MongoDb C# : get all matching sub-documents

Question

Suppose I have the below Merchant document structure

Merchants

[
   {
      "ID":"1",
      "Name":"Merchant 1",
      "Vendors":[
         {
            "ID":"1",
            "Name":"Vendor 1",
            "Currency":"GBP"
         },
         {
            "ID":"2",
            "Name":"Vendor 2",
            "Currency":"EURO"
         },
         {
            "ID":"3",
            "Name":"Vendor 3",
            "Currency":"GBP"
         }
      ]
   },
   {
      "ID":"2",
      "Name":"Merchant 2",
      "Vendors":[
         {
            "ID":"4",
            "Name":"Vendor 4",
            "Currency":"GBP"
         },
         {
            "ID":"5",
            "Name":"Vendor 5",
            "Currency":"EURO"
         },
         {
            "ID":"6",
            "Name":"Vendor 6",
            "Currency":"GBP"
         }
      ]
   }
]

Now I would like to get merchant whose Id is 2 and all it's vendors whose Currency is GBP.

Output would be

{
      "ID":"2",
      "Name":"Merchant 2",
      "Vendors":[
         {
            "ID":"4",
            "Name":"Vendor 4",
            "Currency":"GBP"
         },
         {
            "ID":"6",
            "Name":"Vendor 6",
            "Currency":"GBP"
         }
      ]
   }

How could I do this using .Find method in mongodb & C#?

I am thinking about aggregation framework but I couldn't just make it to work.

Answer 1

db.Merchants.aggregate(

    // Pipeline
    [
        // Stage 1
        {
            $match: {
                "ID": "2"
            }
        },

        // Stage 2
        {
            $project: {
                ID: 1,
                Name: 1,
                Vendors: {
                    $filter: {
                        input: "$Vendors",
                        as: "vendor",
                        cond: {
                            $eq: ["$$vendor.Currency", "GBP"]
                        }
                    }
                }
            }
        },

    ]



);