In many use cases, a user will want to restrict the documents $lookup searches via some additional parameter. For example, they may want:

{
  $lookup: {
    from: 'employees',
    localField: '_id',
    foreignField: 'manager',
    as: 'manages'
  }
}

But, they may only want to look for employees that have "active: true". Currently, they can do this in two ways:

• Follow the $lookup with a $unwind on 'manages', and then a $match.
• Follow the $lookup with a $project and a $filter on 'manages'.
  We will optimize the former case (SERVER-21612) and execute the $match as a query on the foreign collection, but this is not immediately clear from the pipeline.

Another option is, in 3.4, the user could use $graphLookup with a 'maxDepth' of 0 and a 'restrictSearchWithMatch' parameter to achieve the same effect--a poor workaround.

As such, $lookup should allow the user to specify an additional option, 'restrictWithMatch', that is a $match-type filter that is applied to any document from the foreign collection to be returned.

Other features available for $graphLookup that are missing for $lookup:

• localField is a field path only, startWith is an arbitrary expression.