genqlient/generate/unmarshal_helper.go.tmpl

{{/* This template generates a helper for each interface type genqlient must
     unmarshal.  This helper is called by the UnmarshalJSON of each (struct)
     type with a field of the interface type, similar to how it calls custom
     unmarshalers.  The helper itself is fairly simple: it just parses out and
     switches on __typename, then unmarshals into the relevant struct. */}}

func __unmarshal{{.GoName}}(b []byte, v *{{.GoName}}) error {
    if string(b) == "null" {
        return nil
    }

    var tn struct {
        TypeName string `json:"__typename"`
    }
    err := {{ref "encoding/json.Unmarshal"}}(b, &tn)
    if err != nil {
        return err
    }

    switch tn.TypeName {
    {{range .Implementations -}}
    case "{{.GraphQLName}}":
        *v = new({{.GoName}})
        return {{ref "encoding/json.Unmarshal"}}(b, *v)
    {{end -}}
    case "":
        {{/* Likely if we're making a request to a mock server and the author
             of the mock didn't know to add __typename, so give a special
             error. */ -}}
        return {{ref "fmt.Errorf"}}(
            "response was missing {{.GraphQLName}}.__typename")
    default:
        return {{ref "fmt.Errorf"}}(
            `unexpected concrete type for {{.GoName}}: "%v"`, tn.TypeName)
    }
}