$match in $lookup result

With MongoDB 3.4, you can run an aggregation pipeline that uses the $addFields pipeline and a $filter operator to only return the Company array with elements that match the given condition. You can then wrap the $filter expression with the $arrayElemAt operator to return a single document which in essence incorporates the $unwind functionality by flattening the array.

Follow this example to understand the above concept:

db.users.aggregate([
    { "$match": { "UserName": "administrator" } },
    { 
        "$lookup": { 
            "from": 'companies', 
            "localField": 'CompanyID', 
            "foreignField": 'CompanyID', 
            "as": 'Company' 
        } 
    },
    {
        "$addFields": {
            "Company": {
                "$arrayElemAt": [
                    {
                        "$filter": {
                            "input": "$Company",
                            "as": "comp",
                            "cond": {
                                "$eq": [ "$$comp.CompanyName", "edt5" ]
                            }
                        }
                    }, 0
                ]
            }
        }
    }
])