Mongodb在查询中创建别名

Question

mongodb与此查询等效的是什么:

选择"foo"作为bar,id作为tablename的"spec"

Answer 1

可以使用$ project从另一个字段创建具有给定名称和值的新字段:

{
  "_id" : 1,
  title: "abc123",
  isbn: "0001122223334",
  author: { last: "zzz", first: "aaa" },
  copies: 5
}

以下$ project阶段添加了新字段isbn,lastName和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