How to implement UnitOfWork design pattern using petaPOCO

I am trying to implement Repository and UnitOfWork (UOW) patterns with petaPOCO micro ORM. I have completed implementing Repository pattern for the "select" queries. I have decided to follow this post to implement the UOW pattern: PetaPoco + unit of work + repository pattern . The problem I am facing is that the db object in RepositoryBase object always comes as null

public interface IUnitOfWork : IDisposable
{
    void Commit();
    Database Database { get; }
}

public interface IUnitOfWorkProvider
{
    IUnitOfWork GetUnitOfWork(string connectionString);
}

public class PetaPocoUnitOfWork : IUnitOfWork
{
    private readonly Transaction _petaTransaction;
    private readonly Database _database;

    public PetaPocoUnitOfWork(string connectionString)
    {
        _database = new Database(connectionString);
        _petaTransaction = new Transaction(_database);
    }

    public void Dispose()
    {
        UnitOfWork.Current = null;
        _petaTransaction.Dispose();
    }

    public Database Database
    {
        get { return _database; }
    }

    public void Commit()
    {
        _petaTransaction.Complete();
    }
}


public IUnitOfWork GetUnitOfWork(string connectionString)
    {
        if (UnitOfWork.Current != null)
        {
            throw new InvalidOperationException("Existing unit of work.");
        }

        var petaPocoUnitOfWork = new PetaPocoUnitOfWork(connectionString);
        UnitOfWork.Current = petaPocoUnitOfWork.Database;
        return petaPocoUnitOfWork;
    }

public static class UnitOfWork
{
    [ThreadStatic]
    public static IDatabase Current;
}

public abstract class RepositoryBase<T> where T : class
{
    //public Database db = new Database("Northwind");
    protected IDatabase db
    {
        get
        {
            return UnitOfWork.Current;
        }
    }
    public void Delete(T entity)
    {
        db.Delete(entity);
    }

    public void DeleteById(int id)
    {
        db.Delete<T>(id);
    }

    public IQueryable<T> GetAll()
    {
        return db.Query<T>("").AsQueryable();
    }

    public T GetById(int id)
    {
        return db.SingleOrDefault<T>("WHERE CustomerId = @0", id);
    }

    public T GetById(string id)
    {
        return db.SingleOrDefault<T>("WHERE CustomerId = @0", id);
    }



    public void Insert(T entity)
    {

    }

    public IQueryable<T> SearchFor(System.Linq.Expressions.Expression<Func<T, bool>> predicate)
    {
        throw new NotImplementedException();
    }
}

public interface IRepository<T> where T: class 
{
    void Insert(T entity);
    void Delete(T entity);
    IQueryable<T> SearchFor(Expression<Func<T, bool>> predicate);
    IQueryable<T> GetAll();
    T GetById(string id);
}

public interface ICustomerRepository : IRepository<Customers>
{
    CustomerAndOrders GetOrdersAndCustomersByCustomerId(string id);
    List<CustomerOrderHistory> CustomerOrderHistories(string customerId);
}

public class CustomerRepository : RepositoryBase<Customers>, ICustomerRepository
{
    public CustomerAndOrders GetOrdersAndCustomersByCustomerId(string id)
    {
        var customer = this.db.SingleOrDefault<Customers>("WHERE CustomerId = @0", id);
        var orders = this.db.Query<Orders>("WHERE CustomerId = @0", id).ToList();

        //dynamic cust = new Customers();

        var config = new MapperConfiguration(cfg => cfg.CreateMap<Customers, CustomerAndOrders>());
        var mapper = config.CreateMapper();
        CustomerAndOrders customerAndOrders = mapper.Map<CustomerAndOrders>(customer);
        //CustomerAndOrders customerAndOrders = Mapper.Map<Customers, CustomerAndOrders>(customer);

        //cust.Orders = new List<Order>(); 
        if (customerAndOrders != null && orders != null)
        {
            customerAndOrders.Order = new List<Orders>();
            customerAndOrders.Order.AddRange(orders);
        }
        return customerAndOrders;
    }

    public List<CustomerOrderHistory> CustomerOrderHistories(string customerId)
    {
        var history = new List<CustomerOrderHistory>();
        //http://ift.tt/29r1n3X
        //Make sure you have the ; before the execute statement
        var result = db.Fetch<dynamic>(";EXEC CustOrderHist @@CustomerID = @0", customerId);
        if (result != null)
        {
            var customerOrderHistory = new CustomerOrderHistory();
            foreach (var item in result)
            {
                customerOrderHistory.ProductName = item.ProductName;
                customerOrderHistory.Quantity = item.Total;
                history.Add(customerOrderHistory);
            }
        }
        return history;
    }
}