Grant right to use $eval on Mongodb 3.2

One of the side effect of enabling authorization on MongDb is that, even if you create a user with “root” right, this account is not able to execute the $eval command. The simpthom is, when you try to execute $eval you got this error

mongodb Command '$eval' failed: not authorized on jarvis-framework-saga-test to execute command

This happens because $eval is somewhat deprecated, and it should not be used. Since it is a dangerous command, a user should have access to all action on all resources, and you need to create a role that has anyAction on anyResource.

If you really need to use $eval, you should create a role, just connect to the admin database and create a new role with the command.

		role: "executeEval", 
		privileges: [ { 
			resource: { anyResource: true }, 
			actions: [ "anyAction" ] } ], 
		roles: []
 } ) 

Now that you have this new role, just add to all the users that need to use $eval, as an example, if you have a single admin user in admin database, just run this against the admin db.

db.grantRolesToUser("admin", [ { role: "executeFunctions", db: "admin" } ])

And now the admin user can execute $eval against all databases.

Gian Maria.

Secure your MongoDb installation

In last months a lots of rumor spreads about MongoDb and Data Leak because people found lots of MongoDb exposed on the internet without any protection.

The root of the problem is probably a bad default for MongoDb that actually starts without any autentication by default. Developers usually download mongodb, configure without authentication and access MongoDb instance without any knowledge of MongoDb security model. This simplicity of usage can lead to unsecure installation in production.

While this can be tolerable for MongoDb instances that lives in intranets, it is always not a good strategy to leave MongoDb completely unauthenticated.  It turns out that enabling a really basic authentication is really simple even in the community edition.

Once you started your MongoDb instance without authentication just connect with your tool of choice (ex robomongo) and create a user admin in the admin database.

use admin
    user: "admin",
    pwd: "mybeautifulpassword",
    roles: [ { role: "root", db: "admin" } ]

Once this user is created, just stop MongoDb, change configuration to enable authentication.

   authorization: enabled

If authorization is enabled in the configuration file, MongoDb requires that all of your connection to the server is authenticated. There is a nice tutorial in MongoDb site, but basically once authorization is enabled you can authenticate on a single database or to the admin db. With the above instruction I’ve created a user admin on the admin database with the role root. This is the minimum level of authentication you should have, a single user that can do anything. 

This configuration is far to be really secure, but at least avoid to access MongoDb instance without password. It is equivalent to enable only the user “sa” on a Sql Server.

The next step is changing your connection string inside your sofware to specify user and password. The format of the url is this:


As for native authentication in Sql Server, username and password are stored in connection string, and pay attention to the authSource parameter of the connection string. If you omit that parameter C# driver try to authenticate against specified database (newDb in this example) and it fails because the only user is in the admin database. Thanks to the authSource parameter you are able to specify the database to use to authenticate.

You don’t need to change anything else in your code, because the connectionstring contains all the information to authenticate the connection.

To avoid having unsecure instance of mongoDb in production, starts immediately to secure database directly during developing phase, so every person included in the process knows that he need a password to access the database.

Gian Maria.

“Unsupported filter” using ContainsAny in Mongo 2.x driver

Porting code from Legacy driver to new driver syntax is quite annoying for .NET MongoDb driver. In the new Drivers almost everything is changed, and unless you want to still use old legacy syntax creating a mess of new and old syntax, you should convert all the code to the new syntax.

One of the annoying problem is ContainsAny in LINQ compatibility driver. In old drivers, if you have an object that contains an array of strings, and you want to filter for objects that have at least one of the value contained in a list of allowed values you had to resort to this syntax.

  return Containers.AllUnsorted
                .Any(c => c.PathId.Contains(containerIdString) &&

In this situation Aces properties is an HashSet<String> and aceList is a simple String[], the last part of the query uses the ContainsAny extension method from Legacy MongoDb driver. That extension was needed in the past because the old driver has no full support for LINQ Any syntax.

The problem that arise with the new driver is, after migrating code, the above code still compiles because it references the Legacy Drivers, but it throws an “Unsupported Filter” during execution. The solution is really simple, the new driver now support the whole LINQ Any syntax, so you should write:

return Containers.AllUnsorted
	 .Any(c =&gt; c.PathId.Contains(containerIdString) &amp;&amp;
			c.Aces.Any(a =&gt; aceList.Contains(a)));

As you can see, you can now write the Query with standard LINQ syntax without the need to resort to ContainsAny.

While I really appreciate that in the new Drivers LINQ support is improved, it is quite annoying that the old code still compiles but it throws at run-time.

Gian Maria.

Change how MongoDb C# Driver serialize Guid in new driver version

We have code based on the old legacy mongo driver that uses MemberSerializationOptionsConvention class to serialize all Guid as plain strings. This option is really useful because saving Guid in Bson format is often source of confusion (CSUUID, … ) for .NET users. On the contrary, having saving Guid as plain string solves some pain and made your Bson really more readable.

