


How Can You Store Workflows with Diverse Node Types in MongoDB using Mgo (Go)?
Oct 26, 2024 pm 07:58 PMInterface as Model in Mgo (Go): Resolving the Assignability Error
When dealing with workflows comprising nodes of varying types, it may be tempting to leverage Golang interfaces to encapsulate their diverse behaviors. However, storing such workflows in MongoDB poses a unique challenge.
The Assignability Issue
The error encountered during the Find operation stems from the incompatibility between the decoded document and the Node interface. Mgo lacks the type information necessary to determine the specific type of each embedded node. This results in an assignment mismatch.
Alternative Approach: Embedding with Type Information
To overcome this hurdle, consider encapsulating the type information within the embedded nodes. Define a new struct, NodeWithType, which consists of the Node interface and a Type field representing the specific type.
<code class="go">type NodeWithType struct { Node Node `bson:"-"` Type string }</code>
Custom BSON Unmarshaling
To bridge the gap between the decoded document and the Go struct, implement the SetBSON function for NodeWithType. This function will decode the Type field, create an instance of the corresponding type, and delegate the unmarshaling to it.
<code class="go">func (nt *NodeWithType) SetBSON(r bson.Raw) error { // Decode Type field var typeName string err := bson.Unmarshal(r["Type"], &typeName) if err != nil { return err } // Create an instance of the correct type based on Type field var node Node switch typeName { case "EmailNode": node = &EmailNode{} case "TwitterNode": node = &TwitterNode{} } // Unmarshal the remaining fields into the created instance if err = node.SetBSON(r[typeName]); err != nil { return err } // Assign the created instance to the Node field nt.Node = node return nil }</code>
In the Workflow struct, replace the Node slice with a slice of NodeWithType.
<code class="go">type Workflow struct { CreatedAt time.Time StartedAt time.Time CreatedBy string Nodes []NodeWithType }</code>
With this approach, Mgo can now successfully find and unmarshal workflows with embedded nodes of different types.
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