adding polymophic classes

classes.cs

@@ -24,19 +24,29 @@ public class Objective
     public string Description { get; set; }
     public DateTime Target { get; set; }
     public Student Student { get; set; }
-    public ICollection<Track> Tracks { get; set; }
-    public TrackType TrackType { get; set; }
+    public ICollection<Event> Events { get; set; }
+    public ObjectiveType Type { get; set; }
 }
 
-public class Track
+public abstract class Event
 {
     public Guid Id { get; set; }
-    public DateTime Date { get; set; }
+    public DateTime StartDate { get; set; }
+    public DateTime? EndDate { get; set; }
     public Objective Objective { get; set; }
-    public object Data { get; set; }
 }
 
-public enum TrackType
+public class YesNoEvent : Event
+{
+    public bool Occurred { get; set; }
+}
+
+public class FrequencyEvent : Event
+{
+    public int Instances { get; set; }
+}
+
+public enum ObjectiveType
 {
     [Description("Yes/No")]
     YesNo = 0,