How to use interface type as a model in mgo (Go)?

Question

Suppose you have a workflow that consists of multiple embedded nodes of different types. Since nodes are of different types, I thought of using Golang interfaces here and came up with following:

type Workflow struct {
   CreatedAt time.Time
   StartedAt time.Time
   CreatedBy string
   Nodes []Node
}

type Node interface {
  Exec() (int, error)
}

type EmailNode struct {
   From string
   To string
   Subject string
   Body string
}

type TwitterNode struct {
   Tweet string
   Image []byte
}

func (n *EmailNode) Exec() (int, error){
   //send email
   return 0, nil
}

func (n *TwitterNode) Exec() (int, error) {
   //send tweet
   return 0, nil
}

These workflows are stored in MongoDB and I have sample seed data in it. Using mgo, when I try to find a workflow (given its ID):

w = &Workflow{}
collection.FindID(bson.ObjectIdHex(id)).One(w)

I get the error - value of type bson.M is not assignable to type Node.

It also feels a bit weird to me that how would mgo unmarshal embedded Node documents into a Go struct without any type information. May be I need to look at the problem from another point of view.

Any suggestions would be highly appreciated.

Answer 1

You cannot use an interface in a document for the reason you noted. The decoder has no information about the type to create.

One way to handle this is to define a struct to hold the type information:

type NodeWithType struct {
   Node Node `bson:"-"`
   Type string
}

type Workflow struct {
   CreatedAt time.Time
   StartedAt time.Time
   CreatedBy string
   Nodes []NodeWithType
}

Implement the SetBSON function on this type. This function should decode the type string, create a value of the correct type based on that string and unmarshal to that value.

func (nt *NodeWithType) SetBSON(r bson.Raw) error {
}