About initial configuration, qmgo will refer to some of the official mongoDB driver, such as using ReadPref instead of Mgo's Mode

	cfg = Config{
		Uri:              "mongodb://localhost:27017",
		Database:         "user",
		Coll:             "classs",
		ReadPreference:   &ReadPref{Mode: readpref.SecondaryMode, MaxStalenessMS: 500},
	}
	cli, err := Open(context.Background(), &cfg)

• Most API need Context as parameter

• Different implement about ObjectId，for example,ObjectId is string in mgo and [12]byte in qmgo

• Update interfaces(UpdateOne、UpdateAll...) must contain key beginning with '$': Following codes are illegal in qmgo or official mongoDB driver, which is legal in mgo

	filter := bson.M{
		"name": "Alice",
	}
	update := bson.M{
	//	"$set": bson.M{    // this is required in qmgo or official mongoDB driver, not required in mgo
			"name": "Alice1",
			"age":  18,
	//	},
	}
	err = cli.UpdateOne(context.Background(), filter, update)

• The name of interface

    Update(...) -> UpdateOne(...)
    Insert(...) -> InsertOne(...)

• Insert uint32 data become BSON int64 in MongoDB, sometimes cause error when consume it.
  • Issue in qmgo.
  • Issut to fix the difference when call distinct().
  • Issue in Mongo-official-driver.
  • Before official MongoDB driver fix it, you can update (or insert) bson:"foo" tags to look like bson:"foo,minsize".
• Mgo will initial the field to default value of Go when it's not initialed, Qmgo will not. For example, if you insert a field of slice type without initialization, it is []in mongoDB when use Mgo, and null when use Qmgo https://github.com/qiniu/qmgo/issues/117