Arbitrary keys & hierarchies, part 7. Custom key generators

As DataObjects.Net v4 supports wide variety of keys but has default implementation only for key generators with exactly one identity field, there could be scenario in which custom key generator is required. To close the gap DataObjects.Net v4 declares the following abstract class:

public abstract class KeyGenerator
{
  public KeyProviderInfo KeyProviderInfo { get; private set; }

  public abstract Tuple Next();

  public virtual void Initialize() {}

  protected KeyGenerator(KeyProviderInfo keyProviderInfo) {}
}

Custom generator type must inherit KeyGenerator type and implement at least the abstract method KeyGenerator.Next(). All necessary information concerning the structure of key, caching behavior and so on can be found in KeyProviderInfo type. Here is the custom implementation of key generator for Guid type:

public sealed class MyGuidKeyGenerator : KeyGenerator
{
  public override Tuple Next()
  {
    return Tuple.Create(KeyProviderInfo.TupleDescriptor, Guid.NewGuid());
  }

  public MyGuidKeyGenerator(KeyProviderInfo keyProviderInfo)
    : base(keyProviderInfo)
  {}
}

In order to indicate that a hierarchy must be served with custom key generator, KeyGeneratorAttribute was introduced. Here is how it is intended to be  used:

[HierarchyRoot]
[KeyGenerator(typeof(MyGuidKeyGenerator))]
public class Author : Entity
{
  [Field, Key]
  public Guid Id { get; private set; }

  [Field]
  public EntitySet<Book> Books { get; private set; }
}

In scenarios when key generator for a hierarchy is not required at all, this must be set up appropriately:

[HierarchyRoot]
[KeyGenerator(KeyGeneratorKind.None)]
[TableMapping("Metadata.Type")]
[Index("Name", Unique = true)]
public class Type : MetadataBase
{
  [Field, Key]
  public int Id { get; private set; }

  [Field(Length = 1000)]
  public string Name { get; set; }

  public Type(int id, string name) 
    : base(id)
  {
    Name = name;
  }

Here is how system class Metadata.Type is declared in DataObjects.Net v4. Pay attention to KeyGeneratorAttribute usage together with the absence on parameterless constructor. This means that identity field values are provided from the outside and there is actually no need in key generator.

This is the last post in “Arbitrary keys & hierarchies” series. Hope you’'ll find it useful.

Part 6. Identity fields

P.S.
If you want me to blog on some particular topic concerning DataObjects.Net v4 domain – make a request in comments.

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