使用where子句的mongodb中的最大查询

debugcn 发表于 Dev

mongoDB

我想用where子句在MongoDB中创建一个MAX查询。

我尝试这个

db.Collection.aggregate({$group : {_id : "$P_ID", massi : {$max : "$b_val"}}},{P_ID:'XYZ',experiment:'abc'});

我的WHERE子句是 {P_ID:'XYZ',experiment:'abc'}

我不为什么，但是此查询不起作用获取此错误：

assert: command failed: {
    "ok" : 0,
    "errmsg" : "A pipeline stage specification object must contain          exactly one field.",
    "code" : 16435
} : aggregate failed
_getErrorWithCode@src/mongo/shell/utils.js:25:13
doassert@src/mongo/shell/assert.js:13:14
assert.commandWorked@src/mongo/shell/assert.js:287:5
DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1312:5
@(shell):1:1

   2016-10-15T11:55:31.459+0200 E QUERY    [thread1] Error: command failed: {
        "ok" : 0,
        "errmsg" : "A pipeline stage specification object must contain     exactly one field.",
    "code" : 16435
} : aggregate failed :
_getErrorWithCode@src/mongo/shell/utils.js:25:13
doassert@src/mongo/shell/assert.js:13:14
assert.commandWorked@src/mongo/shell/assert.js:287:5
DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1312:5
@(shell):1:1

4J41

Where子句必须处于$match阶段。而且，最好将管道作为数组而不是单独的参数。

查询必须看起来像这样。

db.Collection.aggregate([
    {
        $match : {P_ID:'XYZ',experiment:'abc'}
    },
    {
        $group : {
            _id : "$P_ID", 
            massi : {$max : "$b_val"}
        }
    }
]);

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