Fernando Correia’s Weblog

August 13, 2008

Flex with GAE: adding participants to projects

Filed under: Software Development — Fernando Correia @ 9:45 pm
Tags: , ,

I’m making progress on my endeavor to build a Flex client to a Python service hosted on Google App Engine.

On the last iteration I had the whole architecture set up, using PureMVC on the client side and PyAMF on the service side. The application was able to show, create, edit and delete projects. It also handled project participants, but only in memory, without persisting them.

Now, the project participants are being persisted as Google App Engine model objects, referencing the project model objects.

This is the model definition on the server side:

class ProjectParticipant(db.Model):
    project = db.ReferenceProperty(Project, collection_name='participants')
    name = db.StringProperty()
    created_at = db.DateTimeProperty(auto_now_add=True)
    modified_at = db.DateTimeProperty(auto_now=True)

The service operations are implemented using a standard PyAMF RPC pattern (not REST yet):

class ProjectParticipantsService:
    def get(self, key):
        logging.debug('get %s' % (key))
        return self.to_dto(ProjectParticipant.get(key))

    def insert(self, participant_dto):
        logging.debug('insert %s' % (participant_dto))
        participant = ProjectParticipant()
        participant.project = Project.get(participant_dto.project_key)
        participant.name = participant_dto.name
        return self.to_dto(participant)

I am using the anonymous DTO approach that I described in a previous article:

def to_dto(self, participant):
    if participant is None: return None
    dto = DTO()
    dto._key = str(participant.key())
    dto._id = participant.key().id()
    dto.project_key = str(participant.project.key())
    dto.name = participant.name
    dto.created_at = participant.created_at
    dto.modified_at = participant.modified_at
    return dto

On the client side I have a domain object representing a project participant:

public class Participant
    public var _key:String;
    public var projectKey:String;
    public var name:String;
    public var created_at:Date;
    public var modified_at:Date;

    public static function fromDto(dto:Object = null):Participant
        var participant:Participant = new Participant();
        participant._key = dto._key;
        participant.projectKey = dto.project_key;
        participant.name = dto.name;
        participant.created_at = dto.created_at;
        participant.modified_at = dto.modified_at;
        return participant;

    public function toDto():Object
        var dto:Object = new Object();
        dto._key = _key;
        dto.project_key = projectKey;
        dto.name = name;
        dto.modified_at = modified_at;
        return dto;

    public function toString():String
        return name;

I also follow PureMVC’s approach of using proxy objects to manipulate the model objects, like this:

public class ParticipantsProxy extends Proxy
        public function getProjectParticipants(projectKey:String):ArrayCollection
            if (participants[projectKey] == undefined)
               participants[projectKey] = new ArrayCollection;
            return participants[projectKey] as ArrayCollection;

        public function addParticipant(participant:Participant):void
                new Responder(onAddParticipantResult, onFault), participant.toDto());

        private function onAddParticipantResult(result:Object):void
            var participant:Participant = Participant.fromDto(result);
            var projectParticipants:ArrayCollection = getProjectParticipants(participant.projectKey);
            sendNotification(ApplicationFacade.PARTICIPANT_ADDED, participant);

        // (etc.)

The rest is done on the View layer, like this:

private function onAddParticipant(event:Event):void
    var newParticipant:Participant = new Participant();
    newParticipant.projectKey = participantsPanel.project._key;
    newParticipant.name = StringUtil.trim(participantsPanel.newParticipant.text);
    participantsPanel.newParticipant.text = "";

You can download and browse the source code through github. Feedback is welcome.


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