[{employeeResponses:[{_id:"5d978d372f263f41cc624727",response:"Available to work.",notes:""},{_id:"5d978d372f263f41cc624728",response:"Available to work.",notes:""}]},{employeeResponses:[{_id:"5d978d372f263f41cc624727",response:"Available to work2.",notes:""},{_id:"5d978d372f263f41cc624728",response:"Available to work2.",notes:""}]},{employeeResponses:[{_id:"5d978d372f263f41cc624729",response:"Available to work2.",notes:""},{_id:"5d978d372f263f41cc624720",response:"Available to work2.",notes:""}]}]

db.collection.aggregate([{$addFields:{employeeResponses:{$map:{input:{$filter:{input:"$employeeResponses",cond:{$eq:["$$this._id","5d978d372f263f41cc624727"]}}},in:"$$this.response"}}}},{$group:{_id:null,responses:{$push:{$ifNull:[{$arrayElemAt:["$employeeResponses",0]},"No response"]}}}}])