Mongodb: how to auto-increment a subdocument field?

Question

The following document records a conversation between Milhouse and Bart. I would like to insert a new message with the right num (the next in the example would be 3) in a unique operation. Is that possible ?

    {   user_a:"Bart",
        user_b:"Milhouse",
        conversation:{
            last_msg:2,
            messages:[
                {   from:"Bart",
                    msg:"Hello"
                    num:1
                },
                {   from:"Milhouse",
                    msg:"Wanna go out ?"
                    num:2
                }
            ]
        }
     }

Answer 1

In MongoDB, arrays keep their order, so by adding a num attribute, you're only creating more data for something that you could accomplish without the additional field. Just use the position in the array to accomplish the same thing. Grabbing the X message in an array will provide faster searches than searching for { num: X } .

To keep the order, I don't think there's an easy way to add the num category besides does a find() on conversation.last_msg before you insert the new subdocument and increment last_msg .

Depending on what you need to keep the ordering for, you might consider including a time stamp in your subdocument, which is commonly kept in conversation records anyway and may provide other useful information.

Also, I haven't used it, but there's a Mongoose plugin that may or may not be able to do what you want: https://npmjs.org/package/mongoose-auto-increment

Answer 2

I would recommend using a timestamp rather than a numerical value. By using a timestamp, you can keep the ordering of the subdocument and make other use of it.

Answer 3

您无法创建自动增量字段，但可以使用函数生成和管理序列： http ： //docs.mongodb.org/manual/tutorial/create-an-auto-incrementing-field/