How to convert flat array of objects (with possibly multiple parents) into a nested array of objects

Question

I have an array of tasks that are linked to one-another through parentId's. However any task could have multiple parents. Therefor not necessarily a simple single tree hierarchical structure. What I would like to achieve is instead of parents[], I would like to have a children []. However I cant seem to get my head around how to approach this recursively. Any help would be greatly appreciated.

Here is the Flat JSON array

[
  {
    "_id": "4b04e450-06d5-4453-8d50-d3b2a70d9b2d",
    "task_name": "Parent2",
    "parents": []
  },
  {
    "_id": "a15ca08e-f13b-4d73-a496-ba23832ea233",
    "task_name": "Endpoints",
    "parents": [
      {
        "_id": "97bbf892-8a2a-4f45-befd-4fdbebded04b",
        "task_name": "Parent1"
      },
      {
        "_id": "4b04e450-06d5-4453-8d50-d3b2a70d9b2d",
        "task_name": "Parent2"
      }
    ]
  },
  {
    "_id": "ee78316a-491e-4db5-8f82-13b12b5b86fc",
    "task_name": "Mapping",
    "parents": [
      {
        "_id": "97bbf892-8a2a-4f45-befd-4fdbebded04b",
        "task_name": "Parent1"
      }
    ]
  },
  {
    "_id": "97bbf892-8a2a-4f45-befd-4fdbebded04b",
    "task_name": "Parent1",
    "parents": []
  }
]

What I would like to achieve is the following

[
  {
    "_id": "97bbf892-8a2a-4f45-befd-4fdbebded04b",
    "task_name": "Parent1",
    "children": [
      {
        "_id": "ee78316a-491e-4db5-8f82-13b12b5b86fc",
        "task_name": "Mapping",
        "children": []
      },
      {
        "_id": "a15ca08e-f13b-4d73-a496-ba23832ea233",
        "task_name": "Endpoints",
        "children": []
      }
    ]
  },
  {
    "_id": "4b04e450-06d5-4453-8d50-d3b2a70d9b2d",
    "task_name": "Parent2",
    "children": [
      {
        "_id": "a15ca08e-f13b-4d73-a496-ba23832ea233",
        "task_name": "Endpoints",
        "children": []
      }
    ]
  }
]

What I have tried

function transform(list, idAttr, parentAttr, childrenAttr) {
    if (!idAttr) idAttr = '_id';
    if (!parentAttr) parentAttr = 'parents';
    if (!childrenAttr) childrenAttr = 'children';

    var newArr = [];
    var lookup = {};
    list.forEach(function(obj) {
        lookup[obj[idAttr]] = obj;
        obj[childrenAttr] = [];
    });
    list.forEach(function(obj) {
        if (obj[parentAttr] != null) {
            lookup[obj[parentAttr]][childrenAttr].push(obj);
        } else {
            newArr.push(obj);
        }
    });
    return newArr;
};

This works fine if the parents key in the original array is equal to the _id of the task. However I am not sure how to get this to work for a parent key that has an array of objects as a value.

Answer 1

You can keep the the flat JSON array structure and store only Id's for cross referencing like so (ids are just examples)

    [
        {
            "_id": "97bbf892-8a2a-4f45-befd-4fdbebded04b",
            "task_name": "Task1",
            "children": [
                "ee78316a-491e-4db5-8f82-13b12b5b86fc",
                "a15ca08e-f13b-4d73-a496-ba23832ea233"
            ],
            "parents": [
                "ee78316a-491e-4db5-8f82-13b12b5b86fc",
                "a15ca08e-f13b-4d73-a496-ba23832ea233"
            ]
        },
        {
            "_id": "97bbf892-8a2a-4f45-befd-4fdbebded04b",
            "task_name": "Task2",
            "children": [
                "ee78316a-491e-4db5-8f82-13b12b5b86fc",
                "a15ca08e-f13b-4d73-a496-ba23832ea233"
            ],
            "parents": [
                "ee78316a-491e-4db5-8f82-13b12b5b86fc",
                "a15ca08e-f13b-4d73-a496-ba23832ea233"
            ]
        },
        {
            "_id": "97bbf892-8a2a-4f45-befd-4fdbebded04b",
            "task_name": "Task3",
            "children": [
                "ee78316a-491e-4db5-8f82-13b12b5b86fc",
                "a15ca08e-f13b-4d73-a496-ba23832ea233"
            ],
            "parents": [
                "ee78316a-491e-4db5-8f82-13b12b5b86fc",
                "a15ca08e-f13b-4d73-a496-ba23832ea233"
            ]
        }
    ]

