Database

db={"Campaign":[{"_id":{"$oid":"64eb81337f7d9f6e1107fc3a"},"_cls":"CampaignModel","campaignNumber":"CMPN-00005","account":{"$oid":"64d80694fe75052b39c0e615"},"campaignName":"title","campaignDescription":"wdesc","campaignStartDate":{"$date":"2023-08-17T00:00:00.000Z"},"campaignEndDate":{"$date":"2023-08-27T00:00:00.000Z"},"isWorkOrderGeneratedFlag":false,"isInvoiceGenerated":false,"campaignCost":"20000","clientName":"","attachedBillboardList":[],"accountId":"64d80694fe75052b39c0e615"}],"Booking":[{"_id":{"$oid":"64eb81337f7d9f6e1107fc3b"},"_cls":"BookingModel","campaignId":"64eb81337f7d9f6e1107fc3a","billboardId":"64e0d18aaec5d7ef7bf848dd","startDate":{"$date":"2023-09-01T00:00:00.000Z"},"endDate":{"$date":"2023-09-10T00:00:00.000Z"},"costPerDay":200,"billboard":{"_ref":{"$ref":"BillBoard","$id":{"$oid":"64e0d18aaec5d7ef7bf848dd"}},"_cls":"BillBoardModel"}}]}

Query

db.Campaign.aggregate([{"$lookup":{"from":"Booking",let:{campaignId:"$_id"},pipeline:[{$match:{$expr:{$eq:["$$campaignId",{$toObjectId:"$campaignId"}]}}}],"as":"attaches"}}])

Result