json(references: References): DocumentChanged {
    if (this.attr === "id") {
      throw new Error("'id' field should never change, whatever code just set it is wrong")
    }

    if (this.hint != null)
      return this.hint.json(references)

    const value = this.new_
    const value_json = HasProps._value_to_json(this.attr, value, this.model)
    const value_refs: {[key: string]: any}  = {}
    HasProps._value_record_references(value, value_refs, true) // true = recurse
    if (this.model.id in value_refs && this.model !== value) {
      // we know we don't want a whole new copy of the obj we're
      // patching unless it's also the value itself
      delete value_refs[this.model.id]
    }
    for (const id in value_refs) {
      references[id] = value_refs[id]
    }
    return {
      kind: "ModelChanged",
      model: this.model.ref(),
      attr: this.attr,
      new: value_json,
    }
  }

 static _value_to_json(key, value, optional_parent_object) {
   if (isObject(value) && (key === 'data')) {
     return serialization.encode_column_data(value, optional_parent_object._shapes)
   } else {
     return HasProps._value_to_json(key, value, optional_parent_object)
   }
 }

 static _value_to_json(key: string, value: any, optional_parent_object: any): any {
   if (isPlainObject(value) && key === 'data')
     return encode_column_data(value as any, optional_parent_object._shapes) // XXX: unknown vs. any
   else
     return HasProps._value_to_json(key, value, optional_parent_object)
 }