Retrieve a value from a different collection in MongoDb

Is it possible to copy certain fields from one collection to another?

I am looking to copy the values of 'bar' to the 'foo' collection, excluding the 'type' field. Additionally, I want to add a new _id and an extra field (userId) to 'foo' using Node.js.

collection bar
{
    "_id" : ObjectId("77777777ffffff9999999999"),
    "type" : 0,
    "name" : "Default",
    "index" : 1,
    "layout" : "1",    
}


collection foo
{
    "_id" : NEW OBJECT ID,
//    "type" : 0, NOT INCLUDED IN THIS COLLECTION
    "userId" : ObjectId("77777777ffffff9999999911"),
    "name" : "Default",
    "index" : 1,
    "layout" : "1",    
}

I attempted using db.bar.copyTo("foo");, but it copied the entire collection instead.

javascriptmongodb

Answer №1

One of the best options in this situation is to remove the unwanted field from your collection. You can achieve this by using the $unset operator:

db.collection.update({ },{ "$unset": { "fieldToRemove": 1 } },false,true)

This command will remove the specified field from all documents in your collection with just one statement.

Starting from version 2.6, you can also achieve this with the aggregate method:

db.collection.aggregate([
    { "$project": {
        "field1": 1,
        "field2": 1,
        "field3": 1
    }},
    { "$out": "newCollection" }
])

The new $out stage in aggregation sends the result to a new collection.

Answer №2

To transfer fields between collections, you can employ the following method:

 db.sourceCollection.find().forEach(function(item){

    db.targetCollection.update(
    {"_id" : ObjectId("53205a4a139485ab43e25a72")}, //specify update condition

    { $set:{fieldName:item.fieldName}
});

});

This script iterates through the data using the provided function. It loops through the source collection to extract the data, then updates or inserts it into the target collection document.

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