Flutter how to link arrays by ids

You can use a nested for loop to get those values like the following and resend the loop to the method to get the nested values

getIndexFromNestedList(List<dynamic> mapValue) {
  for (var relation in mapValue) {
    if (relation['sub'] != null) {
      for (var subRelation in relation['sub']) {
        print({relation['id'], subRelation['id']});//you can add it directly to the map here.
        graph.addEdge(relation['id'], subRelation['id']);//like this
        Future.delayed(Duration(milliseconds:1), (){//future is added to finish the first loop then to enter into the inner loops.
          getIndexFromNestedList(relation['sub']);
        });
      }
    }
  }
}

//OUTPUT
{1, 2}
{1, 6}
{2, 3}
{2, 5}
{2, 3}
{2, 5}
{3, 4}
{3, 4}
{3, 4}
{3, 4}

you can use this method to set the values into the graph

getIndexFromNestedList(reList);

EDIT for the current structure you can check if the first value is not null before you access its children like the following

getIndexFromNestedList(List<dynamic> mapValue) {
  if (mapValue != null) {
    for (var relation in mapValue) {
      print(relation['subid']);
      print(relation['relativeEmail']);

      if (relation['subRelatives'] != null) {
        for (var subRelation in relation['subRelatives']) {
          print({relation['subid'], subRelation['subid']});
          // graph.addEdge(relation['subid'], subRelation['subid']); //like this
          Future.delayed(Duration(milliseconds: 1), () {
            getIndexFromNestedList(relation['subRelatives']);
          });
        }
      }
    }
  }
}

Edit on 7AUG

In the question again you missed to check if its a null value and if length is greater than 0. also you are passing relatives to the method.. The list name wassendlocal

Here is the updated method with length check too

  getIndexFromNestedList(List mapValue) {
    if (mapValue != null) {
      for (int i = 0; i < mapValue.length; i++) {
        for (int j = 0; j < mapValue[i]['subRelatives'].length; j++) {
          Map subList = mapValue[i]['subRelatives'][j];
          print({mapValue[i]['subid'], subList['subid']});
          if (subList['subRelatives'].length > 0) {
            Future.delayed(Duration(milliseconds: 10), () {
              getIndexFromNestedList(mapValue[i]['subRelatives']);
            });
          }
        }
      }
    }
  }

//Output
flutter: {1, 2}
flutter: {1, 5}
flutter: {2, 3}
flutter: {2, 4}

Tested this multiple times and removed all duplicates too.

I got your answer as per your requirement, maybe this helpful for you

void main(){
  var check =getIndexFromList(sendlocal);
}

getIndexFromList(List sendlocal) {
    if (sendlocal != null) {
      for (var relation in sendlocal) {
        print('subid:- ${relation['subid']}');
        print('relativeEmail:- ${relation['relativeEmail']}');

        if (relation['subRelatives'] != null && relation['subRelatives'].length > 0) {
          for (var subRelation in relation['subRelatives']) {
            
            //its your required response
            print({relation['subid'], subRelation['subid']});
            
            Future.delayed(Duration(milliseconds: 2), () {
              getIndexFromList(relation['subRelatives']);
            });
          }
        }
      }
    }
  }


var sendlocal = [
  {
    "firstName": "tree",
    "lastName": "tree",
    "relativeEmail": "tree@gmail.com",
    "relativeType": 0,
    "subid": 1,
    "subRelatives": [
      {
        "firstName": "julia2",
        "lastName": "Michle",
        "relativeEmail": "test@hotmail.com3",
        "relativeType": 2,
        "subid": 2,
        "subRelatives": [
          {
            "firstName": "john",
            "lastName": "bravo",
            "relativeEmail": "johny@gmail.com",
            "relativeType": 1,
            "subRelatives": [],
            "subid": 3,
          },
          {
            "firstName": "simith",
            "lastName": "bravo",
            "relativeEmail": "johny@gmail.com",
            "relativeType": 1,
            "subRelatives": [],
            "subid": 4,
          },
        ],
      },
      {
        "firstName": "julia3",
        "lastName": "Michle",
        "relativeEmail": "test3@hotmail.com",
        "relativeType": 2,
        "subRelatives": [],
        "subid": 5,
      },
    ],
  },
]; 

The response is something like that,

 subid:- 1
    relativeEmail:- tree@gmail.com
    {1, 2}
    {1, 5}
    subid:- 2
    relativeEmail:- test@hotmail.com3
    {2, 3}
    {2, 4}
    subid:- 5
    relativeEmail:- test3@hotmail.com
    subid:- 2
    relativeEmail:- test@hotmail.com3
    {2, 3}
    {2, 4}
    subid:- 5
    .............
    .............