Mongodb 在查询中创建别名
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Mongodb creating alias in a query
提问by Fatmuemoo
What is the mongodb's equivalent to this query:
mongodb 与此查询的等价物是什么：
SELECT "foo" as bar,
idas "spec" from tablename
SELECT "foo" as bar,
idas "spec" from tablename
回答by Andrey Dolgikh
It is possible to create new field with given name and value taken from another field with $project:
可以使用 $project 使用从另一个字段中获取的给定名称和值创建新字段：
{
"_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:
以下 $project 阶段添加了新字段 isbn、lastName 和 copySold：
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
http://docs.mongodb.org/manual/reference/operator/aggregation/project/#pipe._S_project
回答by user1735921
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.
您可以使用任何运算符，如 toUpper 或 toLower 或 concat，或者您认为可以使用并创建别名的任何其他运算符。
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)
示例：在以下示例中 created_time 是集合中的一个字段。（我不擅长语法，所以你可以纠正它，但这是方法）
{$project {
"ALIAS_one" : {"$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.
因此，以这种方式使用运算符，您可以创建任意数量的别名。
回答by Edward
you can use this, maybe help
你可以用这个，也许有帮助
database data
数据库数据
{ "_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
mongo 命令
db.zhb_test.aggregate(
[{
$group: {
_id: {
$dateToString: {
format: "%Y-%m",
date: "$lastmodified"
}
},
count: {
$sum: 1
}
}
},
{
$project: {
"month": "$_id",
count: 1,
"_id": 0
}
}])
result
结果
{ "count" : 2, "month" : "2018-02" }
{ "count" : 1, "month" : "2018-03" }
