ASP.NET MVC Many-to-Many Relationship Using My Own Table
I am new to Code First approach with entity structure, and I know that I have many, many relationships like the entities below, EF will automatically create a table of intermediates:
class Post {
...
public virtual ICollection<Category> Categories {get; set;}
...
}
class Category {
...
public virtual ICollection<Post> Posts {get; set;}
...
}
However, if I need to have additional data fields in the staging table, one of the possible ways (which I like right now, maybe because I don't know about better ways) would be to define a new Entity of my own, for example:
class Posts_Categories {
public int Id {get; set;}
public int CategoryId {get; set;}
public int PostId {get; set;}
public string Exrtafield1 {get; set;}
public int ex extraField2 {get; set;}
...
public virtual Post Post {get; set;}
public virtual Category Category {get; set;}
}
Using this approach, EF creates my custom intermediate table, but it also creates another of its own called "PostsCategories" which contains only a foreign key for Post_Id and another for Category_Id.
How can I make it not create an extra one and use the one I defined? Is this a good way to manage Many to Many relationships with additional data fields?
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you should use one-to-many relation like this:
public class Post
{
public System.Int32 PostId { get; set; }
[InverseProperty("Post")]
public virtual ICollection<Posts_Category> PostCategories { get; set; }
}
public class Category
{
public System.Int32 CategoryId { get; set; }
[InverseProperty("Category")]
public virtual ICollection<Posts_Category> PostCategories { get; set; }
}
public class Posts_Category
{
public System.Int32 PostId { get; set; }
public System.Int32 CategoryId { get; set; }
[ForeignKey("PostId")]
[InverseProperty("PostCategories")]
public virtual Post Post { get; set; }
[ForeignKey("CategoryId")]
[InverseProperty("PostCategories")]
public virtual Category Category { get; set; }
}
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I needed to expand on Iraj's answer a bit in order for it to work. Another modification is that I include by default ApplicationUser
as one of my tables.
So the ratio ApplicationUser
1- & infin; IdeaVote
& infin; -1 Idea
(i.e. there are users and ideas, users can vote on ideas, and each vote is represented by a link between ApplicationUser
and Idea
.
public class Idea
{
[Key]
public int Id { get; set; }
// This is an ordinary data field
public string Text { get; set; }
[InverseProperty("Idea")]
public virtual ICollection<IdeaVote> Votes { get; set; }
}
public class IdeaVote
{
[Key]
public int Id { get; set; }
public int IdeaId { get; set; }
[ForeignKey("IdeaId")]
[InverseProperty("Votes")]
public virtual Idea Idea { get; set; }
public string UserId { get; set; }
[ForeignKey("UserId")]
[InverseProperty("Votes")]
public virtual ApplicationUser User { get; set; }
}
public class ApplicationUser : IdentityUser
{
[InverseProperty("User")]
public virtual ICollection<IdeaVote> Votes { get; set; }
// Default stuff
}
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It's okay to create this PostsCategories table for the relationship between them, and you will want it. If c is a category you should be able to do things like c.Posts
Usually you would not create your own table manually for this. What data would you store in the "additional" fields? I would probably move the fields to one of the other tables and discard them. Most many relationship tables do not have additional fields.
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