Wednesday, September 25, 2013

A tutorial - implementing a friends list

I've been playing with Parse in my spare time recently, and a couple of topics seem to come up quite often: relationships and some kind of "friends list" functionality.

A "friends list" is a great example of a many-to-many relationship. Seems to me that a tutorial of how to add a "friends list" would solve both these questions, so that's what I'm doing.


First lets think of what a "friends list" should entail:
  • Find a person
  • Add them to my friends list
That's it! We're done, right? Well often you want to allow people to approve/reject being friended. You'll want to have the friendship be both ways (if I add you to my list, then you get to add me to yours). There's all sorts of extra things to consider.

Revised Requirements

Here's an expanded list, lists first:
  • Find a person
    • Highlight if I already have them in my friends list, or have asked
  • List requests from others waiting for my approve/reject action
  • List my friends
  • List my requests that are pending
  • List my rejections
  • List requests I have rejected
Now for actions we want to be able to perform:
  • Request to add someone as a friend
  • Approve a request
  • Reject a request
  • Un-friend
  • Things we could add later:
    • Un-reject (change to approved, in case someone makes a mistake)
    • Re-request (possibly with a time-window to prevent abuse)
We will leave off things like showing how many friends you have in common etc.

Given these requirements, a Parse class like the following should give us what we want:

Class: FriendRequest
RequestFrom (reference: User)
RequestTo (reference: User)
Status (string: requested/approved/rejected/etc)

So, lets see how we can handle the above lists and actions:

Find a person (highlight existing status)

Once your existing "find a person" query is run, you would need an array of User objects. Using this list we could ask for all FriendRequest objects where RequestFrom is in the list or RequestTo is in the list.

List requests from others waiting for my approver/reject action

Query FriendRequest where the current user is the RequestTo and the status is "requested".

List my friends

Query FriendRequest where the current user is RequestFrom or RequestTo and the status is "approved".

List my requests that are pending

Query FriendRequest where the current user is RequestFrom and the status is "requested".

List my rejections

Query FriendRequest where the current user is RequestFrom and the status is "rejected".

List requests I have rejected

Query FriendRequest where the current user is RequestTo and the status is "rejected".

Well, looks like we can handle all the requests, what about the actions?

Request to add someone as a friend

First make sure there's no existing record with the current User and the target User as RequestFrom/RequestTo or the other way around.

If that requirement is met then you just need to create a new FriendRequest object with the following properties:
RequestFrom: current User
RequestTo: target User
Status: requested

Approve a request

Change the Status from "requested" to "approved".

Reject a request

Change the Status from "requested" to "rejected".


Change the Status from "approved" to "rejected".


In theory it looks like everything is covered, though I welcome comments and feedback if you think I'm missed anything or made a mistake.

I'll post some sample code (JavaScript and maybe some Objective-C too) once I get my code formatting working again.

Things I'm playing with now

Wow, it's been a while since I've updated this blog. Looks like my code-formatting broke too.

Anyway, lots of things have changed, but here's what I am looking at recently:
  •, a data backend with SDKs for all major platforms: JavaScript, iOS, Andriod, .NET as well as REST
  • LINQ, specifically PredicateBuilder that lets you build OR logic, as well as building expressions and having them translated into SQL calls
I'll see if I can post some tutorials on using Parse some time soon.

Thursday, October 29, 2009

Running Windows 7 in a VM on OS X 10.6

Anyone that’s worked with me recently will know that I’m a “switcher”, someone who has changed from being a Mac hater / Windows lover to a more realistic lover of what’s good in both OS’s (it helps that Mac released OS X, as most of my pain was with OS 7-9 clients on a Windows NT network). That said all the computer in my house are Macs, except for an old Toshiba T200 TabletPC that my wife uses to play games with the kids. This gives me the ability to run any operating system I like on all the computers.

