relationships.md

Relationships

Sometimes your models are connected with each other. The following person class has a list of other persons called "friends". There could also be a list of other objects like "pets".

class Person extends HiveObject {
  String name;
  
  int age;
  
  List<Person> friends;
  
  Person(this.name, this.age);
}

You could just use a regular list to store the persons, but updating a person would be rather complicated because the person objects would be stored redundantly.

HiveLists

HiveLists provide an easy way to solve the problem above. They allow you to store a "link" to the actual object.

import 'package:hive/hive.dart';

void main() async {
  Hive.registerAdapter(PersonAdapter());
  var persons = await Hive.openBox<Person>('personsWithLists');
  persons.clear();
  
  var mario = Person('Mario');
  var luna = Person('Luna');
  var alex = Person('Alex');
  persons.addAll([mario, luna, alex]);
  
  mario.friends = HiveList(persons); // Create a HiveList
  mario.friends.addAll([luna, alex]); // Update Mario's friends
  mario.save(); // make persistent the change,
  print(mario.friends);
  
  luna.delete(); // Remove Luna from Hive
  print(mario.friends); // HiveList updates automatically
}

@HiveType()
class Person extends HiveObject {
  @HiveField(0)
  String name;

  @HiveField(1)
  HiveList friends;

  Person(this.name);

  String toString() => name; // For print()
}

class PersonAdapter extends TypeAdapter<Person> {
  @override
  final typeId = 0;

  @override
  Person read(BinaryReader reader) {
    return Person(reader.read())..friends = reader.read();
  }

  @override
  void write(BinaryWriter writer, Person obj) {
    writer.write(obj.name);
    writer.write(obj.friends);
  }
}

First, we store the three persons, Mario, Luna, and Alex in the persons box. HiveLists can only be used with objects which are currently in a box.

Next, we create a HiveList which contains Mario's friends. The HiveList constructor needs the HiveObject, which will contain the list. The list must not be moved to another HiveObject. The second parameter is the box, which contains the items of the list.

When you delete an object from a box, it is also deleted from all HiveLists. If you delete an object from a HiveList, it still remains in the box.