Database
[{topicId:"LSCv0mnb",createdBy:"Sam",posts:[{postId:"nNP8UDxL",createdBy:"Sam"},{postId:"aJ8UxLa",createdBy:"Bob"}]},{topicId:"hlsTOgAX",createdBy:"Victoria",posts:[{postId:"E3cJa0Nm",createdBy:"Sam"},{postId:"Rt0xQnAy",createdBy:"Jessica"}]}]
Query
db.collection.aggregate([{$match:{createdBy:"Sam"}},{$addFields:{createdBy:"[deleted]",posts:{$map:{input:"$posts",in:{postId:"$$this.postId",createdBy:{$cond:[{$eq:["$$this.createdBy","Sam"]},"[deleted]","$$this.createdBy"]}}}}}}])