$group:{_id:"$data.books.tags"}}])