Make node tree with recursive table with Express and Mongo

Question

I'm working in a REST api with ExpressJS and Mongo and I have a collection with N quantity of levels.

So to solve this problem I'm using an recursive table (or collection) in mongo where a field is the id and every register has a parent_id which is at the same level as it's childs. To explain better this, here is an E-R representation

So as you se, mongo will save the data like this json (accounts level 0 has null parent)

[
  { "id": "45TYYU", "parent_id": null, "name":"account 1", "type": 1, "category": 1 },
  { "id": "45TYYXT", "parent_id": "45TYYU", "name":"account 2", "type": 1, "category": 1 },
  { "id": "45TYYPZ", "parent_id": "45TYYU", "name":"account 3", "type": 1, "category": 1 },
  { "id": "45TYYPZRE", "parent_id": "45TYYPZ", "name":"account 4", "type": 1, "category": 1 },
  { "id": "45TYYPZSX", "parent_id": "45TYYPZ", "name":"account 5", "type": 1, "category": 1 },
  { "id": "45TYYPZGP", "parent_id": "45TYYXT", "name":"account 6", "type": 1, "category": 1 }
]

account 2 and account 3 are children of account 1, while account 4 and account 5 are children of account tree and account 6 is child of account 2 ... but every register is at the same logical level only identifying through parent_id.

so I need to transform this data into a GET method to restructure it like this:

[
    { 
        "id": "45TYYU",
        "parent_id": null,
        "name":"account 1",
        "type": 1,
        "category": 1,
        "children": [
            { 
                "id": "45TYYXT",
                "parent_id": "45TYYU",
                "name":"account 2",
                "type": 1,
                "category": 1,
                "children": [
                    { "id": "45TYYPZGP", "parent_id": "45TYYXT", "name":"account 6", "type": 1, "category": 1 }
                ]
            },
            { 
                "id": "45TYYPZ",
                "parent_id": "45TYYU",
                "name":"account 3",
                "type": 1,
                "category": 1,
                "children": [
                    { "id": "45TYYPZRE", "parent_id": "45TYYPZ", "name":"account 4", "type": 1, "category": 1 },
                    { "id": "45TYYPZSX", "parent_id": "45TYYPZ", "name":"account 5", "type": 1, "category": 1 }
                ]
            }
        ]
    },
    { 
        "id": "45TFJK",
        "parent_id": null,
        "name":"account 7",
        "type": 1,
        "category": 1,
        "children": [
            { 
                "id": "47HJJT",
                "parent_id": "45TFJK",
                "name":"account 8",
                "type": 1,
                "category": 1
            },
            { 
                "id": "47YHJU",
                "parent_id": "45TFJK",
                "name":"account 8",
                "type": 1,
                "category": 1
            }
        ]
    }
]

Yes... the parents level 0 has null parent_id and I want to put it's children inside an array called "children" and then send like this in the GET response to my UI

What is the best way to do this in expressJS? Is there a library or component out there that allows me to do this?

Thank you

Answer 1

You can use $graphLookup and other useful array operators,

• $match filter that records only have parent_id is null
• $graphLookup to get child records and depth number in depthField level
• $unwind deconstruct children array and allow to not remove empty children
• $sort by depth level field level in descending order
• $group by id field and reconstruct children array

db.collection.aggregate([
  { $match: { parent_id: null } },
  {
    $graphLookup: {
      from: "collection",
      startWith: "$id",
      connectFromField: "id",
      connectToField: "parent_id",
      depthField: "level",
      as: "children"
    }
  },
  {
    $unwind: {
      path: "$children",
      preserveNullAndEmptyArrays: true
    }
  },
  { $sort: { "children.level": -1 } },
  {
    $group: {
      _id: "$id",
      parent_id: { $first: "$parent_id" },
      name: { $first: "$name" },
      type: { $first: "$type" },
      category: { $first: 1 },
      children: { $push: "$children" }
    }
  },

• $addFields now find the nested level children and allocate to its level,
  • $reduce to iterate loop of children array.
  • initialize default field level default value is -1, presentChild is [], prevChild is [] for the conditions purpose
  • $let to initialize fields:
    • prev as per condition if both level are equal then return prevChild otherwise return presentChild
    • current as per condition if both level are equal then return presentChild otherwise []
  • in to return level field and prevChild field from initialized fields
    • presentChild $filter children from prev array and return, merge current objects with children array using $mergeObjects and concat with current array of let using $concatArrays
• $addFields to return only presentChild array because we only required that processed array

  {
    $addFields: {
      children: {
        $reduce: {
          input: "$children",
          initialValue: { level: -1, presentChild: [], prevChild: [] },
          in: {
            $let: {
              vars: {
                prev: {
                  $cond: [
                    { $eq: ["$$value.level", "$$this.level"] },
                    "$$value.prevChild",
                    "$$value.presentChild"
                  ]
                },
                current: {
                  $cond: [{ $eq: ["$$value.level", "$$this.level"] }, "$$value.presentChild", []]
                }
              },
              in: {
                level: "$$this.level",
                prevChild: "$$prev",
                presentChild: {
                  $concatArrays: [
                    "$$current",
                    [
                      {
                        $mergeObjects: [
                          "$$this",
                          {
                            children: {
                              $filter: {
                                input: "$$prev",
                                as: "e",
                                cond: { $eq: ["$$e.parent_id", "$$this.id"] }
                              }
                            }
                          }
                        ]
                      }
                    ]
                  ]
                }
              }
            }
          }
        }
      }
    }
  },
  {
    $addFields: {
      id: "$_id",
      children: "$children.presentChild"
    }
  }
])

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