Or if you want to achieve it the way you currently desire you can use this function. Not completely tested but you get the point

var tasks =     [
  {
    "_id": "4b04e450-06d5-4453-8d50-d3b2a70d9b2d",
    "task_name": "Parent2",
    "parents": []
  },
  {
    "_id": "a15ca08e-f13b-4d73-a496-ba23832ea233",
    "task_name": "Endpoints",
    "parents": [
      {
        "_id": "97bbf892-8a2a-4f45-befd-4fdbebded04b",
        "task_name": "Parent1"
      },
      {
        "_id": "4b04e450-06d5-4453-8d50-d3b2a70d9b2d",
        "task_name": "Parent2"
      }
    ]
  },
  {
    "_id": "ee78316a-491e-4db5-8f82-13b12b5b86fc",
    "task_name": "Mapping",
    "parents": [
      {
        "_id": "97bbf892-8a2a-4f45-befd-4fdbebded04b",
        "task_name": "Parent1"
      }
    ]
  },
  {
    "_id": "97bbf892-8a2a-4f45-befd-4fdbebded04b",
    "task_name": "Parent1",
    "parents": []
  }
];

function transformTasks(tasks){
    var transformed = {};
    var transformedAsArray = [];
    for(var taskIndex in tasks){
        var task = tasks[taskIndex];
        if(transformed[task["_id"]] === undefined){
            transformed[task["_id"]] = {_id:task["_id"],task_name:task["task_name"],children:[]};
            transformedAsArray.push(transformed[task["_id"]]);
        }
        for(var parentIndex in task["parents"]){
            var parentTask = task["parents"][parentIndex];
            if(transformed[parentTask["_id"]] === undefined){
                transformed[parentTask["_id"]] = {_id:parentTask["_id"],task_name:parentTask["task_name"],children:[]};
                transformedAsArray.push(transformed[parentTask["_id"]]);
            }
            transformed[parentTask["_id"]].children.push(transformed[task["_id"]]);

        }
    }

    return transformedAsArray;
}

console.log(transformTasks(tasks));

Answer 2

You could take an object with the _id as key and store the parents as well with their relation. At the end take the children of the items without parents.

var data = [{ _id: "4b04e450-06d5-4453-8d50-d3b2a70d9b2d", task_name: "Parent2", parents: [] }, { _id: "a15ca08e-f13b-4d73-a496-ba23832ea233", task_name: "Endpoints", parents: [{ _id: "97bbf892-8a2a-4f45-befd-4fdbebded04b", task_name: "Parent1" }, { _id: "4b04e450-06d5-4453-8d50-d3b2a70d9b2d", task_name: "Parent2" }] }, { _id: "ee78316a-491e-4db5-8f82-13b12b5b86fc", task_name: "Mapping", parents: [{ _id: "97bbf892-8a2a-4f45-befd-4fdbebded04b", task_name: "Parent1" }] }, { _id: "97bbf892-8a2a-4f45-befd-4fdbebded04b", task_name: "Parent1", parents: [] }],
    tree = function (data, root) {
        var t = {};
        data.forEach(({ parents, ...o }) => {
            Object.assign(t[o._id] = t[o._id] || {}, o);
            if (!parents.length) {
                t[root] = t[root] || {};
                t[root].children = t[root].children || [];
                t[root].children.push(t[o._id]);
                return;
            }
            parents.forEach(p => {
                Object.assign(t[p._id] = t[p._id] || {}, p);
                t[p._id].children = t[p._id].children || [];
                t[p._id].children.push(t[o._id]);
            });
        });
        return t[root].children;
    }(data);

console.log(tree);

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