How to create something like virtual field & hardcode a value in mongodb aggregation $project? -


consider want show following document:

   {      "_id" : objectid("512bc95fe835e68f199c8686"),      "authorname": "dave",       "virtualfield" : "hardcoded_value"    }

actual document stored in mongodb

   {      "_id" : objectid("512bc95fe835e68f199c8686"),      "author": "dave",      "score" : 80    }

can :

collection.aggregate([     { $project: {         _id: 1,         "authorname": "$author",         "virtualfield": "hardcoded_value"        }     }     ], function (err, doc) {         if (err) return console.error(err);         console.dir(doc);     } );

could tell how same?

note: don't want same after retrieving document.

got error :

[mongoerror: exception: field path references must prefixed '$' ('hardcoded_value ]   name: 'mongoerror',   errmsg: 'exception: field path references must prefixed \'$\' (\'hardcoded_value\'',   code: 15982,   ok: 0 }

getting following error on using: "virtualfield": {$concat: ["hardcoded_value"]}

{ [mongoerror: exception: invalid operator '$concat']   name: 'mongoerror',   errmsg: 'exception: invalid operator \'$concat\'',   code: 15999,   ok: 0 }

you should use concat here: http://docs.mongodb.org/manual/reference/aggregation/#exp._s_concat

like so:

"virtualfield": {$concat: ["hardcoded_value"]}

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