$graphLookup: {
  from: `a collection that this gets results from`;
  startWith: `connect-to val(s) to search with`;
  connectFromField: `a field in each doc in the FROM coll used to perform the next recursive query`;
  connectToField: `sets the field in each doc in the FROM coll that is queried against in each recursive query`;
  as: `field in the output doc that holds the resulting arr of results`;
  maxDepth: optional`max number of recursive depth`;
  depthField: optional`the field name that holds the number of recursive iterations to REACH this specific node, 0 for first lookup`;
  restrictSearchWIthMatch: optional`a match condition`;
}

// mock show data
db.parent_refs.find()

// mock res
// each doc has a "parent reference"
{_id: 4, name: "Calros Sangrana", title: "CRO", reports_to: 1 }
{_id: 5, name: "Henrietta Washington", title: "VP Eng", reports_to: 2 }
{_id: 6, name: "Sarah Silverstein", title: "VP Web Apps", reports_to: 5 }
{_id: 7, name: "Wendy Alfredson", title: "VP App Components", reports_to: 5 }

db.parent_reference.aggregate([
  {
    // get the doc of interest, the top person is "elliot"
    $match: {
      name: 'Eliot',
    },
  },
  {
    // get all descendent docs from the parent doc
    $graphLookup: {
      // same collection, a self-lookup
      from: 'parent_reference',
      // starting with the id
      startWith: '$_id',
      // searching on the _id field
      connectFromField: '_id',
      // using this to match for subsequent iterations
      connectToField: 'reports_to',
      // store res under this field
      as: 'all_reports',
    },
  },
]);

// parent reference data
{
"_id" : 9,
"name" : "Shannon",
"title" : "VP Education",
"reports_to" : 5
}
{ "_id" : 1, "name" : "Dev", "title" : "CEO" }
{ "_id" : 7, "name" : "Elyse", "title" : "COO", "reports_to" : 2 }
{ "_id" : 6, "name" : "Ron", "title" : "VP PM", "reports_to" : 2 }
{ "_id" : 4, "name" : "Carlos", "title" : "CRO", "reports_to" : 1 }
{ "_id" : 5, "name" : "Andrew", "title" : "VP Eng", "reports_to" : 2 }
{ "_id" : 3, "name" : "Meagen", "title" : "CMO", "reports_to" : 1 }
{
"_id" : 10,
"name" : "Dan",
"title" : "VP Core Engineering",
"reports_to" : 5
}
{ "_id" : 2, "name" : "Eliot", "title" : "CTO", "reports_to" : 1 }
{
"_id" : 11,
"name" : "Cailin",
"title" : "VP Cloud Engineering",
"reports_to" : 5
}
{ "_id" : 8, "name" : "Richard", "title" : "VP PS", "reports_to" : 1 }


db.parent_reference.aggregate([
  {
    $match: {
      name: 'Shannon'
    }
  },
  {
    $graphLookup: {
      from: 'parent_reference',
      startWith: '$reports_to',
      connectFromField: 'reports_to',
      connectToField: '_id',
      as: 'bosses'
    }
  }
])

// child-reference data
{
"_id" : 5,
"name" : "Andrew",
"title" : "VP Eng",
"direct_reports" : [
  "Cailin",
  "Dan",
  "Shannon"
]
}
{ "_id" : 7, "name" : "Elyse", "title" : "COO" }
{ "_id" : 6, "name" : "Ron", "title" : "VP PM" }
{ "_id" : 4, "name" : "Carlos", "title" : "CRO" }
{ "_id" : 10, "name" : "Dan", "title" : "VP Core Engineering" }
{
"_id" : 1,
"name" : "Dev",
"title" : "CEO",
"direct_reports" : [
  "Eliot",
  "Meagen",
  "Carlos",
  "Richard",
  "Kristen"
]
}
{ "_id" : 8, "name" : "Richard", "title" : "VP PS" }
{ "_id" : 11, "name" : "Cailin", "title" : "VP Cloud Engineering" }
{ "_id" : 3, "name" : "Meagen", "title" : "CMO" }
{ "_id" : 9, "name" : "Shannon", "title" : "VP Education" }
{
"_id" : 2,
"name" : "Eliot",
"title" : "CTO",
"direct_reports" : [
  "Andrew",
  "Elyse",
  "Ron"
]
}


// attempt to get all reports
db.child_reference.aggregate([
  {$match: { name: "Dev" }},
  {$graphLookup: {
    from: 'child_reference',
    startWith: '$direct_reports',
    connectFromField: 'direct_reports',
    connectToField: 'name',
    as: 'all_reports'
  }}
])

db.child_reference.aggregate([
  { $match: { name: 'Dev' } },
  {
    $graphLookup: {
      from: 'child_reference',
      startWith: '$direct_reports',
      connectFromField: 'direct_reports',
      connectToField: 'name',
      as: 'two_level_reports',
      maxDepth: 1,
      // ALSO THIS!
      // output how many 'levels' deep the doc is
      depthField: 'hierarchy_level_from_start',
    },
  },
]);

db.air_alliances.aggregate([
  {
    $match: { name: 'OneWorld' },
  },
  {
    $graphLookup: {
      startWith: '$airlines',
      from: 'air_airlines',
      connectFromField: 'name',
      connectToField: 'name',
      as: 'airlines',
      maxDepth: 0,
      restrictSearchWithMatch: {
        country: { $in: ['Germany', 'Spain', 'Canada'] },
      },
    },
  },
  {
    $graphLookup: {
      startWith: '$airlines.base',
      from: 'air_routes',
      connectFromField: 'dst_airport',
      connectToField: 'src_airport',
      as: 'connections',
      maxDepth: 1,
    },
  },
  {
    $project: {
      validAirlines: '$airlines.name',
      'connections.dst_airport': 1,
      'connections.airline.name': 1,
    },
  },
  { $unwind: '$connections' },
  {
    $project: {
      isValid: { $in: ['$connections.airline.name', '$validAirlines'] },
      'connections.dst_airport': 1,
    },
  },
  { $match: { isValid: true } },
  { $group: { \_id: '$connections.dst_airport' } },
]);