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  • How do I avoid a race condition in my Rails app?

    - by Cathal
    Hi, I have a really simple Rails application that allows users to register their attendance on a set of courses. The ActiveRecord models are as follows: class Course < ActiveRecord::Base has_many :scheduled_runs ... end class ScheduledRun < ActiveRecord::Base belongs_to :course has_many :attendances has_many :attendees, :through => :attendances ... end class Attendance < ActiveRecord::Base belongs_to :user belongs_to :scheduled_run, :counter_cache => true ... end class User < ActiveRecord::Base has_many :attendances has_many :registered_courses, :through => :attendances, :source => :scheduled_run end A ScheduledRun instance has a finite number of places available, and once the limit is reached, no more attendances can be accepted. def full? attendances_count == capacity end attendances_count is a counter cache column holding the number of attendance associations created for a particular ScheduledRun record. My problem is that I don't fully know the correct way to ensure that a race condition doesn't occur when 1 or more people attempt to register for the last available place on a course at the same time. My Attendance controller looks like this: class AttendancesController < ApplicationController before_filter :load_scheduled_run before_filter :load_user, :only => :create def new @user = User.new end def create unless @user.valid? render :action => 'new' end @attendance = @user.attendances.build(:scheduled_run_id => params[:scheduled_run_id]) if @attendance.save flash[:notice] = "Successfully created attendance." redirect_to root_url else render :action => 'new' end end protected def load_scheduled_run @run = ScheduledRun.find(params[:scheduled_run_id]) end def load_user @user = User.create_new_or_load_existing(params[:user]) end end As you can see, it doesn't take into account where the ScheduledRun instance has already reached capacity. Any help on this would be greatly appreciated.

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  • Does Ruby/Rails require more unit testing than say a PHP app?

    - by Blankman
    I don't find the unit testing push in the PHP market like I see/read in the ruby/rails arena. Could one just as easily NOT unit test in ruby/rails as in php, or is ruby just too bendable and breakable that it "more" recommended to test in ruby than in php? Meaning there are large code bases like vBulletin, and from what I can tell, they don't unit test. I hope you understand what I am asking here, not the pros/cons of testing, or whether one should test or not, but rather, does one language need to be tested more than another? maybe its easy to write buggy code, or break during refactoring etc.

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  • problem with converting simple code from Winform to silverlight app.

    - by Sara
    Hi. I have this code for window form application and I have been attempting to convert it to a Silverlight application but it does not work!. There is a Textbox and I attached KeyDown event handler to it. when the user press the arrow key ( left or right) while the focus on the textbox, it will write . or -. When it is window form i used e.KeyCode and Keys.Right and its works great but when it is silverlight I used e.Key and key.Right and the program doesn't work good because the arrows do the 2 functions moving and write ./-. How I can work this out in Silverlight? (My English not good) The code ( window form): private void textBox1_KeyDown(object sender, KeyEventArgs e) { if (sender is TextBox) { TextBox textBox = (TextBox)sender; if (e.KeyCode == Keys.Left || e.KeyCode == Keys.Right) { e.Handled = true; char insert; if (e.KeyCode == Keys.Left) { insert = '.'; } else { insert = '-'; } int i = textBox.SelectionStart; textBox.Text = textBox.Text.Insert(i, insert.ToString()); textBox.Select(i + 1, 0); } } } (and Silverlight): private void textBox1_KeyDown(object sender, KeyEventArgs e) { if (sender is TextBox) { TextBox textBox = (TextBox)sender; if (e.Key == Key.Left || e.Key == Key.Right) { e.Handled = true; char insert; if (e.Key == Key.Left) { insert = '.'; } else { insert = '-'; } int i = textBox.SelectionStart; textBox.Text = textBox.Text.Insert(i, insert.ToString()); textBox.Select(i + 1, 0); } } } I don't understand, is there huge different effect between using Keycode/Keys and Key/Key or because something else?

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  • How do stream rows from a MSSQL table into a .NET app, in 10000 row chunks?

    - by Gravitas
    I have a table with 300 million rows in Microsoft SQL Server 2008 R2. There is a clustered index on the date column [DataDate] which means that the entire table is ordered by the date column. How do I stream out the data from this table, into my .NET application, in 10000 row chunks? Environment: Using C#. Have to be able to pause the data stream at any point, to allow the client to process the rows. Unfortunately, cannot use a select * from as this will select the entire table (its 50GB - it won't fit into memory).

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  • C++ MFC server app with sockets crashes and I cannot find the fault, help!

