$match in $lookup result

Below answer is for mongoDB 3.6 or later.

Given that:

• You have a collection users with a field CompanyID and a collection of companies with a field CompanyID

• you want to lookup Companies on Users by matching CompanyID, where additionally:

  • each User must match condition: User.UserName equals administrator
  • each Company on User must match condition: CompanyName equals edt5

The following query will work for you:

  db.users.aggregate([
    { $match: { UserName: 'administrator' } },
    {
      $lookup: {
        from: 'companies',
        as: 'Company',
        let: { CompanyID: '$CompanyID' },
        pipeline: [
          {
            $match: {
              $expr: {
                $and: [
                  { $eq: ['$CompanyID', '$$CompanyID'] },
                  { $eq: ['$CompanyName', 'edt5'] },
                ]
              }
            }
          }
        ]
      }
    },
  ])

Explanation: This is the way to perform left join queries with conditions more complex than simple foreign / local field equality match.

Instead of using localField and foreignField, you use:

• let option where you can map local fields to variables,
• pipeline option where you can specify aggregation Array.

In pipeline you can use $match filter, with $expr, where you can reuse variables defined earlier in let.

More info on $lookup

Nice tutorial

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
                ]
            }
        }
    }
])