GraphLookup in MongoDB

{
   $graphLookup: {
      from: collection,
      startWith: expression,
      connectFromField: string,
      connectToField: string,
      as: string,
      maxDepth: number,
      depthField: string,
      restrictSearchWithMatch: document
   }
}
        

  { _id: 1, name: 'a', friends: [2, 3] },
  { _id: 2, name: 'b', friends: [1, 3, 4] },
  { _id: 3, name: 'c', friends: [1, 3] },
  { _id: 4, name: 'd', friends: [2, 5] },
  { _id: 5, name: 'e', friends: [4] }
        

db.customers.aggregate([
  { $match: { _id: 1 } }, // Only look at 'a'
  {
    $graphLookup: {
      from: 'customers', // Use the customers collection
      startWith: '$friends', // Start looking at the document's `friends` property
      connectFromField: 'friends', // A link in the graph is represented by the friends property...
      connectToField: '_id', // ... pointing to another customer's _id property
      maxDepth: 1, // Only recurse one level deep
      as: 'connections' // Store this in the `connections` property
    }
  }
]); 

{
    "_id" : 1,
    "name" : "a",
    "friends" : [
        2,
        3
    ],
    "connections" : [
        {
            "_id" : 4,
            "name" : "c",
            "friends" : [
                2,
                5
            ]
        },
        {
            "_id" : 1,
            "name" : "a",
            "friends" : [
                2,
                3
            ]
        },
        {
            "_id" : 3,
            "name" : "c",
            "friends" : [
                1,
                3
            ]
        },
        {
            "_id" : 2,
            "name" : "b",
            "friends" : [
                1,
                3,
                4
            ]
        }
    ]
}