db.foo.insert({a:1});
db.bar.insert({a:1});

db.foo.aggregate([{"$lookup":{"from":"bar","localField":"x","foreignField":"y","as":"bar"}},{"$unwind":"$bar"},{"$match":{"bar.b":0}}]).explain();

  stages: [
    {
      '$cursor': {
        queryPlanner: {
          namespace: 'tpch.foo',
          parsedQuery: {},
          indexFilterSet: false,
          queryHash: '4E45B701',
          planCacheShapeHash: '4E45B701',
          planCacheKey: '36FF8BFB',
          optimizationTimeMillis: 0,
          cursorType: 'regular',
          maxIndexedOrSolutionsReached: false,
          maxIndexedAndSolutionsReached: false,
          maxScansToExplodeReached: false,
          prunedSimilarIndexes: false,
          winningPlan: {
            isCached: false,
            usedJoinOptimization: false,
            stage: 'COLLSCAN',
            nss: 'tpch.foo',
            direction: 'forward'
          },
          rejectedPlans: []
        }
      }
    },
    {
      '$lookup': {
        from: 'bar',
        as: 'bar',
        localField: 'x',
        foreignField: 'y',
        let: {},
        pipeline: [
          { '$match': { b: { '$eq': 0 } } }
        ],
        unwinding: { preserveNullAndEmptyArrays: false }
      }
    }
  ],

db.foo.aggregate([{"$lookup":{"from":"bar","localField":"x","foreignField":"y","as":"bar", pipeline: [{$match: {b: 0}}]}},{"$unwind":"$bar"}]).explain();