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Although i have found a similar question on stackOverFlow MongoDB copy a field to another collection with a foreign key

I want to copy a field name from userdetails collection to user collection where userId in userDetails equals _id in user.

user collection

 
 "_id" : ObjectId("5b97743bbff66e0be66283cc"), 
 "username" : "mmi_superadmin", 
 "accId" : "acc1" 
 
 
 "_id" : "c21d580ea3ca5c7a1664bd5feb57f0c8", 
 "username" : "client", 
 "accId" : "acc1" 
 

userDetail collection

 "_id" : ObjectId("5b97743bbff66e0be66283cd"), 
 "userId" : "5b97743bbff66e0be66283cc", 
 "name" : "mmi_superadmin" 
 
 
 "_id" : "5bab8a60ef86bf90f1795c44", 
 "userId" : "c21d580ea3ca5c7a1664bd5feb57f0c8", 
 "name" : "RAHUL KUMAR TIWARI" 
 

Here is my query :

db.userDetails.find().forEach(
function(x) 
 db.user.update( _id :x.userId, $set: name:x.name);

);

This query is partially working. It only updates user documents where _id is of type string. User document with _id as ObjectId are not getting updated.

Please check your documents _id's (because in your example some _id's is not valid documents _id's. for example c21d580ea3ca5c7a1664bd5feb57f0c8 not a mongo _id) and use this query:

let usersIds = ;
db.user.find("_id": $type: 7).forEach(doc => 
 usersIds.push(doc._id + '')

 db.userDetail.find(
 userId: 
 $in: usersIds
 
 ).forEach(doc => 
 db.user.update(
 
 "_id": ObjectId(doc.userId)
 ,
 
 $set: 
 "name": doc.name
 
 ,
 
 multi: false,
 upsert: false
 
 )
 )

)

if you have any question feel free to ask