It should come as no surprise then that I eagerly tried Windows 7 back when the first beta was made available to the public. I chose to run it in Boot Camp, mostly because I was testing how well my games ran, since I had been running them in XP. When I was doing “serious” stuff (Visual Studio 2008, SQL etc) I mostly ran in a VM in Mac mode, as it gave me greater flexibility.

After my terrible experience with Vista (twice it became unrecoverable, causing me a lot of problems) I went cautiously, but was pleasantly surprised to find that Windows 7 (even as Beta and then RC) was pretty stable, quite responsive, and ran everything rather well.

My only real pain point was having to reboot into Boot Camp for anything that required decent graphics, and to get the full Aero experience.

To my great delight VM Ware and Parallels recently release new versions of their VM tools, I quickly tried both, as I have purchased previous versions of both and found each to have nice features.

Parallels 4

I was disappointed to discover no DirectX support in the latest offering, unless I missed the feature somewhere. That said, the features continue to improve and it feels more solid.

VM Ware Fusion 3

This release is huge, they’ve changed a lot and I’m quite impressed. I’ve so far tried it on a 2009 Mac Book Pro 13”, and a iMac (late 2007 model I think, with Core 2 Extreme 2.8). Both ran it quite well, Windows 7 works with Aero, and yes even games seem to work well, although given that those two computers have limited graphics power to start with I wasn’t expecting much.

Once I can convince my wife to get off my Mac Pro for long enough I’ll test the Direct 3D support on there, since with 8 cores and 6Gb RAM with a 9800GT card I expect to be able to get good performance even in a VM.

Wednesday, October 14, 2009

First week at a new job

Well I’ve started a new contract this week, but this time I’m not inflicted with draconian web filtering. Maybe now I’ll be able to do some more blogging, posting little tid-bits of information that seem interesting and get back in touch with some of my peers online.

I was with ABB Grain (now a yet-to-be-named part of Viterra, perhaps Viterra Australia?) for around 15 months, in that time I assembled a team of great developers and as a team we produced a great product that our users love. Thanks to everyone there that made those 15 months so memorable for me.

I’ve discovered that I can even run MSN Messenger now, but we’re under a pretty tight deadline so I doubt I’ll have time to chat or answer questions for the next couple of weeks, so I won’t bother signing in until things calm down a bit.

Oh, Ben Laan, this means you have to do your blog post now :P

Monday, July 21, 2008

Time Tracker - a demo Sync application

I've created a little demo application now that I've had a chance to get some better real-world experience with creating a Microsoft Sync Framework for ADO.NET application.

I feel that the first rule of creating an application that will work in offline mode using Sync is to get your basic application working without sync, talking to a local CE database. It's important to keep in mind all the usual concurrency issues involved with creating a database application such as:

  • multiple users updating the same row
  • updating something that someone else deleted
  • deleting something that someone else updated
  • etc...

These issues become even more of a problem when using Sync since the time between the user pressing "Save" and when it actually gets written to the central database can be a long time if they're offline. Also due to this disconnected nature you'll need to handle it at the server, which means you might want to create a "conflict" table to store any client-side changes that are going to be rejected so that user can be notified that their change failed for some reason and given the chance to merge or retry their update.

In this demo app I'll keep it simple, we'll assume that whoever makes the last change to a row wins, since each person should only be updating their own time-sheet.

Our app flow is as follows:

  1. Pick an employee from a list
  2. Select a day (date) to add/edit/delete entries for
  3. Add/edit/delete entries
  4. Save changes to this day's entries
  5. Go back to (2) and pick another day or exit (you could also select another employee if you want and start again)

You'll notice that there's no ability to do any updates to the list of Employees, Projects or BillingInfos (list of billing codes), these are considered management tasks and should be done by a separate application (which I might build later if people are interested).

I like to have a "Splash Window", especially since .NET (and WPF even more so) applications can take a while to start, and it'll come in handy later when we'll want to deal with checking the local DB, possibly showing a config window, or even asking for a login.

