How to create a json in dart (Exception Instance of '_CompactLinkedHashSet) - json

Question

I'm trying to convert some objects to json to send it to DB, this is the code I have now:
Map<String, dynamic> bag = {};
bag = {
'colors' : [
for(int i = 0; i < product.colors.length; i ++){
'id': product.colors[i].id,
'desc': product.colors[i].desc,
'sizes': [
for(int j=0; j < product.colors[i].sizes.length; j ++){
product.colors[i].sizes[j].toJson()
}
]
}]
};
print(json.encode(bag));
but the result I get is this:
Converting object to an encodable object failed: Instance of '_CompactLinkedHashSet<Map<String, dynamic>>'
How can I solve it?
Product class:
final String id;
final String desc;
final List<Colors> colors;
Colors:
final String id;
final String desc;
final List<Prices> prices;
Prices:
final String id;
final String desc;
final List<Types> types;
Types:
final String id;
final String desc;
final List<Sizes> sizes;
Sizes:
final String id;
final int stock;
final String desc;
final bool filled;
int quantity;
final double price;
I want the json like this:
colors:
id : 12
desc : black
sizes: [
id : 0202202
stock : 10
desc: Full Size
filled: true
quantity : 2
price : 200
]

Answer 1

The way you are creating the JSON is way too complicated. Try instead add a toJson() method on each of your classes. This method is automatically called by the Dart JSON encoder if it hits an object that is not compatible with JSON.
Here is an example:
import 'dart:convert';
class Product {
final String id;
final String desc;
final List<Colors> colors;
Product({
required this.id,
required this.desc,
required this.colors,
});
Map<String, Object> toJson() => {
'id': id,
'desc': desc,
'colors': colors,
};
}
class Colors {
final int id;
final String desc;
final List<Prices> prices;
Colors({
required this.id,
required this.desc,
required this.prices,
});
Map<String, Object> toJson() => {
'id': id,
'desc': desc,
'prices': prices,
};
}
class Prices {
final String id;
final String desc;
final List<Types> types;
Prices({
required this.id,
required this.desc,
required this.types,
});
Map<String, Object> toJson() => {
'id': id,
'desc': desc,
'types': types,
};
}
class Types {
final String id;
final String desc;
final List<Sizes> sizes;
Types({
required this.id,
required this.desc,
required this.sizes,
});
Map<String, Object> toJson() => {
'id': id,
'desc': desc,
'sizes': sizes,
};
}
class Sizes {
final String id;
final int stock;
final String desc;
final bool filled;
int quantity;
final double price;
Sizes({
required this.id,
required this.stock,
required this.desc,
required this.filled,
required this.quantity,
required this.price,
});
Map<String, Object> toJson() => {
'id': id,
'stock': stock,
'desc': desc,
'filled': filled,
'quantity': quantity,
'price': price,
};
}
void main() {
final product = Product(id: '1', desc: 'Some text', colors: [
Colors(id: 1, desc: '', prices: [
Prices(id: '1', desc: '', types: [
Types(id: '1', desc: '', sizes: [
Sizes(
id: '1',
stock: 5,
desc: '',
filled: true,
quantity: 2,
price: 1.0)
])
])
]),
Colors(id: 2, desc: '', prices: [
Prices(id: '1', desc: '', types: [
Types(id: '1', desc: '', sizes: [
Sizes(
id: '5',
stock: 5,
desc: '',
filled: true,
quantity: 2,
price: 1.0),
Sizes(
id: '50',
stock: 5,
desc: '',
filled: true,
quantity: 2,
price: 1.0),
])
])
])
]);
final bag = {
'colors': [
for (final colors in product.colors)
{
'id': colors.id,
'desc': colors.desc,
'sizes': [
for (final prices in colors.prices)
for (final types in prices.types) ...types.sizes
]
}
]
};
print(const JsonEncoder.withIndent(' ').convert(bag));
}
Output:
{
"colors": [
{
"id": 1,
"desc": "",
"sizes": [
{
"id": "1",
"stock": 5,
"desc": "",
"filled": true,
"quantity": 2,
"price": 1.0
}
]
},
{
"id": 2,
"desc": "",
"sizes": [
{
"id": "5",
"stock": 5,
"desc": "",
"filled": true,
"quantity": 2,
"price": 1.0
},
{
"id": "50",
"stock": 5,
"desc": "",
"filled": true,
"quantity": 2,
"price": 1.0
}
]
}
]
}
A trick here is that Dart will call object inside a structure returned from another toJson() call. So e.g. Product.toJson returns a Map<String, Object> which contains a list of colors. Dart are then calling the toJson() on each of these colors and so on.

Answer 2

This should help you, it's better to convert to json when you have maps, so you need to convert the class and it's subclasses to maps, and in the end it will be simple to convert to json.
Product class to map:
class Product {
final String id;
final String desc;
final List<Colors> colors;
Product(this.id, this.desc, this.colors);
Map toMap() {
return {
'id': this.id,
'desc': this.desc,
'colors': this.colors.map((color) => color.toMap()).toList(),
};
}
}
Colors class to map:
class Colors {
final String id;
final String desc;
final List<Sizes> sizes;
Colors(this.id, this.desc, this.sizes);
Map toMap() {
return {
'id': this.id,
'desc': this.desc,
'sizes': this.sizes.map((size) => size.toMap()).toList(),
};
}
}
Sizes class to map:
class Sizes {
final String id;
final int stock;
final String desc;
final bool filled;
int quantity;
final double price;
Sizes(this.id, this.stock, this.desc, this.filled, this.quantity, this.price);
Map toMap() {
return {
'id': this.id,
'stock': this.stock,
'desc': this.desc,
'filled': this.filled,
'quantity': this.quantity,
'price': this.price,
};
}
}
Result:
Product product = Product(...);
Map map = product.toMap();
String json = jsonEncode(map);