How Can You Embed Go Interfaces in MongoDB Models Using mgo?

Embedding Interfaces in MongoDB Models with mgo

When working with a workflow system that involves nodes of varying types, it's common to use Go interfaces to represent the different node types. However, this approach presents a challenge when storing these workflows in MongoDB using mgo.

Consider the following example:

<code class="go">type Workflow struct {
   CreatedAt time.Time
   StartedAt time.Time
   CreatedBy string
   Nodes []Node
}

type Node interface {
  Exec() (int, error)
}</code>

Where Node is an interface with a single method, Exec() that can be implemented by different node types.

To store a workflow in MongoDB, you may attempt to find it by ID:

<code class="go">w = &Workflow{}
collection.FindID(bson.ObjectIdHex(id)).One(w)</code>

However, you will encounter an error because mgo cannot unmarshal embedded Node interfaces directly into a Go struct. This is because it lacks the necessary type information to create the appropriate concrete type for each node.

To address this issue, you can define a struct that explicitly holds both the node type and the actual node value:

<code class="go">type NodeWithType struct {
   Node Node `bson:"-"`
   Type string
}

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

By using bson:"-", the Node subfield is excluded from automatic decoding and instead handled manually in the SetBSON function:

<code class="go">func (nt *NodeWithType) SetBSON(r bson.Raw) error {
    // Decode the node type from the "Type" field
    // Create a new instance of the concrete node type based on the decoded type
    // Unmarshal the remaining document into the concrete node type
    //...
}</code>

This approach allows mgo to identify the type of each node and create the appropriate concrete instance during decoding, resolving the type information issue.

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