MongoDB concatenate strings from two fields into a third field - mongodb

Question

How do I concatenate values from two string fields and put it into a third one?
I've tried this:
db.collection.update(
{ "_id": { $exists: true } },
{ $set: { column_2: { $add: ['$column_4', '$column_3'] } } },
false, true
)
which doesn't seem to work though, and throws not ok for storage.
I've also tried this:
db.collection.update(
{ "_id": { $exists : true } },
{ $set: { column_2: { $add: ['a', 'b'] } } },
false, true
)
but even this shows the same error not ok for storage.
I want to concatenate only on the mongo server and not in my application.

Answer 1

You can use aggregation operators $project and $concat:
db.collection.aggregate([
{ $project: { newfield: { $concat: [ "$field1", " - ", "$field2" ] } } }
])

Answer 2

Unfortunately, MongoDB currently does not allow you to reference the existing value of any field when performing an update(). There is an existing Jira ticket to add this functionality: see SERVER-1765 for details.
At present, you must do an initial query in order to determine the existing values, and do the string manipulation in the client. I wish I had a better answer for you.

Answer 3

You could use $set like this in 4.2 which supports aggregation pipeline in update.
db.collection.update(
{"_id" :{"$exists":true}},
[{"$set":{"column_2":{"$concat":["$column_4","$column_3"]}}}]
)

Answer 4

Building on the answer from #rebe100x, as suggested by #Jamby ...
You can use $project, $concat and $out (or $merge) in an aggregation pipeline.
https://docs.mongodb.org/v3.0/reference/operator/aggregation/project/
https://docs.mongodb.org/manual/reference/operator/aggregation/concat/
https://docs.mongodb.com/manual/reference/operator/aggregation/out/
For example:
db.collection.aggregate(
[
{ $project: { newfield: { $concat: [ "$field1", " - ", "$field2" ] } } },
{ $out: "collection" }
]
)
With MongoDB 4.2 . . .
MongoDB 4.2 adds the $merge pipeline stage which offers selective replacement of documents within the collection, while $out would replace the entire collection. You also have the option of merging instead of replacing the target document.
db.collection.aggregate(
[
{ $project: { newfield: { $concat: [ "$field1", " - ", "$field2" ] } } },
{ $merge: { into: "collection", on: "_id", whenMatched: "merge", whenNotMatched: "discard" }
]
)
You should consider the trade-offs between performance, concurrency and consistency, when choosing between $merge and $out, since $out will atomically perform the collection replacement via a temporary collection and renaming.
https://docs.mongodb.com/manual/reference/operator/aggregation/merge/
https://docs.mongodb.com/manual/reference/operator/aggregation/merge/#merge-out-comparison

Answer 5

**
in my case this $concat worked for me ...
**
db.collection.update( { "_id" : {"$exists":true} },
[ {
"$set" : {
"column_2" : { "$concat" : ["$column_4","$column_3"] }
}
}
]

Answer 6

let suppose that you have a collection name is "myData" where you have data like this
{
"_id":"xvradt5gtg",
"first_name":"nizam",
"last_name":"khan",
"address":"H-148, Near Hero Show Room, Shahjahanpur",
}
and you want concatenate fields (first_name+ last_name +address) and save it into "address" field like this
{
"_id":"xvradt5gtg",
"first_name":"nizam",
"last_name":"khan",
"address":"nizam khan,H-148, Near Hero Show Room, Shahjahanpur",
}
now write query will be
{
var x=db.myData.find({_id:"xvradt5gtg"});
x.forEach(function(d)
{
var first_name=d.first_name;
var last_name=d.last_name;
var _add=d.address;
var fullAddress=first_name+","+last_name+","+_add;
//you can print also
print(fullAddress);
//update
db.myData.update({_id:d._id},{$set:{address:fullAddress}});
})
}

Answer 7

You can also follow the below.
db.collectionName.find({}).forEach(function(row) {
row.newField = row.field1 + "-" + row.field2
db.collectionName.save(row);
});

Answer 8

Find and Update Each Using For Loop
Try This:
db.getCollection('users').find({ }).forEach( function(user) {
user.full_name = user.first_name + " " + user.last_name;
db.getCollection('users').save(user);
});
Or Try This:
db.getCollection('users').find({ }).forEach( function(user) {
db.getCollection('users').update(
{ _id: user._id },
{ $set: { "full_name": user.first_name + " " + user.last_name } }
)
});