MongoDB - Aggregate with nested array not working

Question

I have 2 collections using a simple nested array. An example of the records looks like so:

menus

{ 
    "_id" : ObjectId("620323fe80ec16abea7a0205"), 
    "name" : "A new menu", 
    "description" : "", 
    "menuData" : [
        {
            "catName" : "Cat 2", 
            "items" : [
                "61ded42461b1d8966c5efc45", 
                "61ded55001e3cafb8db19198", 
                "61df9adf2441b6113033f341", 
                "61dfa8c82441b6113033f738"
            ]
        }, 
        {
            "catName" : "Cat 1", 
            "items" : [
                "62020691adda4aab89f1742d", 
                "61dfa8c82441b6113033f738"
            ]
        }
    ], 
    "status" : "active", 
}

and I'm trying to complete a lookup on the menu items array (menuData.items). These are object IDs from another collection.

menuitems One example is:

{ 
    "_id" : ObjectId("61ded55001e3cafb8db19198"), 
    "name" : "Coca-Cola 600ml", 
    "description" : "Refreshing taste...ahhh", 
    "price" : 3.95, 
    "tags" : [
        "drinks"
    ], 
    "options" : [

    ], 
    "status" : "active",  
}

Using the following aggregate, I do not get the output of the lookup

db.getCollection("menus").aggregate(
[
    { 
        "$match" : { 
            "_id" : ObjectId("620323fe80ec16abea7a0205")
        }
    }, 
    { 
        "$addFields" : { 
            "newField" : "$menuData.items"
        }
    }, 
    { 
        "$unwind" : { 
            "path" : "$newField"
        }
    }, 
    { 
        "$lookup" : { 
            "from" : "menuitems", 
            "localField" : "newField", 
            "foreignField" : "_id", 
            "as" : "items"
        }
    }
]
);

Output

{ 
    "_id" : ObjectId("620323fe80ec16abea7a0205"), 
    "name" : "A new menu", 
    "description" : "", 
    "menuData" : [
        {
            "catName" : "Cat 2", 
            "items" : [
                "61ded42461b1d8966c5efc45", 
                "61ded55001e3cafb8db19198", 
                "61df9adf2441b6113033f341", 
                "61dfa8c82441b6113033f738"
            ]
        }, 
        {
            "catName" : "Cat 1", 
            "items" : [
                "62020691adda4aab89f1742d", 
                "61dfa8c82441b6113033f738"
            ]
        }
    ], 
    "status" : "active", 
    "createdBy" : ObjectId("61bb07c778e39ca45c161d81"), 
    "createdByModel" : "Administrator", 
    "createdAt" : ISODate("2022-02-09T02:16:30.108+0000"), 
    "updatedAt" : ISODate("2022-02-09T04:26:00.837+0000"), 
    "__v" : NumberInt(0), 
    "newField" : [
        "61ded42461b1d8966c5efc45", 
        "61ded55001e3cafb8db19198", 
        "61df9adf2441b6113033f341", 
        "61dfa8c82441b6113033f738"
    ], 
    "items" : [

    ]
}

As you can see, items are empty. I tried to complete this without $addFields and $unwind - with the same result.

Any help is greatly appreciated.

Answer 1

You need $lookup with pipeline .

1. Converting ObjectId to string ( newFields hold set of Id strings while menuItems document Id is an ObjectId )
2. Check the converted id string is within the newFields array via $in .

{
  "$lookup": {
    "from": "menuitems",
    let: {
      newField: "$newField"
    },
    pipeline: [
      {
        $match: {
          $expr: {
            $in: [
              {
                $toString: "$_id"
              },
              "$$newField"
            ]
          }
        }
      }
    ],
    "as": "items"
  }
}

Sample Demo on Mongo Playground