So, our app is very simple:

  • SplashWindow - simple window to let people know the app is starting, will be handy later for other things
  • TimeSheetWindow - where everything happens
  • LocalDatabase.sdf - SQL CE database with our 4 tables
  • CreateLinqToSql.cmd – a batch file to generate the LINQ to SQL classes
  • Data\LocalDatabase.cs – generated code for LINQ to SQL
  • Data\LocalDataContext.cs – partial class, uses connection string from config
  • Data\Employee.cs – partial class, added a FullName property
  • Data\TimeEntry.cs – partial class, sets ID to a new GUID when created

You’ll notice the user experience isn’t very satisfying and data validation is quite minimal, but it’s enough for this sample. One possible way to improve it is to use the Xceed DataGrid for WPF (free edition), which also includes a nice DatePicker control.

You can download the application from Time Tracker Demo on Google Code, if you have TortoiseSVN or similar you'll be able to easily download the code and try it out, this version is branch “Stage1”.

Thursday, July 17, 2008

Using SQL Express (or Standard/Enterprise) at the Client

First off, why would anyone want to use SQL Express (or one of it's bigger brothers) at the client? For us there's the case of a Site (grain silo) that wants offline functionality (if the link to head-office goes down), but still have changes on one PC be visible on all others. This is a perfect case for all the apps at that site to talk to a local SQL Server, and have a service running on the server that will sync with head-office when the connection is up.

We took a look at the SQL Express Client Synchronization using Sync Services for ADO.NET sample provided, which was very helpful. Some of the coding standards were not quite how I would have liked, but as a proof-of-concept it did the job.

After a few false starts we finally got it working in our trial application. There's one very interesting issue that we found: all your tables in your SyncAgent must be defined as BiDirectional, otherwise the SqlExpressClientSyncProvider sample class ignores them!

I was concerned that this might allow changes to download only tables, but if your server provider (which in our trial app is inside a WCF service) doesn't set the UpdateCommand or DeleteCommend then these changes are silently swallowed by the server. The client thinks the changes have been made server-side, but the server doesn't actually do anything.

This represents the final stage in proving our architecture to management, so now we'll be going full-steam ahead next week, with two new staff members starting on Monday: Nigel Spencer and Richard Hollon. That leaves only one person left for our "dream team" who will be starting in a few weeks.

Wednesday, July 2, 2008

Sync for ADO.NET CTP2 - Batch Anchor proc is broken?

In the documentation for the CTP2 it gives a sample stored procedure called usp_GetNewBatchAnchor, but from my tests it doesn't seem to work.
I had to make a few changes in my version:
  • parameters are defined as "timestamp" data type, changed them to "binary(8)"
  • for consistency I changed the "out" suffix on 2 of the 3 output params to be "output" like the 3rd one
  • multiple checks of "IF @sync_batch_count <= 0" need to be replaced with "IF @sync_batch_count IS NULL OR @sync_batch_count <= 0"
NOTE: you'll also need to change the tables it's using to check for the min timestamp value, in my case the following:
SELECT @sync_last_received_anchor = MIN(TimestampCol)
        SELECT MIN(CreateVersion) AS TimestampCol FROM dbo.BillingInfo
        SELECT MIN(UpdateVersion) AS TimestampCol FROM dbo.BillingInfo
        SELECT MIN(CreateVersion) AS TimestampCol FROM dbo.Employees
        SELECT MIN(UpdateVersion) AS TimestampCol FROM dbo.Employees
        SELECT MIN(CreateVersion) AS TimestampCol FROM dbo.Projects
        SELECT MIN(UpdateVersion) AS TimestampCol FROM dbo.Projects
        SELECT MIN(CreateVersion) AS TimestampCol FROM dbo.TimeEntries
        SELECT MIN(UpdateVersion) AS TimestampCol FROM dbo.TimeEntries
        SELECT MIN(DeleteVersion) AS TimestampCol FROM dbo.TimeEntries_Tombstone
    ) MinTimestamp

Before I made the above changes I was getting erratic behaviour depending on the batch size specified. Now everything seems to be working as expected.