Golang Firebase管理员SDK

Assuming a Firebase Datastore Db that is...

Collection("Collection1").
Document("documentX").
Collection("CollectionA").
field("FieldB")

In Golang, using firebase admin SDK... What would the code be to get value of FieldB in GoLang? this is what I've tried...

// dsnapp, err :=     client.Collection("Collection1/documentX").Get(ctx)
// dsnapp, err := client.Collection("Collection1").Document("documentX").Collection["CollectionA"].Get(ctx)

dsnapp2, err := client.Collection("Collection1").Doc("documentX").Get(ctx)
m = dsnapp2.Data()
fmt.Printf("222  Document data: %#v
", m["CollectionA"]["FieldB")

and assorted other techniques... any suggestions?