如何使用golang获取mongodb中数组中的最后一个元素？

Question

I'm working in go language. I'm using mgo driver to get the data from mongodb. I want last entry from the events array.

There will be huge data in future. so I don't want to read the whole record but just want specific data from the record.

db.events.find({"_id":"59ce53b9-970a-44a2-8419-b41a99120b25"},{"events":{$slice:-1}}).pretty()

this is working in mongo shell. I want this to work in go lang.

This is the sample data, from which I want last entry present in events.

    {
    "_id" : "59ce53b9-970a-44a2-8419-b41a99120b25",
    "version" : 9,
    "events" : [
        {
            "event_type" : "customer:added",
            "data" : {
                "id" : "59ce53b9-970a-44a2-8419-b41a99120b25",
                "name" : "arjun"
            },
            "timestamp" : ISODate("2017-11-20T12:21:34.910Z"),
            "aggregate_type" : "customer",
            "_id" : "59ce53b9-970a-44a2-8419-b41a99120b25",
            "version" : 1
        },
        {
            "event_type" : "customer:address-updated",
            "data" : {
                "id" : "59ce53b9-970a-44a2-8419-b41a99120b25",
                "address" : "bangalore"
            },
            "timestamp" : ISODate("2017-11-20T12:22:08.496Z"),
            "aggregate_type" : "customer",
            "_id" : "59ce53b9-970a-44a2-8419-b41a99120b25",
            "version" : 2
        }
    ]
}

Answer 1

What you pass as the 2nd argument to find() is a projection.

Projections in mgo can be specified using the Query.Select() method.

So your query in mgo simply looks like this:

sess := ... // Acquire MongoDB session
c := sess.DB("dbname").C("events")

var doc bson.M

err := c.FindId("59ce53b9-970a-44a2-8419-b41a99120b25").
    Select(bson.M{"events": bson.M{"$slice": -1}}).
    One(&doc)

if err != nil {
    // Handle error
}
fmt.Println(len(doc["events"].([]interface{}))) // This prints 1
fmt.Println(doc)