How to query data from xDB from facets

GetSampleOrders.cs

public class GetSampleOrders : ReportProcessorBase
{
    private readonly QueryBuilder _contactsQueryBuilder = new QueryBuilder()
    {
        collectionName = "Contacts", // Name of the collection containing contacts in MongoDb
        QueryParms =
        {
            { "_id", "@contactid" },
        },
        Fields =
        {
            "_id",
            "KeyInteractions_SampleOrder", // path so the Sample Order Element in our custom xDB Facet
            "System_VisitCount"
        }
    };

    public override void Process(ReportProcessorArgs args)
    {
        Guid contactId = args.ReportParameters.ContactId;
        DataTable contactQueryExpression =
            base.GetTableFromContactQueryExpression(_contactsQueryBuilder.Build(), contactId, new Guid?());
        Assert.IsTrue(contactQueryExpression.Rows.Count >= 1,
            string.Format("Contact with id {0} was not found.", (object) contactId));
        int? nullable = DataRowExtensions.Field<int?>(contactQueryExpression.Rows[0], "System_VisitCount");
        if (nullable == null) 
            Log.Debug(string.Format("No VisitCount exists for contact: {0}", (object) contactId));
        args.QueryResult = contactQueryExpression;
    }
}