TwitterUser Class

The TwitterUser class contains various properties, such as the screen name of the Twitter user. It also includes a collection of TimelineItem objects that are maintained using an EntitySet. An EntitySet provides for deferred loading and manages the relationship of the collection side of one-to-many relationships between entities.

The TwitterUser constructor initializes the EntitySet, specifying two handlers: one that is called when a TimelineItem is added to its collection; the other when a TimelineItem is removed (see Listing 29.2).

The TwitterUser class also uses the Index attribute, which tells the DataContext to create a database index for the secondary column ScreenName.

Indexes improve the speed of data retrieval operations by allowing the database engine to quickly locate a record. Without an index, potentially all rows in a table need to be scanned, an expensive O(n) operation.¹

1. O(n) describes algorithm complexity. For more information on Big O notation see http://en.wikipedia.org/wiki/Big_O_notation.

LISTING 29.2. TwitterUser Class (excerpt)

Click here to view code image

[Table]
[Index(Columns = "ScreenName")]
public class TwitterUser : NotifyPropertyChangeBase
{
    public TwitterUser()
    {
        timelineItems = new EntitySet<TimelineItem>(
                                AttachTimelineItem, DetachTimelineItems);
    }

    string id;

    [Column(IsPrimaryKey = true)]
    public string Id
    {
        get
        {
            return id;
        }
        set
        {
            Assign(ref id, value);
        }
    }
    string screenName;

    [Column]
    public string ScreenName
    {
        get
        {
            return screenName;
        }
        set
        {
            Assign(ref screenName, value);
        }
    }

... (ImageUrl and Description properties omitted)

    readonly EntitySet<TimelineItem> timelineItems;

    [Association(
        Storage = "timelineItems",
        OtherKey = "TwitterUserId")]
    public EntitySet<TimelineItem> TimelineItems
    {
        get
        {
            return timelineItems;
        }
        set
        {
            timelineItems.Assign(value);
        }
    }

    void AttachTimelineItem(TimelineItem entity)
    {
        OnPropertyChanging("TimelineItems", timelineItems, timelineItems);
        entity.TwitterUser = this;
        OnPropertyChanged("TimelineItems");
    }

    void DetachTimelineItems(TimelineItem entity)
    {
        OnPropertyChanging("TimelineItems", timelineItems, null);
        entity.TwitterUser = null;
        OnPropertyChanged("TimelineItems");
    }
}

The PropertyChanging event is raised manually before the assignment of the TimelineItem.TwitterUser property, and the PropertyChanged event is raised after it has been assigned. These calls indicate to the change tracking infrastructure that the TwitterUser object is dirty and differs from the stored value in the database.