How to get only queried array object from an array in mongo db with regex?

Question

I have my data saved in mongo db like this

  {
 "_id": "5eff4376e036e45de9dbc6df",
 "social_connections":{
     "friend":{
         "friends":[
            {
                "_id": "5eff42dee036e45de9dbc6d3",
                "user_name": "x",
                "name": "Viper King"
            },
            {
                "_id": "5eff40efe036e45de9dbc6c9",
                "user_name": "z",
                "name": "Brad Prasad Pitt"
            },
            {
                "_id": "5eff50337508da5ff40bf36e",
                "user_name": "test",
                "name": "Test"
            }

         ]
         
     },
     "followers":{
        "following":[
            {
                "_id": "5eff42dee036e45de9dbc6d3",
                "user_name": "x",
                "name": "Viper King"
            },
            {
                "_id": "5eff40efe036e45de9dbc6c9",
                "user_name": "z",
                "name": "Brad Prasad Pitt"
            },
            {
                "_id": "5eff50337508da5ff40bf36e",
                "user_name": "test",
                "name": "Test"
            }
        ]
     }
 }
 }

I want to make a mongo db query in which I would pass the _id of the document that is 5eff4376e036e45de9dbc6df and a regex that is 'Vi', now I want all those array objects whose name contains 'Vi'. My expected output is:-

  {
 "_id": "5eff4376e036e45de9dbc6df",
 "social_connections":{
     "friend":{
         "friends":[
            {
                "_id": "5eff42dee036e45de9dbc6d3",
                "user_name": "x",
                "name": "Viper King"
            }

         ]
         
     },
     "followers":{
        "following":[
            {
                "_id": "5eff42dee036e45de9dbc6d3",
                "user_name": "x",
                "name": "Viper King"
            }
        ]
     }
 }
 }

You can also make a query which only return _id of those whose name contains 'Vi'

Use regular dot notation for the query + docs.mongodb.com/manual/reference/operator/projection/… to retrieve only the matching array elements. — D. SM
– D. SM, Commented Jul 13, 2020 at 4:15
You want to regex search 'Vi' in both friends and following.friends? You need to user the aggregation pipe where you should first use the $unwind both the array objects and then regex match them and use $regex to match your string then user $project to get the required output. — Romit
– Romit, Commented Jul 13, 2020 at 4:35

mickl · Accepted Answer · 2020-07-13 07:51:17Z

You can use $filter to get a new filtered array and $regexMatch to apply your regular expression:

db.collection.aggregate([
    {
        $project: {
            "social_connections.friend.friends": {
                $filter: {
                    input: "$social_connections.friend.friends",
                    as: "friend",
                    cond: {
                        $regexMatch: { input: "$$friend.name", regex: "Vi" }
                    }
                }
            },
            "social_connections.followers.following": {
                $filter: {
                    input: "$social_connections.followers.following",
                    as: "follower",
                    cond: {
                        $regexMatch: { input: "$$follower.name", regex: "Vi" }
                    }
                }
            }
        }
    }
])

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