In trying to map a simple ISet<string> within an Active Record class my app kept bombing while trying to perform the SchemaExport during app initialization. Here's the error I get resulting from some call in ActiveRecordStarter.cs:

The given key was not present in the dictionary.

I knew this was related to the following property in my User class:

[HasMany(typeof(System.String),"UserId","UserRoles",

    Access = PropertyAccess.FieldCamelcase,

    RelationType = RelationType.Set

    )]

public virtual ISet<System.String> Roles

{

    get { return roles; }

}

Not until I went in to manually create my table did it dawn on me what I was missing:

    Element = "Role"

DOH! I should have gone and looked at other mappings I have created by hand before chasing this down...so now it looks like:

[HasMany(typeof(System.String),"UserId","UserRoles",

    Access = PropertyAccess.FieldCamelcase,

    RelationType = RelationType.Set,

    Element = "Role"

    )]

public virtual ISet<System.String> Roles

{

    get { return roles; }

}

All is well again.