Mongodb creating alias in a query - mongodb

Question

What is the mongodb's equivalent to this query:
SELECT "foo" as bar, id as "spec" from tablename

Answer 1

It is possible to create new field with given name and value taken from another field with $project:
{
"_id" : 1,
title: "abc123",
isbn: "0001122223334",
author: { last: "zzz", first: "aaa" },
copies: 5
}
The following $project stage adds the new fields isbn, lastName, and copiesSold:
db.books.aggregate(
[
{
$project: {
title: 1,
isbn: {
prefix: { $substr: [ "$isbn", 0, 3 ] },
group: { $substr: [ "$isbn", 3, 2 ] },
publisher: { $substr: [ "$isbn", 5, 4 ] },
title: { $substr: [ "$isbn", 9, 3 ] },
checkDigit: { $substr: [ "$isbn", 12, 1] }
},
lastName: "$author.last",
copiesSold: "$copies"
}
}
]
)
http://docs.mongodb.org/manual/reference/operator/aggregation/project/#pipe._S_project

Answer 2

You can use any operator like toUpper or toLower or concat or any other operator you feel like which you think you can work on and create an alias.
Example:
In the following example created_time is a field in the collection.
(I am not good with syntax so you can correct it, but this is the approach)
{$project {
"ALIAS_one" : {"$concat" : "$created_time"},
"ALIAS_two" : {"$concat" : "$created_time"},
"ALIAS_three" : {"$concat" : "$created_time"}
}}
So using an operator in that fashion you can create as many as aliases as your like.

Answer 3

you can use this, maybe help
database data
{ "_id" : "5ab0f445edf197158835be63", "userid" : "5aaf15c28264ee17fe869ad8", "lastmodified" : ISODate("2018-03-21T07:04:41.735Z") }
{ "_id" : "5ab0f445edf197158835be64", "userid" : "5aaf15c28264ee17fe869ad8", "lastmodified" : ISODate("2018-02-20T12:31:08.896Z") }
{ "_id" : "5ab0f445edf197158835be65", "userid" : "5aaf15c28264ee17fe869ad7", "lastmodified" : ISODate("2018-02-20T02:31:08.896Z") }
mongo command
db.zhb_test.aggregate(
[{
$group: {
_id: {
$dateToString: {
format: "%Y-%m",
date: "$lastmodified"
}
},
count: {
$sum: 1
}
}
},
{
$project: {
"month": "$_id",
count: 1,
"_id": 0
}
}])
result
{ "count" : 2, "month" : "2018-02" }
{ "count" : 1, "month" : "2018-03" }