Convert Collection to Capped with mgo

Question

I would like to convert a mongo collection to capped using gopkg.in/mgo.v2.

I am able to create a capped collection from scratch - as follows:

# Create a Capped Collection
sess.DB("").C("my_collection").Create(&mgo.CollectionInfo{Capped: true, MaxBytes: tenMB, MaxDocs: 10})

I can't work out how to get the stats for an existing collection or how to run convertToCapped command.

Step 1 - Get Collection Stats:

# Mongo
db.getCollection('my_collection').stats();

# mgo // I need to find out how to do this.

Step 2 - Convert to Capped

# Mongo
db.runCommand({"convertToCapped": "my_collection", size: 1000000});

# mgo // I need to find out how to do this.
if err := sess.DB("").Run(bson.D{{"convertToCapped", "my_collection"}, {"size", "1000"}}, nil); err != nil {
        println(err.Error()) // invalid command spec
        os.Exit(1)
}

Answer 1

1. Get Collection Stats

This is not part of the mgo package, but it is available as a runnable MongoDB command: collStats.

Running collStats with mgo:

var doc bson.M
err := mgr.sess.DB("").Run(bson.D{
    {Name: "collStats", Value: "my_collection"},
}, &doc)
if err != nil {
    fmt.Println("Error:", err)
    return
}

fmt.Println("Result:", doc)
fmt.Println("Capped:", doc["capped"])
// Acquire capped info as a bool value:
capped, ok := doc["capped"].(bool)
if ok {
    fmt.Println("Capped bool value:", capped)
}

Example output (veeeeeeery long so chunked):

Result: map[ok:1 size:36 storageSize:4096 totalIndexSize:4096  ...chunked...]
Capped: true
Capped bool value: true

2. Converting to Capped

The simple problem with your attempt is that the value of the "size" parameter must be a number and not a string. Quoting from the convertToCapped doc:

The command has the following syntax:

  { convertToCapped: <collection>, size: <capped size> }

convertToCapped takes an existing collection (<collection>) and transforms it into a capped collection with a maximum size in bytes, specified by the size argument (<capped size>).

Correct version:

var doc bson.M
err := mgr.sess.DB("").Run(bson.D{
    {Name: "convertToCapped", Value: "my_collection"},
    {Name: "size", Value: 1<<20}, // 1 MB
}, &doc)
if err != nil {
    fmt.Println("Error:", err)
    return
}

fmt.Println("Result:", doc)

Example output:

Result: map[ok:1]