Bind comments to my entites in EF - asp.net-mvc

Question

I´am just in the beginning of creating a comment system for my website. I´am using EF and I want to bind a few of my tables to the Comments table. We can say that I have a Car entity and a Bike entity in two separate tables, and I would like to bind a collection of comments of these two tables.
In my mind I have a picture that the comments table would contain:
CommentID | EntityID | CommentText
1 Bike_2 Hello world..
2 Car_2 --
3 Bike_3 --
Am I thinking right? How do a setup this with entity framework?
Best regards.

Answer 1

(The following is for Entity Framework 4.1 to 4.3.1 and Code-First/DbContext.)
The type of mapping which comes closest to your idea is Table-per-Type (TPT) inheritance mapping. It would look like this:
public abstract class EntityWithComments
{
public int Id { get; set; }
public ICollection<Comment> Comments { get; set; }
}
public class Comment
{
public int Id { get; set; }
public string CommentText { get; set; }
public int EntityId { get; set; }
public EntityWithComments Entity { get; set; }
}
public class Car : EntityWithComments
{
public string Manufacturer { get; set; }
public string Color { get; set; }
}
public class Bicycle : EntityWithComments
{
public int Weight { get; set; }
public bool HasThreeWheels { get; set; }
}
EntityWithComments is a base class for Car and Bicycle and perhaps other entities. Then you have a derived DbContext class:
public class MyContext : DbContext
{
public DbSet<EntityWithComments> EntitiesWithComments { get; set; }
public DbSet<Comment> Comments { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<Car>()
.ToTable("Cars");
modelBuilder.Entity<Bicycle>()
.ToTable("Bicycles");
}
}
As a result you have four tables in the database:
A Comments table which looks like your proposal but EntityId won't refer directly to the Cars and Bicycles tables. Instead it refers to the base type table EntitiesWithComments.
A table EntitiesWithComments representing the abstract base class and which only has a single column, namely the Id column.
A table Cars with a one-to-one shared primary key constraint between the Id and the Id in table EntitiesWithComments
A table Bicycles with a one-to-one shared primary key constraint between the Id and the Id in table EntitiesWithComments
You can then - for example - load all blue cars:
using (var ctx = new MyContext())
{
var blueCars = ctx.EntitiesWithComments.OfType<Car>()
.Where(c => c.Color == "Blue")
.ToList();
}
Because the EntitiesWithComments base table does not contain any column except the Id there is no join between the tables necessary. The generated SQL looks like this and only touches the table for the derived type:
SELECT
'0X0X' AS [C1],
[Extent1].[Id] AS [Id],
[Extent1].[Manufacturer] AS [Manufacturer],
[Extent1].[Color] AS [Color]
FROM [dbo].[Cars] AS [Extent1]
WHERE N'Blue' = [Extent1].[Color]
(I guess, the strange 0X0X value in this query is kind of a type descriptor EF uses to check if the returned rows are really cars, but I am not sure.)
If you want to load all bicycles with three wheels including their comments the following query works:
using (var ctx = new MyContext())
{
var bicyclesWithThreeWheelsWithComments = ctx.EntitiesWithComments
.Include(e => e.Comments)
.OfType<Bicycle>()
.Where(b => b.HasThreeWheels)
.ToList();
}