在mongodb shell JavaScript forEach函数中编写关注

Question

The mongo shell defaults to safe writes, which from my understanding happens at the end of every carriage return. What if you have code in a loop like this:

db.coll1.find().forEach(function(doc){
    db.coll2.update({"blah": doc._id}, {$set: {"blahblah": doc.value}});
});

Does the db.getLastError() occur for every single update or only at the very end of the for loop on the last update? Or does it happen at the end of the for loop for every updated document, all at one time?

Answer 1

The shell actually has w:1 (safe writes) when in interactive mode, when running in a loop it will not call getLastError until the end.

As reference you can actually see this comment by @Asya who works for MongoDB Inc.

the shell uses safe in that it called getLastError after every "command" (ie carriage return). If you are writing data, say, in a loop then GLE will only be called once at the end. Provide more details about how you plan to populate collection from the shell - maybe the right thing will already happen

Setting MongoDB's write concern in shell / shell script

Answer 2

Adding to that answer from Sammaye, if you wanted to call GLE after each update call you would do something like this:

db.coll1.find().forEach(function(doc){
    db.coll2.update({"blah": doc._id}, {$set: {"blahblah": doc.value}}, true);
    if(db.getLastError(1)){
        printjson(db.getLastError(1)); 
        printjson("failed to update ID: "+doc._id)
    }; 
});

You'll get a dupe error at the end of the loop because of the carriage return if the last op fails, but otherwise it's pretty much what you would expect. If you want an easy test to reproduce just set the value field to be a string, then use $inc instead of $set - that will fail on non-numerics.