Here is the old code that runs with the old legacy driver.

var conventions = new ConventionPack
        new MemberSerializationOptionsConvention(
            typeof (Guid),
            new RepresentationSerializationOptions(BsonType.String)
ConventionRegistry.Register("guidstring", conventions, t =&gt; true);

In the new Driver, these classes are removed, so we need to find a different way to serialize Guid as strings. Here is a possible solution.

public static void RegisterMongoConversions(params String[] protectedAssemblies)
    var guidConversion = new ConventionPack();
    guidConversion.Add(new GuidAsStringRepresentationConvention(protectedAssemblies.ToList()));
    ConventionRegistry.Register("guidstring", guidConversion, t =&gt; true);

This class uses a custom class called GuidAsStringRepresentationConvention, used to instruct Mongo serializer to serialize Guid as plain String. This class accepts a list of string that represents a list of protected assemblies. The reason behind it is that we do not want this convention to be applied to every type loaded into memory, and we want to avoid to change how Guid are serialized for types stored in assembly we do not have control to.

Here is the code of the class.

/// <summary>
/// A convention that allows you to set the serialization representation of guid to a simple string
/// </summary>
public class GuidAsStringRepresentationConvention : ConventionBase, IMemberMapConvention
    private List protectedAssemblies;

    // constructors
    /// <summary>
    /// Initializes a new instance of the  class.
    /// </summary>  
    public GuidAsStringRepresentationConvention(List protectedAssemblies)
        this.protectedAssemblies = protectedAssemblies;

    /// <summary>
    /// Applies a modification to the member map.
    /// </summary>
    /// The member map.
    public void Apply(BsonMemberMap memberMap)
        var memberTypeInfo = memberMap.MemberType.GetTypeInfo();
        if (memberTypeInfo == typeof(Guid))
            var declaringTypeAssembly = memberMap.ClassMap.ClassType.Assembly;
            var asmName = declaringTypeAssembly.GetName().Name;
            if (protectedAssemblies.Any(a =&gt; a.Equals(asmName, StringComparison.OrdinalIgnoreCase)))

            var serializer = memberMap.GetSerializer();
            var representationConfigurableSerializer = serializer as IRepresentationConfigurable;
            if (representationConfigurableSerializer != null)
                BsonType _representation = BsonType.String;
                var reconfiguredSerializer = representationConfigurableSerializer.WithRepresentation(_representation);

The code is really simple, it scan each memberMap to verify if the memberMap is of type GUID, but if the type is declared in a protected assembly, simply return, because we want to leave serialization as-is. If the type is not in a protected assembly and is of type GUID, we simply change the representation of the serialization to be a BsonType.String.

The result is that now, each property of type GUID, will be serialized as plain string into MongoDb.

Gian Maria.

Why I’m not a great fan of LINQ query for MongoDb

I’m not a great fan of LINQ provider in Mongo, because I think that developers that start using only LINQ misses the best part of working with a Document Database. The usual risk is: developer always resort to LINQ queries to load-modify-save a document instead of using all powerful update operators available in Mongo.

Despite this consideration, if you need to retrieve full document content, sometimes writing a LINQ query is the simplest approach, but, as always, not every valid LINQ statement you can write can be translated to MongoQuery. This is the situation of this query.

//apply security filtering.
documentsQuery = documentsQuery
  .Where(d =&gt; d.Aces.Any(a =&gt; permittingAces.Contains(a)))
  .Where(d =&gt; !d.Aces.Any(a =&gt; denyingAces.Contains(a)));

I need to filter all documents, finding documents where Aces property (is a simple HashSet<String>) contains at least one of the aces in permittingAces list but should not contain any aces listed in denyingAces collection. While this is a perfectly valid LINQ query, if you try to issue it to Mongo you got a:

Any is only support for items that serialize into documents. The current serializer is StringSerializer and must implement IBsonDocumentSerializer for participation in Any queries.

You can use Any with sub-objects, but expressing an Any condition on an array of string is not supported. To overcome this limitation, .NET provider for MongDb provide a convenient ContainsAny extension operator to write previous query.

documentsQuery = documentsQuery
  .Where(d =&gt; d.Aces.ContainsAny(permittingAces))
  .Where(d =&gt; !d.Aces.ContainsAny(denyingAces));

This LINQ query works perfectly, and if you are curious how this query translated to standard MongoQuery, you can use the GetMongoQuery() method, as I’ve described in previous post.

This simple example shows you some of the limitation that you can encounter using LINQ provider in MongoDb, and my suggestion is to always prefer using standard MongoQuery because it gives you lots of more flexibility, especially for update operations.

Another reason in the past to stay away from the LINQ provider is that the older version of the driver, still used by large amount of persons, had a really bad implementation of the Select LINQ operator, because the projection is done client side, as stated here:


Select does not result in fewer fields being returned from the server. The entire document is pulled back and passed to the native Select method. Therefore, the projection is performed client side.

This is a great problem, because the whole document is always returned from the server, using more bandwidth and more resource server side. Remember that one of the standard optimization when you issue query to MongoDb instance is reducing the amount of field you are loading from your document. If you use old LINQ provider and you are doing Select to retrieve less field from the server, you are wasting your time, because you are loading always the whole document.

Gian Maria.