type_converters.md

title	description
Type converters	Store more complex data in columns with type converters

Moor supports a variety of types out of the box, but sometimes you need to store more complex data. You can achieve this by using TypeConverters. In this example, we'll use the the json_serializable package to store a custom object in a text column. Moor supports any Dart type for which you provide a TypeConverter, we're using that package here to make the example simpler.

import 'dart:convert';

import 'package:json_annotation/json_annotation.dart' as j;
import 'package:moor/moor.dart';

part 'database.g.dart';

@j.JsonSerializable()
class Preferences {
  bool receiveEmails;
  String selectedTheme;

  Preferences(this.receiveEmails, this.selectedTheme);

  factory Preferences.fromJson(Map<String, dynamic> json) =>
      _$PreferencesFromJson(json);

  Map<String, dynamic> toJson() => _$PreferencesToJson(this);
}

Next, we have to tell moor how to store a Preferences object in the database. We write a TypeConverter for that:

// stores preferences as strings
class PreferenceConverter extends TypeConverter<Preferences, String> {
  const PreferenceConverter();
  @override
  Preferences mapToDart(String fromDb) {
    if (fromDb == null) {
      return null;
    }
    return Preferences.fromJson(json.decode(fromDb) as Map<String, dynamic>);
  }

  @override
  String mapToSql(Preferences value) {
    if (value == null) {
      return null;
    }

    return json.encode(value.toJson());
  }
}

Finally, we can use that converter in a table declaration:

class Users extends Table {
  IntColumn get id => integer().autoIncrement()();
  TextColumn get name => text()();

  TextColumn get preferences =>
      text().map(const PreferenceConverter()).nullable()();
}

The generated User class will then have a preferences column of type Preferences. Moor will automatically take care of storing and loading the object in select, update and insert statements. This feature also works with [compiled custom queries]({{ "/queries/custom" | absolute_url }}).