title: Filter in golang mongodb
body: <p>Currently I learn to create restful api with golang and mongodb. Actually I am beginner in both. I use <a href="https://godoc.org/go.mongodb.org/mongo-driver" rel="noreferrer">mongodb-go-driver</a> and I learn to use filter when we want to use <code>find()</code> function. But I have some that I don't understand. What is the different between <code>filter := bson.M{"_id": "abcd"}</code> and <code>filter := bson.M{{"_id": "abcd"}}</code>? Thank you</p>