如何将字符串转换为日期然后在猫鼬查询中减去日期？

Question

Can I subtract string to string with date format?

Example:

I have two fields: request and response (String format: yyyy/mm/dd HH:mm:ss).

I have record:

{
_id: 1,
request: 2015/12/28 12:10:10,
response: 2015/12/28 12:10:15
}

and expect result:

{
_id: 1,
duration: 5
}

Can we convert string to date then using $subtract (aggregation) to subtract two value?

I try to search and found nothing yet.

Answer 1

To convert the string to date format you would have to iterate the results from a find() operation and update the collection within the loop:

db.collection.find({}).forEach(function(doc) { 
    db.collection.update(
        { "_id": doc._id }, 
        {
            "$set": { 
                "request": new Date(doc.request), 
                "response": new Date(doc.response) 
            }
        }
    );
});

Perfomance with the above update operation can be compromised if dealing with large collections, however using the Bulk API can streamline the updates for maximised efficiency by reducing the amount of update operations sent to the sever, sending once every 1000 queued operations:

var bulk = db.collection.initializeOrderedBulkOp(),   
    counter = 0;

db.collection.find({
    "request": { "$exists": true, "$type": 2 }, 
    "response": { "$exists": true, "$type": 2 }
}).forEach(function(doc) {     
    bulk.find({ "_id": doc._id }).updateOne({
        "$set": { 
            "request": new Date(doc.request), 
            "response": new Date(doc.response) 
        }
    });

    counter++;
    if (counter % 1000 == 0) {
        // Execute per 1000 operations and re-initialize every 1000 update statements
        bulk.execute();
        bulk = db.collection.initializeOrderedBulkOp();
    }
});

// Clean up queues
if (counter % 1000 != 0){
    bulk.execute();
}

Having updated the fields to proper date formats, you can then run the following aggregation pipeline to get the desire results:

db.collection.aggregate([
    {
        "$project": {
            "duration": { "$subtract": [ "$response", "$request" ] }
        }
    }
])