how to create collection (1:n) relation

问题:

I'm implementing SQLite database in my Windows Store application (WinRT). I want to relation between two tables (1:n) Book (1) - Chapter (n)

class Book
{
    [SQLite.AutoIncrement, SQLite.PrimaryKey]
    public int Id { get; set; }
    public String Title { get; set; }
    public String Description { get; set; }
    public String Author { get; set; }
    public List<Chapter> Chapters { get; set; }

    public Book() 
    {
        this.Chapeters = new List<Chapter>();
    }
}

I get

-       $exception  {"Don't know about     System.Collections.Generic.List`1[Audioteka.Models.Chapter]"}    System.Exception {System.NotSupportedException}

+       [System.NotSupportedException]  {"Don't know about System.Collections.Generic.List`1[Audioteka.Models.Chapter]"}    System.NotSupportedException

+       Data    {System.Collections.ListDictionaryInternal} System.Collections.IDictionary {System.Collections.ListDictionaryInternal}
    HelpLink    null    string
    HResult -2146233067 int
+       InnerException  null    System.Exception
    Message "Don't know about System.Collections.Generic.List`1[Audioteka.Models.Chapter]"  string

What am I doing wrong ?

回答1:

Just to follow up on my comment with a bit more research - SQLite-net doesn't support anything which can't be directly mapped to the database. See here for why:

The ORM is able to take a .NET class definition and convert it to a SQL table definition. (Most ORMs go in the other direction.) It does this by examining all public properties of your classes and is assisted by attributes that you can use to specify column details.

You can look into using a different ORM to actually access your data (I use Vici Coolstorage), if that's what you're trying to do, or simply remove the List<Chapters> from your class and add a BookID field to the Chapters class. That's how the database would represent it.

For purposes of working with it, you could add one of these to your class:

List<Chapters> Chapters { 
  get { 
     return db.Query<Chapters> ("select * from Chapters where BookId = ?", this.Id); 
  } 
}

or

List<Chapters> Chapters { 
  get { 
     return db.Query<Chapters>.Where(b => b.BookId == this.Id); 
  } 
}

That would at least let you pull the list easily, although it would be slow because it hits the database every time you access it.

回答2:

Take a look at SQLite-Net Extensions. It provides complex relationships on top of SQLite-Net by using reflection.

Example extracted from the site:

public class Stock
{
    [PrimaryKey, AutoIncrement]
    public int Id { get; set; }
    [MaxLength(8)]
    public string Symbol { get; set; }

    [OneToMany]      // One to many relationship with Valuation
    public List<Valuation> Valuations { get; set; }
}

public class Valuation
{
    [PrimaryKey, AutoIncrement]
    public int Id { get; set; }

    [ForeignKey(typeof(Stock))]     // Specify the foreign key
    public int StockId { get; set; }
    public DateTime Time { get; set; }
    public decimal Price { get; set; }

    [ManyToOne]      // Many to one relationship with Stock
    public Stock Stock { get; set; }
}