如果创建两个收藏

首先存储要为其创建自动递增编号的字段，例如

First to store the field for whichyou want to create autoincrement number like

// Inserting 2 records for which we want to create auto-increment numbers
db.autoincrementcollection.insert({
   field_id: "register_id",
   nextId: 0
},
{
   field_id: "user_id",
   nextId: 0
});

// Function is used to get the next autoincremented value
function getNextId(field)
{
   var ret = db.autoincrementcollection.findAndModify({
        query: { field_id: field },
        update: { $inc: { nextId: 1 } },
        new: true
   });    
   return ret.nextId;
}

第二，您要在其中使用递增的数字，例如

Second in which you want to use the incremented number like

// inserting data in registration collection with register_id auto-increment number
db.registration.insert({
     _id: getNextId("register_id"),
     fname: "Rohan",
     lname:"Kumar"
});
// inserting data in user collection with user_id auto-increment number
db.users.insert({
     _id: getNextId("user_id"),
     name: "Rohan Kumar"
});

findAndModify()可能无法在使用 PHP 的代码中工作，然后可以使用 update()

findAndModify() may not work while code using PHP then you can use find() and/or update()

有关创建的文档 -自动递增字段

Refer Docs for create-an-auto-incrementing-field