如何在mongodb中的纯JSON中通过Id查询文档

Question

In Mongodb (2.6.1), I need to query a document by _id using pure json (without using ObjectIds). As mentioned in the mongodb extended json , I was expecting db.collection.findOne({"_id": {"$oid": "51b6eab8cd794eb62bb3e131"}}) to work but it does not. It even throw the following exception.

Can't canonicalize query: BadValue unknown operator: $oid

Anyone knows how to do it?

Answer 1

The extended JSON syntax is intended as a "transfer" format so that if for example you are sending JSON ouput to a remote client there is still a way to determine the actual implemented type such as ObjectId, Date, Binary etc.

The only place AFIAK where this is implemented is within the C# driver which provides a json parser utility method which would take JSON with the extended syntax fields and then "cast" those into objects of the required type.

So in much the same way you can implement your own parser routine to do much the same thing, it is just a matter of testing the key values for something that represents the type of object specified in the key. Given a sample fragment:

{ "_id": { "$oid": "51b6eab8cd794eb62bb3e131" } }

In simplified form without recursively checking by depth:

data = JSON.parse( json );

for ( k in data ) {
    if ( data[k].hasOwnProperty("$oid") )
        data[k] = new ObjectId( data[k]["$oid"] );
    // etc
}

So just because you may be using JavaScript it doesn't mean the "extended syntax" is valid as a query source, but you can as with other languages post-process the parsed JSON into the valid object notation required by that language and the query interface.

Similar "casting" is performed by some drivers on "string" values supplied against an _id field in order to cast to the correct object type required by the BSON wire protocol.