Mongodb：如何在一次操作中更新一些元素并删除数组的其余部分？

Question

To an array I want to update some elements and keep array's size no great than N. How to do it in an single operation? Lets' make an example to make my question clear:

1)db.test.insertOne({a:[1, 100, 3, 5, 600]});
{ 
    "_id" : ObjectId("5fc5a14e1b800790b21afa65"), 
    "a" : [
        1.0, 
        100.0, 
        3.0, 
        5.0, 
        600.0
    ]
}
2) change it to
{ 
    "_id" : ObjectId("5fc5a14e1b800790b21afa65"), 
    "a" : [
        1.0, 
        100.0, 
        5.0
    ]
}

As shown above, a[2]->5, and N=3, only keeps a[0],a[1],a[2]. I have to make these happen in one single operation to make data consistent(don't consider multiple clients) How could I do? Thanks!

Answer 1

In your case, I'd recommend to follow this logic:

1. If the array in the memory has not been changed - do nothing.
2. If your array was changed and it's small then just update the whole array.
3. If your array was changed only a little bit and it's pretty big then calculate in your app what exactly elements were changed and create an update with corresponding operations for everyone (remove, update, insert). Be careful, it's an error-prone approach.

If your app runs into case number 3 "often", it means you designed a wrong data structure to solve your problem.