Mongodb creating alias in a query

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.