Querying a mongo DB collection into a struct

Defining this struct

type SymbolMCAddrPort struct {
	    ID        bson.ObjectId `bson:"_id,omitempty"`
        Symbol    string
        MCAddr 	  string
        MCPort 	  int
}

session, err := mgo.Dial("10.0.0.61")
if err != nil {
    panic(err)
}

defer session.Close()

csap := session.DB("FX").C("MCAddrPortPairs")

If I say

var resultsSMP bson.M
err = csap.Find(bson.M{"Symbol": "EUR/USD"}).One(&resultsSMP)
fmt.Println(resultsSMP)

I correctly see

map[_id:ObjectIdHex("56fc34e961fed32064e656b0") Symbol:EUR/USD MCAddr:239.0.0.222 MCPort:345]

But if I say

resultsSMP := SymbolMCAddrPort{}
err = csap.Find(bson.M{"Symbol": "EUR/USD"}).One(&resultsSMP)

if err != nil {
	panic(err)
}

fmt.Println(resultsSMP)

I just see

{ObjectIdHex("56fc34e961fed32064e656b0")   0}

I note that the ID is correct, but I can't get the rest of the fields in the struct?

Use tags to hint Unmarshal what the key names for each field are.

type SymbolMCAddrPort struct {
   ID        bson.ObjectId `bson:"_id,omitempty"`
   Symbol    string `bson:"Symbol"`
   MCAddr    string `bson:"MCAddr"`
   MCPort    int `bson:"MCPort"`
}

From documentation of Unmarshal,

> The lowercased field name is used as the key for each exported field,
> but this behavior may be changed using the respective field tag.

So by default, when you are using a struct, it expects keys to be lowercased values of field names. When the key name should be anything else field tags have to be used to specify the key name.