Flutter - Sembast Database insert List of Objects

Question

I'm about to use the database "Sembast" in Flutter. Simple objects with data types like string and int are working properly. However, it becomes problematic when using Lists.

I have created an example and oriented myself on the following tutorial: https://resocoder.com/2019/04/06/flutter-nosql-database-sembast-tutorial-w-bloc/ In my example, there are fruits and leaves as objects. A fruit contains a list of leaves.

class Fruit {
  final String id;
  final String name;
  final bool isSweet;
  final List<Leaves> leaves;
...
}

class Leaves {
  final String id;
  final String name;
...
}

//Create a sample object
var leaveOne = Leaves(id: "1", name: "leaveOne");
var leaveTwo = Leaves(id: "2", name: "leaveTwo");
var leaveThree = Leaves(id: "3", name: "leaveThree");

var leavesList = List<Leaves>();
leavesList.add(leaveOne);
leavesList.add(leaveTwo);
leavesList.add(leaveThree);

var fruit = Fruit(id: "1", name: "Apple", isSweet: true, leaves: leavesList);
_fruitDao.insert(fruit);


// The fruitDao.insert makes following
Future insert(Fruit fruit) async {
  await _fruitStore.add(await _db, fruit.toJson());
}

The JSON looks like that: {id: 1, name: Apple, isSweet: true, leaves: [Instance of 'Leaves', Instance of 'Leaves', Instance of 'Leaves']}

The ERROR is following: [ERROR:flutter/lib/ui/ui_dart_state.cc(148)] Unhandled Exception: Invalid argument(s): value Instance of 'Leaves' unsupported type Leaves

alextk · Accepted Answer · 2019-05-13 11:43:11Z

7

As pointed at, Instance of 'Leaves' is not a valid type so each Leave must be converted as well. Hard to guess what you are doing without seeing your toJson() but something like this should work (could be largely optimized):

class Fruit {
  final String id;
  final String name;
  final bool isSweet;
  final List<Leaves> leaves;

  Fruit({this.id, this.name, this.isSweet, this.leaves});

  Map<String, dynamic> toJson() => <String, dynamic>{
        'id': id,
        'name': name,
        'isSweet': isSweet,
        'leaves': leaves?.map((leave) => leave.toJson())?.toList(growable: false)
      };
}

class Leaves {
  final String id;
  final String name;

  Leaves({this.id, this.name});

  Map<String, dynamic> toJson() => <String, dynamic>{'id': id, 'name': name};
}

and your json should look like something this this:

{
  "id": "1",
  "name": "Apple",
  "isSweet": true,
  "leaves": [
    {
      "id": "1",
      "name": "leaveOne"
    },
    {
      "id": "2",
      "name": "leaveTwo"
    },
    {
      "id": "3",
      "name": "leaveThree"
    }
  ]
}

edited May 13, 2019 at 11:43

answered May 13, 2019 at 10:44

alextk

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1 Comment

max Over a year ago

Thanks for reply! I'm using "json_serializable" - package. So this generates me the code for the function toJson() and fromJson(). If I edit this generated file (usually you should not do this), and paste your code to map the leaves ('leaves': leaves?.map((leave) => leave.toJson())?.toList(growable: false)) - it's working fine.

user9990112 · Accepted Answer · 2019-06-17 19:25:48Z

3

Here is an example in addition to @alextk answer with converting to and from without any code generation or library's.

class Fruit {
  final String id;
  final String name;
  final bool isSweet;
  final List<Leaves> leaves;

  Fruit({this.id, this.name, this.isSweet, this.leaves});

  Map<String, dynamic> toMap() {
    return {
      'id': id,
      'name': name,
      'isSweet': isSweet,
      'leaves': leaves.map((leave) => leave.toMap()).toList(growable: false)
    };
  }

  static Fruit fromMap(Map<String, dynamic> map) {
    return Fruit(
      id: map['id'],
      name: map['name'],
      isSweet: map['isSweet'],
      leaves: map['leaves'].map((mapping) => Leaves.fromMap(mapping)).toList().cast<Leaves>(),
    );
  }
}

class Leaves {
  final String id;
  final String name;

  Leaves({this.id, this.name});

  Map<String, dynamic> toMap() {
    return {
      'id': id,
      'name': name,
    };
  }

  static Leaves fromMap(Map<String, dynamic> map) {
    return Leaves(
      id: map['id'],
      name: map['name'],
    );
  }
}

answered Jun 17, 2019 at 19:25

user9990112

1 Comment

Fardeen Khan Over a year ago

Instead of using a static fromMap method you can very well use a Factory method

Murat Kurbanov · Accepted Answer · 2022-11-20 12:06:24Z

2

For cases when you use freezed, I found this solution:

...
...
@freezed
class Fruit with _$Fruit {
//add this line
@JsonSerializable(explicitToJson: true)
const factory Fruit({...}) = _Fruit;

factory Fruit.fromJson(Map<String, dynamic> json) => _$FruitFromJson(json);
}

answered Nov 20, 2022 at 12:06

Murat Kurbanov

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