Mongodb在查询中创建别名 [英] Mongodb creating alias in a query

问题描述

此查询等效于mongodb:

What is the mongodb's equivalent to this query:

从表名中选择"foo"作为栏，将id作为"spec"

SELECT "foo" as bar, id as "spec" from tablename

推荐答案

可以使用$ project创建具有给定名称和值的新字段，该字段取自另一个字段:

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
}

接下来的$ project阶段添加了新字段isbn，lastName和copysSold:

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/聚合/项目/#pipe._S_project

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