db={"members":[{"membershipNumber":"N03XXXXXX"}],"an_users":[{"memref":"N03XXXXXX","user_id":"48XXXXXX"}],"emails":[{"email_id":"407XXX","actions_count":17,"administrative_title":"test","bounce":["3440XXXX"],"click":["38294CCC","418FFFF","48XXXXXX","38eGGGG"]}]}

db.members.aggregate([{$lookup:{from:"an_users",let:{membershipNumber:"$membershipNumber"},pipeline:[{$match:{$expr:{$eq:["$memref","$$membershipNumber"]},}},{"$lookup":{"from":"emails","localField":"user_id","foreignField":"click","as":"clicks"}},{"$project":{"_id":1,"membershipNumber":1,"an_user_id":"$user_id","n_email_clicks":{$size:"$clicks"}}}],as:"details"}},{$replaceRoot:{newRoot:{$mergeObjects:[{$arrayElemAt:["$details",0]},"$$ROOT"]}}},{$project:{details:0}}])