total rewrite to interface handling; not complete but it compiles

generate/unmarshal.go

@@ -0,0 +1,51 @@
+package generate
+
+var unmarshalTemplate = mustTemplate("unmarshal.go.tmpl")
+
+type templateData struct {
+	// Go type to which the method will be added
+	Type string
+	// Abstract fields of the type, which need special handling.
+	Fields []abstractField
+}
+
+type abstractField struct {
+	// Name of the field, in Go and JSON
+	GoName, JSONName string
+	// Concrete types the field might take.
+	ConcreteTypes []concreteType
+}
+
+type concreteType struct {
+	// Name of the type, in Go and GraphQL
+	GoName, GraphQLName string
+}
+
+func (builder *typeBuilder) maybeWriteUnmarshal(fields []selection) error {
+	data := templateData{Type: builder.typeName}
+	for _, field := range fields {
+		typedef := builder.schema.Types[field.Type().Name()]
+		if typedef.IsAbstractType() {
+			fieldInfo := abstractField{
+				GoName:   upperFirst(field.Alias()),
+				JSONName: field.Alias(),
+			}
+			for _, typedef := range builder.schema.GetPossibleTypes(typedef) {
+				fieldInfo.ConcreteTypes = append(fieldInfo.ConcreteTypes,
+					concreteType{
+						// TODO: lies!  We might have added "1" or something.
+						GoName:      upperFirst(typedef.Name),
+						GraphQLName: typedef.Name,
+					})
+			}
+			data.Fields = append(data.Fields, fieldInfo)
+		}
+	}
+
+	if len(data.Fields) == 0 {
+		return nil
+	}
+
+	builder.WriteString("\n\n")
+	return unmarshalTemplate.Execute(builder, data)
+}