I am using the Dart library json_path to filter JSON objects. When I try to filter objects based on a field value, the filter works if the object values are stored inside an array, while it does not if it is a regular field of the parent object, see the example. I tried several syntax variations. All work on the array but not on the single object, what am I missing?

import 'dart:convert';

import 'package:json_path/json_path.dart';

void main() {
  // This example works, note the inner array with just one element
  final docArray = jsonDecode('''
{
  "store": {
     "auto":    
     [  
      {
      "price": 10
    }  
    ]
  }
}    ''');

  // This example does not work, single field instead of array
  final docObject = jsonDecode('''
{
  "store": {
     "auto":     
      {
      "price": 10
    }  
  }
}    ''');
  print('\nauto   Object :');
  JsonPath(r'$.store.auto[[email protected] < 20]').readValues(docObject).forEach(print);

  print('auto   Array:');
  JsonPath(r'$.store.auto[[email protected] < 20]').readValues(docArray).forEach(print);
}

Output:

auto object :

auto Array: {price: 10}

Process finished with exit code 0

I am using the Dart library json_path to filter JSON objects. When I try to filter objects based on a field value, the filter works if the object values are stored inside an array, while it does not if it is a regular field of the parent object, see the example. I tried several syntax variations. All work on the array but not on the single object, what am I missing?

import 'dart:convert';

import 'package:json_path/json_path.dart';

void main() {
  // This example works, note the inner array with just one element
  final docArray = jsonDecode('''
{
  "store": {
     "auto":    
     [  
      {
      "price": 10
    }  
    ]
  }
}    ''');

  // This example does not work, single field instead of array
  final docObject = jsonDecode('''
{
  "store": {
     "auto":     
      {
      "price": 10
    }  
  }
}    ''');
  print('\nauto   Object :');
  JsonPath(r'$.store.auto[[email protected] < 20]').readValues(docObject).forEach(print);

  print('auto   Array:');
  JsonPath(r'$.store.auto[[email protected] < 20]').readValues(docArray).forEach(print);
}

Output:

auto object :

auto Array: {price: 10}

Process finished with exit code 0

• arrays
• dart
• jsonpath

• I'm not sure that I understand what you expect to get back from that first JSON path query, as you're already selecting the auto property. If you want all properties of store with a price < 20, that'd be JsonPath(r'$.store[[email protected] < 20]'). – Frank van Puffelen Commented Nov 20, 2024 at 19:15

1 Answer 1

With the $.store.auto in your expression, you already point to one property in store (auto). So that's a single node, meaning that there is no array to filter on (and a filter expression always returns an array).

If you want to get a list of the properties that have a price < 20, you can do:

JsonPath(r'$.store[[email protected] < 20]')

It's a bit easier to see in this screenshot:

I added a second property in the store object, and added a name attribute to them to make it easier to see what nodes are being selected.

My test bed on Zapp.run: https://zapp.run/edit/jsonpath-testing-zs9u06qxs9v0