Add functionality for converting Session models to json

lib/models/session.dart

@@ -1,4 +1,9 @@
+import 'package:json_annotation/json_annotation.dart';
+
+part 'session.g.dart';
+
 /// Represents the metadata for data collection session.
+@JsonSerializable()
 class Session {
   /// Unique identifier for the participant
   final String participantID;
@@ -24,4 +29,9 @@ class Session {
     return "Session(participantID: $participantID, sessionID: $sessionID, "
         "startTime: $startTime, endTime: $endTime)";
   }
+
+  /// Convert the [Session] object to its json representation.
+  /// This method is particularly useful when uploading data to Firebase and
+  /// similar no-sql dbs.
+  Map<String, dynamic> toJson() => _$SessionToJson(this);
 }

test/models/session_test.dart

@@ -15,4 +15,20 @@ void main() {
         "endTime: ${session.endTime})";
     expect(session.toString(), strRep);
   });
+
+  test(
+    "Session.toJson() returns a valid json representation",
+    () {
+      final Session session = Session(
+        participantID: '101',
+        sessionID: '001',
+        startTime: DateTime.now(),
+        endTime: DateTime.now(),
+      );
+
+      final Map<String, dynamic> sessionJson = session.toJson();
+
+      expect(sessionJson, isMap);
+    },
+  );
 }