您如何计算Node内MongoDB集合中的文档数量？

Question

Quite an odd situation, I've tried numerous solutions and I can't seem to crack it.

A lot of the resources I've found only include mongo console commands, I'm not sure how you'd write this in Node.js.

The reason I'm trying to work this out is I'm trying to make each documents 'id' iterate from the last, so the attempt is to find the amount of documents in a collection, add one and then use that number as the 'id' for the new document.

Answer 1

To get the number of documents in a collection, use db.collection.find({}).count() .

However, what you are trying to do will not work. When you have a lot of parallel accesses to the database, then it is possible that multiple threads do this at the same time, receive the same count and will thus insert a document with the same id. According to the CAP theorem , a distributed database like MongoDB can not provide this kind of consistency.

What you should do instead is rely on MongoDB ObjectId 's as unique identifiers for documents. MongoDB generates these automatically for each document when you don't provide an own value for _id . ObjectId's are globally unique (unique enough for any practical purpose), so you won't get any collisions. They also begin with a timestamp, so when you order by _id you get a roughly chronological order (as previously stated, a strict chronological order is impossible to provide by a distributed system).

As a rule of thumb, whenever you would use AUTO_INCREMENT in SQL, you would likely use ObjectId's in MongodDB.