[{_id:ObjectId("60a5531cd6186f39b42d4344"),comments:[{comment:"test1",client_id:"1111"},{comment:"test2",client_id:"1111"},{comment:"test3",client_id:"3333"}],clients:[{client_id:"1111",name:"name 1111"},{client_id:"3333",name:"name 3333"}]}]

db.collection.aggregate([{$addFields:{comments:{"$map":{"input":"$comments","as":"first","in":{"$mergeObjects":["$$first",{$arrayElemAt:[{$map:{input:{$filter:{input:"$clients",as:"second",cond:{$eq:["$$first.client_id","$$second.client_id"]}}},in:{name:"$$this.name"}}},0]}]}}}}}])