Why does renaming a field in a child array of objects in mongodb returning an array when using $project in aggregate function?

Question

My query goes like this im using aggregate function with lookup:

this.document.aggregate([
  {
    $lookup : {
      from: "projects",
      localField: "_id",
      foreignField: "organization_id",
      as: "projects",
    }, 
  },
  {
    $lookup : {
      from: "accounts",
      localField: "accounts",
      foreignField: "_id",
      as: "organization_accounts",
    },
  },
  {
    "$project": {
      "_id" : false,
      "id": "$_id",
      "name": 1,
      "email": "$email",
      "projects.id": "$projects._id",
      "projects.name" : 1,
      "projects.organization_id" : 1,
      "organization_accounts._id" : 1,
      "organization_accounts.first_name" : 1,
      "organization_accounts.last_name" : 1,
      "organization_accounts.email" : 1,
      "organization_accounts.main_org_id" : 1,
    }
  }
])

But the output goes like this:

  "id": "5ec69acbc072871bb03dd773",
  "email": "organization email",
  "name": "Organization name",
  "projects": [
    {
      "name": "project 1",
      "organization_id": "5ec69acbc072871bb03dd773",
      "id": [
        "5ec6a08342ad5b0a28ba6876"
      ]
    }
  ],
  "organization_accounts": []

I've tried applying the solution from this question but it doesnt seem to work or idk.

The projects.id field should be a string not an array.

PS: Just new to mongoDB

Answer 1

You have to $unwind the embedded arrays. You will get the cartesian product of array length for each document. Example

this.document.aggregate([
  {
    $lookup : {
      from: "projects",
      localField: "_id",
      foreignField: "organization_id",
      as: "projects",
    }, 
  },
  {
    $lookup : {
      from: "accounts",
      localField: "accounts",
      foreignField: "_id",
      as: "organization_accounts",
    },
  },
  {
    "$project": {
      "_id" : false,
      "id": "$_id",
      "name": 1,
      "email": "$email",
      "projects.id": "$projects._id",
      "projects.name" : 1,
      "projects.organization_id" : 1,
      "organization_accounts._id" : 1,
      "organization_accounts.first_name" : 1,
      "organization_accounts.last_name" : 1,
      "organization_accounts.email" : 1,
      "organization_accounts.main_org_id" : 1,
    }
  },
   {
    $unwind: "$projects"
  },
  {
    $unwind: "$projects.id"
  }
])