How to add Clone and Copy functions in mocking MongoDB in Go?

Question

I read this article and it has good guides for mocking MongoDB in Go. But there are some problems in Clone() and Copy() methods. I create this interfaces and structs:

type ISession interface {
    DB(name string) IDatabase
    Close()
    Clone() ISession
    Copy() ISession
}

type IDatabase interface {
    C(name string) ICollection
}

type MongoSession struct {
    dbSession *mgo.Session
}

func (s MongoSession) DB(name string) IDatabase {
    return &MongoDatabase{Database: s.dbSession.DB(name)}
}

func (s MongoSession) Clone() ISession {
    //return session.clone
    return s.dbSession.Clone()
}

func (s MongoSession) Copy() ISession {
    return s.dbSession.Copy()
}

But I got this error

cannot use s.dbSession.Clone() (type *mgo.Session) as type ISession in return argument: *mgo.Session does not implement ISession (wrong type for Clone method) have Clone() *mgo.Session want Clone() ISession

cannot use s.dbSession.Copy() (type *mgo.Session) as type ISession in return argument: *mgo.Session does not implement ISession (wrong type for Clone method) have Clone() *mgo.Session want Clone() ISession

How can I add Clone() and Copy() methods to interface?

Answer 1

MongoSession.Copy() and MongoSession.Clone() must return a value that implements ISession. Basically you create MongoSession type exactly for this: to implement ISession.

mgo.Session does not implement your ISession interface (e.g. because its Session.Clone() method has a return type of *mgo.Session and not ISession). You should create and return a new value of MongoSession, in which you can wrap the copied or cloned *mgo.Session value.

Like this:

func (s MongoSession) Clone() ISession {
    return MongoSession{dbSession: s.dbSession.Clone()}
}

func (s MongoSession) Copy() ISession {
    return MongoSession{dbSession: s.dbSession.Copy()}
}