Auto calculating fields in mongodb

问题:

Let's say there are documents in MongoDB, that look something like this:

{
    "lastDate" : ISODate("2013-14-01T16:38:16.163Z"),
    "items":[
        {"date":ISODate("2013-10-01T16:38:16.163Z")},
        {"date":ISODate("2013-11-01T16:38:16.163Z")},
        {"date":ISODate("2013-12-01T16:38:16.163Z")},
        {"date":ISODate("2013-13-01T16:38:16.163Z")},
        {"date":ISODate("2013-14-01T16:38:16.163Z")}        
    ]
}

Or even like this:

{
    "allAre" : false,
    "items":[
        {"is":true},
        {"is":true},
        {"is":true},
        {"is":false},
        {"is":true}        
    ]
}

The top level fields "lastDate" and "allAre" should be recalculated every time the data in array changes. "lastDate" should be the biggest "date" of all. "allAre" should be equal to true only if all the items have "is" as true.

How should I build my queries to achieve such a behavior with MongoDB?
Is it considered to be a good practice to precalculate some values on insert, instead of calculating them during the get request?

回答1:

MongoDB cannot make what you are asking for with 1 query. But you can make it in two-step query.

First of all, push the new value into the array:

db.Test3.findOneAndUpdate(
{_id: ObjectId("58047d0cd63cf401292fe0ad")},
{$push: {"items":  {"date": ISODate("2013-01-27T16:38:16.163+0000")}}},
{returnNewDocument: true},
function (err, result) {

}
);

then update "lastDate" only if is less then the last Pushed.

  db.Test3.findOneAndUpdate (
   {_id: ObjectId("58047d0cd63cf401292fe0ad"), "lastDate":{$lt: ISODate("2013-01-25T16:38:16.163+0000")}},
   {$set: {"lastDate": ISODate("2013-01-25T16:38:16.163+0000")}},
   {returnNewDocument: true},
   function (err, result) {
   }
  ); 

the second parameter "lastDate" is needed in order to avoid race condition. In this way you can be sure that inside "lastDate" there are for sure the "highest date pushed".

Related to the second problem you are asking for you can follow a similar strategy. Update {"allAre": false} only if {"_id":yourID, "items.is":false)}. Basically set "false" only if some child of has a value 'false'. If you don't found a document with this property then do not update nothing.

// add a new Child to false
db.Test4.findOneAndUpdate(
{_id: ObjectId("5804813ed63cf401292fe0b0")},
{$push: {"items":  {"is": false}}},
{returnNewDocument: true},
 function (err, result) {

}
);

// update allAre to false if some child is false
db.Test4.findOneAndUpdate (
   {_id: ObjectId("5804813ed63cf401292fe0b0"), "items.is": false},
   {$set: {"allAre": false}},
   {returnNewDocument: true},
   function (err, result) {
   }
  );