ManyToMany Mapping using conventions

[faboudib]

I am using FluentNH with automapping and conventions. I set up a Many-to-Many convention as such:

    public class HasManyToManyConvention : IHasManyToManyConvention
    {
        public void Apply(IManyToManyCollectionInstance instance)
        {
            instance.Key.Column(instance.EntityType.Name + "Id");
            instance.Relationship.Column(instance.Relationship.StringIdentifierForModel + "Id");
        }
    } 

Assuming these are my tables:
User(Id,Username,Password),
Role(Id,Name)

Upon database generation, the associative table is being generated fine:
UserInRole(UserId,RoleId)

However, it has no composite primary key set (containing UserId and RoleId). Any way to set this from the convention?