Invoke db.eval in FindAndModify using MongoDB C# Client

Question

I have the following Document:

{
"_id": 100,
"Version": 1,
"Data": "Hello"
}

I have a function which return a number from a sequence:

function getNextSequence(name) {
   var ret = db.Counter.findAndModify(
          {
            query: { _id: name },
            update: { $inc: { value: 1 } },
            new: true,
            upsert: true
          }
   );

   return ret.value;
}

I can use this for optimistic concurrency by performing the following Mongo command:

db.CollectionName.findAndModify({
    query: { "_id" : NumberLong(100), "Version" : 1 },
    update: { "$set" : { 
         "Data": "Here is new data!",
         "Version" : db.eval('getNextSequence("CollectionName")') } 
    },
    new: true
    }
);

This will update the document (as the _id and Version) match, with the new Data field, and also the new number out of the eval call.

It also returns a modified document, from which I can retrieve the new Version value if I want to make another update later (in the same 'session').

My problem is:

You cannot create an Update document using the MongoDB C# client that will serialize to this command.

I used:

 var update = Update.Combine(
                new UpdateDocument("$set", doc),
                Update.Set(versionMap.ElementName, new BsonJavaScript("db.eval('getNextSequence(\"Version:CollectionName\")')")))
                );

If you use what I first expected to perform this task, BsonJavascript, you get the following document, which incorrectly sets Version to a string of javascript.

update: { "$set" : { 
         "Data": "Here is new data!",
         "Version" : { "$code" : "db.eval('getNextSequence(\"Version:CollectionName\")')" }
           }
 }

How can I get MongoDB C# client to serialize an Update document with my db.eval function call in it?

I have tried to add a new BsonValue type in my assembly which I would serialize down to db.eval(''); However there is a BsonType enum which I cannot modify, without making a mod to MongoDB which I would not like to do incase of any issues with the change, compatibility etc.

I have also tried simply creating the Update document myself as a BsonDocument, however FindAndModify will only accept an IMongoUpdate interface which a simply a marker that at present I find superfluous.

I have just tried to construct the command manually by creating a BsonDocument myself to set the Value: db.eval, however I get the following exception: A String value cannot be written to the root level of a BSON document.

I see no other way now than drop down to the Mongo stream level to accomplish this.

James · Accepted Answer · 2015-01-22 12:27:31Z

So I gave up with trying to get Mongo C# Client to do what I needed and instead wrote the following MongoDB function to do this for me:

db.system.js.save(
   {
     _id : "optimisticFindAndModify" ,
     value : function optimisticFindAndModify(collectionName, operationArgs) {
           var collection = db.getCollection(collectionName);
           var ret = collection.findAndModify(operationArgs);
           return ret;
        }
   }
);

This will get the collection to operate over, and execute the passed operationArgs in a FindAndModify operation.

Because I could not get the shell to set a literal value (ie, not a "quoted string") on a javascript object, I had to to this in my C# code:

var counterName = "Version:" + CollectionName;
var sequenceJs = string.Format("getNextSequence(\"{0}\")", counterName);

var doc = entity.ToBsonDocument();
doc.Remove("_id");
doc.Remove(versionMap.ElementName);
doc.Add(versionMap.ElementName, "SEQUENCEJS");

var findAndModifyDocument = new BsonDocument
{
    {"query", query.ToBsonDocument()},
    {"update", doc},
    {"new", true},
    {"fields", Fields.Include(versionMap.ElementName).ToBsonDocument() }
};

// We have to strip the quotes from getNextSequence.
var findAndModifyArgs = findAndModifyDocument.ToString();
findAndModifyArgs = findAndModifyArgs.Replace("\"SEQUENCEJS\"", sequenceJs);

var evalCommand = string.Format("db.eval('optimisticFindAndModify(\"{0}\", {1})');", CollectionName, findAndModifyArgs);
var modifiedDocument = Database.Eval(new EvalArgs
{
    Code = new BsonJavaScript(evalCommand)
});

The result of this is that I can now call my Sequence Javascript, the getNextSequence function, inside the optimisticFindAndModify function.

Unforunately I had to use a string replace in C# as again there is no way of setting a BsonDocument to use the literal type db.eval necessary, although Mongo Shell likes it just fine.

All is now working.

EDIT:

Although, if you really want to push boundaries, and are actually awake, you will realize this same action can be accomplished by performing an $inc on the Version field.... and none of this is necessary....

However: If you want to follow along to the MongoDB tutorial on how they to say to implement concurrency, or you just want to use a function in a FindAndModify, this will help you. I know I'll probably refer back to it a few times in this project!

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