使用Golang的mgo库在执行$group聚合操作时如何返回结果值？

Question

如何使聚合查询返回在$group语句中使用的字段值。

代码如下：

type TheGroup struct{
    Id bson.ObjectId `json:"id,omitempty" bson:"_id,omitempty"` 
    Totalamount int
    Dayofyear int
    Actualyear string
    Transactiondate string
    Count int
}

var results []TheGroup

o1 := bson.M{"$match" :bson.M{"transactiontype": transactiontype},}
o2 := bson.M{"$group" : bson.M{"_id": bson.M{"day": "$dayofyear", "year":"$actualyear"},"totalamount":bson.M{"$sum":"$qty"}, "count":bson.M{"$sum":1}},}
operations := []bson.M{o1, o2}
pipe := collection.Pipe(operations)
err1 := pipe.All(&results)

if err := json.NewEncoder(w).Encode(results); err != nil {
panic(err)
}

输出结果如下：

[{"Totalamount":2061,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":679},{"Totalamount":8705,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":2145},{"Totalamount":8156,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":2806},{"Totalamount":9865,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":3294},{"Totalamount":9619,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":3102},{"Totalamount":9975,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":3457},{"Totalamount":14839,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":4036},{"Totalamount":5100,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":1699},{"Totalamount":9649,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":2854},{"Totalamount":11457,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":3220},{"Totalamount":12643,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":3860},{"Totalamount":10301,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":3620},{"Totalamount":7681,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":2816},{"Totalamount":8130,"Dayofyear":0,"Actualyear":"","Transactiondate":"","Count":3023}]

我认为我明白了为什么Dayofyear/Actualyear没有被填充 - 因为在$group遍历匹配的文档时，文档值没有进行聚合 - 但是我该如何使它们填充呢？

- mommabear

---

为什么输出中没有 _id？dayofyear 和 actualyear 应该在 _id 中。 - dgiugg

我已经添加了输出生成的方式。我应该用不同的方法吗？ - mommabear

Answer 1

在Mongo shell中运行以下管道应该给您正确的结果：

pipeline = [
    {
        "$match": { "transactiontype": transactiontype }
    },
    { 
        "$group": {
            "_id": {
                "year": { "$year": "$transactiondate" },
                "dayOfYear": { "$dayOfYear": "$transactiondate" }               
            },
            "totalamount": { "$sum": "$qty" }, 
            "count": { "$sum": 1 },
            "date": { "$first": "$transactiondate"}
        }
    },
    {
        "$project": {
            "_id": 0,
            "totalamount": 1,
            "dayOfYear": "$_id.dayOfYear",
            "actualyear": { "$substr": [ "$_id.year", 0, 4 ] },
            "transactiondate": { 
                "$dateToString": { "format": "%Y-%m-%d %H:%M", "date": "$date" } 
            },
            "count": 1
        }
    }
]
db.collection.aggregate(pipeline)

等效的 mGo 表达式如下（未经测试）：

pipeline := []bson.M{   
    bson.M{
        "$match": bson.M{ "transactiontype": transactiontype }
    },
    bson.M{
        "$group": bson.M{
            "_id": bson.M{
                "year": bson.M{ "$year": "$transactiondate" },
                "dayOfYear": bson.M{ "$dayOfYear": "$transactiondate" }             
            },
            "totalamount": bson.M{ "$sum": "$qty" }, 
            "count": bson.M{ "$sum": 1 },
            "date": bson.M{ "$first": "$transactiondate"}
        }
    },
    bson.M{
        "$project": bson.M{
            "_id": 0,
            "totalamount": 1,
            "dayOfYear": "$_id.dayOfYear",
            "actualyear": bson.M{ "$substr": []interface{}{ "$_id.year", 0, 4 } },
            "transactiondate": bson.M{ 
                "$dateToString": bson.M{ "format": "%Y-%m-%d %H:%M", "date": "$date" } 
            },
            "count": 1
        }
    }   
}

pipe := collection.Pipe(pipeline)

- chridam

---

我收到了 无法将bson.M文字（类型为bson.M）用作数组或切片文字中的bson.D类型 的错误信息（类似情况）。 - arnoudhgz

@arnoudhgz 将 bson.D 更改为 bson.M，这样可以编译。 - chridam