db={claims:[{_id:ObjectId("64d79a9b5239b08bb7c00bb0"),claim_message:"hola",files:[],added_by:ObjectId("64d4f8a3fd3dfa4fd2a37607"),company:ObjectId("64d4b30dfd3dfa4fd2a37283"),services:[ObjectId("64d4b0f4fd3dfa4fd2a3720f")],status:"open",createdAt:ISODate("2023-08-12T14:43:39.343Z"),updatedAt:ISODate("2023-08-12T18:57:40.162Z"),actions:[{files:null,added_by:ObjectId("64d4f8a3fd3dfa4fd2a37607"),message:"testing"},{files:null,added_by:ObjectId("64d4f8a3fd3dfa4fd2a37607"),message:"new reply"},{files:null,added_by:ObjectId("64d4f8a3fd3dfa4fd2a37879"),message:"another reply"},]}],users:[{_id:ObjectId("64d4f8a3fd3dfa4fd2a37607"),account_type:"CLIENT",email:"linked@gmail.com",},{_id:ObjectId("64d4f8a3fd3dfa4fd2a37879"),account_type:"CLIENT",email:"visitor@gmail.com",}]}

db.claims.aggregate([{$lookup:{from:"users",let:{actions:"$actions"},as:"actionUsers",pipeline:[{$match:{$expr:{$in:["$_id","$$actions.added_by"]}}}]}},{$set:{actions:{$map:{input:"$actions",in:{$mergeObjects:["$$this",{$arrayElemAt:["$actionUsers",{$indexOfArray:["$actionUsers._id","$$this.added_by"]}]}]}}},actionUsers:"$$REMOVE"}}])