Linq mapping to multiple tables - asp.net-mvc

Question

I working my way through the Apress "Pro ASP.NET MVC Framework" ( http://www.apress.com/book/view/9781430210078 ) book, and in an example the author creates a link to a db table (as well as a fake repository) using linq like this:-
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Linq.Mapping;
namespace DomainModel.Entities
{
[Table(Name = "Products")]
public class Product
{
[Column(IsPrimaryKey = true, IsDbGenerated = true, AutoSync = AutoSync.OnInsert)]
public int ProductID { get; set; }
[Column] public string Name { get; set; }
[Column] public string Description { get; set; }
[Column] public decimal Price { get; set; }
[Column] public string Category { get; set; }
public string this[string propName]
{
get {
if ((propName == "Name") && string.IsNullOrEmpty(Name))
return "Please enter a product name";
if ((propName == "Description") && string.IsNullOrEmpty(Description))
return "Please enter a description";
if ((propName == "Price") && (Price < 0))
return "Price must not be negative";
if ((propName == "Category") && string.IsNullOrEmpty(Category))
return "Please specify a category";
return null;
}
}
public string Error { get { return null; } } // Not required }
}
creating an interface:-
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using DomainModel.Entities;
namespace DomainModel.Abstract
{
public interface IProductsRepository
{
IQueryable Products { get; }
void SaveProduct(Product product);
void DeleteProduct(Product product);
}
}
a fake repository (not included) and then a real DB connection repository:-
using System.ComponentModel;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using DomainModel.Abstract;
using System.Data.Linq;
using DomainModel.Entities;
namespace DomainModel.Concrete
{
public class SqlProductsRepository : IProductsRepository
{
private Table productsTable;
public SqlProductsRepository(string connectionString)
{
productsTable = (new DataContext(connectionString)).GetTable();
}
public IQueryable Products
{
get { return productsTable; }
}
public void SaveProduct(Product product)
{
EnsureValid(product, "Name", "Description", "Category", "Price");
// If it's a new product, just attach it to the DataContext
if (product.ProductID == 0)
productsTable.InsertOnSubmit(product);
else {
// If we're updating an existing product, tell the DataContext
// to be responsible for saving this instance
productsTable.Attach(product);
// Also tell the DataContext to detect any changes since the last save
productsTable.Context.Refresh(RefreshMode.KeepCurrentValues, product);
}
productsTable.Context.SubmitChanges();
}
public void DeleteProduct(Product product)
{
productsTable.DeleteOnSubmit(product);
productsTable.Context.SubmitChanges();
}
private void EnsureValid(IDataErrorInfo validatable, params string[] properties)
{
if (properties.Any(x => validatable[x] != null))
throw new InvalidOperationException("The object is invalid.");
}
}
off to a DB table "Products" with the column name specified and it works nicely. I am using this technique in a real-world app, as it handles the DB access layer nicely, but I need to be able to access several tables from my object. How can I do this - do I need to split my object up into a hierarchy that reflects its tables or can I access more than 1 table from the main object, with other additional objects hanging off it that have their own tables? If so then how do I create the ORM links between the objects and tables?
Cheers
MH

Answer 1

Just add the related tables to your repository interface, just as you have for Product and then create the concrete implementations in your repository class, again just as you have for Product.
I've used the same pattern on my app, I have two repositories, each handling 5-10 tables. There are two distinct groups of tables which are related, hence two repositories.
I would change the SQLRepository constructor thus:
public SqlProductsRepository(string connectionString)
{
DataContext dc = new DataContext(connectionString);
productsTable = dc.GetTable<Product>();
}
You can then extend it easily thus e.g.:
private Table<Order> ordersTable;
public SqlProductsRepository(string connectionString)
{
DataContext dc = new DataContext(connectionString);
productsTable = dc.GetTable<Product>();
ordersTable = dc.GetTable<Order>();
}
IQueryable<Order> Orders
{
get { return from o in ordersTable select o; }
}
EDIT - Answering comment
Here's an example of how to deliver subordinate objects (related tables) via this method:
[Table(Name="Projects")]
public class Project
{
[Column(IsPrimaryKey = true, IsDbGenerated = true, AutoSync = AutoSync.OnInsert)]
public Guid ID { get; set; }
[Column]
public String Name { get; set; }
[Column]
public bool Active { get; set; }
[Association(ThisKey="ID", OtherKey = "ProjectID")]
private EntitySet<ProjectDate> _projectDates = new EntitySet<ProjectDate>();
public IQueryable<ProjectDate> ProjectDates
{
get { return _projectDates.AsQueryable(); }
}
}
And the ProjectDate class for completeness
[Table(Name="ProjectDates")]
public class ProjectDate
{
[Column(IsPrimaryKey = true, IsDbGenerated = true, AutoSync = AutoSync.OnInsert)]
public Guid ID { get; set; }
[Column]
public Guid ProjectID { get; set; }
[Column]
public DateTime TargetDate { get; set; }
[Column(CanBeNull = true)]
public DateTime? ActualDate { get; set; }
[Column(CanBeNull=true, IsDbGenerated = true)]
public DateTime? Created { get; set; }
private EntityRef<Project> _project;
[Association(ThisKey = "ProjectID", Storage = "_project", OtherKey = "ID")]
public Project Project
{
get { return _project.Entity; }
set { _project.Entity = value; ProjectID = _project.Entity.ID; }
}
}