Monday, October 26, 2009

Arbitrary keys & hierarchies, part 6. Identity fields

DataObjects.Net v4 supports the following .Net types to be used in identity fields:

  • Boolean;
  • Byte, SByte, Int16, UInt16, Int32, UInt32, Int64, UInt64;
  • String, Char;
  • Double, Single, Decimal;
  • Guid, DateTime, TimeSpan;
  • Reference to Entity (is stored as Key)

Usage of Structure and EntitySet<T> types is not allowed, however restriction for Structure usage could disappear in future versions of DataObjects.Net.

Identity fields are set once for all persistent hierarchy, so all descendants of hierarchy root share the same Key structure. In order to set up structure of Key KeyAttribute class should be used:

public class Animal : Entity
  [Field, Key]
  public int ID { get; private set; }

  public int Name { get; set; }

Key attribute must be placed on each identity field. Also note, that identity field must be immutable, it is prohibited for identity field to have a public, protected or internal setter.

For persistent types with complex keys (which have more than one identity field) explicit identity field order is strictly recommended, because the particular order of list of properties, got with the help of .NET reflection, is not guaranteed at all.

public class BookReview : Entity
  [Field, Key(1)]
  public Person Reviewer { get; private set; }

  [Field, Key(0)]
  public Book Book { get; private set; }

  [Field(Length = 4096)]
  public string Text { get; set; }

  public BookReview(Book book, Person reviewer)
    : base(book, reviewer)

Pay attention to KeyAttribute usage (Key(0), Key(1) lines). In this example it also used to set the position of identity field within complex key. So for BookReview type the structure of Key is {Book, Person}. Also note that values for both identity fields are required in BookReview constructor and passed to the base constructor of Entity where key is constructed.

Part 5. Key providers, Part 7. Custom key generators


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