    - by usermeister
    My program has one dialog and two sockets. Both sockets are derived from CAsyncSocket, one is for listening, other is for receiving data from client. My program crashes when client tries to connect to server application and server needs to initialize receiving socket. This is my MFC dialog class. class CFileTransferServerDlg : public CDialog { ... ListeningSocket ListenSock; ReceivingSocket* RecvSock; void OnAccept(); // called when ListenSock gets connection attempt ... }; This is my derived socket class for receiving data that calls parent dialogs method when event is signaled. class ReceivingSocket : public CAsyncSocket { CFileTransferServerDlg* m_pDlg; // for accessing parent dialogs controls virtual void OnReceive(int nErrorCode); } ReceivingSocket::ReceivingSocket() { } This is dialogs function that handles incoming connection attempt when listening socket gets event notification. This is where the crash happens. void CFileTransferServerDlg::OnAccept() { RecvSock = new ReceivingSocket; /* CRASH */ } OR void CFileTransferServerDlg::OnAccept() { ReceivingSocket* tmpSock = new ReceivingSocket; tmpSock->SetParentDlg(this); CString message; if( ListenSock.Accept(*tmpSock) ) /* CRASH */ { message.LoadStringW(IDS_CLIENT_CONNECTED); m_txtStatus.SetWindowTextW(message); RecvSock = tmpSock; } } My program crashes when I try to create a socket for receiving file sent from client application. OnAccept starts when Listening socket signals incoming connection attempt, but my application then crashes. I've tried running it on another computer and connection attempt was succesful. What could be wrong? Error in debug mode: Unhandled exception at 0x009c30e1 in FileTransferServer.exe: 0xC0000005: Access violation reading location 0xccccce58.

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  • Building SL4 + RIAServices app takes too long on VS2010.

    - by adlanelm
    Got a Win7 box with VS2010 Premium installed on it. Building desktop apps works just fine. But we got this solution with 15 SL4 and 21 desktop projects... Building the SL part of it takes too long. This is very irritating and encourages to drop TDD since every time I run a test it takes ~3 seconds for msbuild to find out that nothing changed and the project should be skipped. The projects are very small and there's nothing fancy in them and we hadn't any problems before we switched from VS2008+SL3. I've heard people complaining abound VS2010 speed in general, but nothing about SL4 build time. Is anyone experiencing same problems and is there any workaround for this?

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  • How can I write a "Hello World" app in assembly language?

    - by SLC
    I've often heard of applications written using the language of the gods, assembly language. I've never tried though, and I don't even have a clue how to do it. If I wanted to dabble, how would I go about doing it? I know absolutely nothing about what is required, although presumably some kind of compiler and Notepad. Just purely out of curiousity, what would I need to write a "Hello World!" application?

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  • Can I write a module/filter that gets fired before the web app get's run in Tomcat?

    - by Blankman
    Can I write a module/filter that gets put into the processing pipleline in Tomcat BEFORE the web application even gets run? Something that I could turn on/off for each web application that tomcat is handling. is this possible? So basically it would be a re-usable filter that hooks into the web pipeline that could alter the requests behavior or perform/modify the requests. One example would be to log all ip's, or redirect based on the url, block the request, etc.

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  • Looking for suggestions about an architecture of a MultiThreaded app.

    - by Dimitri
    Hello everyone. I am looking to develop a multithreaded application that will be running in unconditional loop and processing high volume of data. High volume is 2000+ records per minute. Processing will involve data retrieval, calculations and data updates. I need the application to perform so that there is virtually no back log, meaning i need to be able to finish up all of the 2000 points in one minute or even faster. Our current implementation features a multithreaded application that is spawn multiple times (from 10 to 20) and we are noticing that it's not handling data as expected and i even feel that instances of the application compete with each other for processor time and eventually if not slowing, not benefiting each other for sure. I would like to know what would be the best approach: have a single instance running but maximize threads that can run simultaneously? or is there other ways i don't know? I'm open to suggestions. Thank you in advance

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  • Detect if NSString contains a URL and generate a "link" to open inside the app a Safari View

    - by Cy
    I have am reading a twitter feed in my iPhone application and can do it correctly, but I'd like to evolve it in a way to detect if the whole NSString contains any URL or URLs and create a "link" that will open a UIWebView within the same application. Can you guide me on how to perform this task? -(void) setTextTwitter:(NSString *)text WithDate:(NSString*)date { [text retain]; [textTwitter release], textTwitter = nil; textTwitter = text; [date retain]; [dateTwitter release], dateTwitter = nil; dateTwitter = date; [self setNeedsDisplay]; }

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  • Where do I place the launch image "Default.png" in my iPhone app's project directory?

    - by Dr Dork
    In the Apple documentation, it says... iPhone OS looks for a Default.png file, located in the top level of the application bundle. but I'm having a hard time figuring out how to get the image in that location in the bundle (I'm new to all this). I'm assuming my image will end up in the right place in the bundle when I compile it so long as I add it to the project correctly and place it in the right location in the project directory. My questions are... Where in my project directory should I place the "Default.png" image? Do I need add it to the project through Xcode? If so, how? Thanks so much in advance for all your help! I'm going to continue researching this question right now.

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  • How do I run an interactive command line Python app inside of Emacs on Win32?

    - by sludge
    If I use M-x shell and run the interactive Python interpreter, Emacs on Windows does not return any IO. When I discovered M-x python-shell, I regained hope. However, instead of running the interactive Python shell, I want to run a specific Python script that features an interactive CLI. (See Python's cmd module for details). Is there a way of launching a Python script in Emacs that is interactive? (stdout, stdin, stderr)

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  • Using an embedded DB (SQLite / SQL Compact) for Message Passing within an app?

    - by wk1989
    Hello, Just out of curiosity, for applications that have a fairly complicated module tree, would something like sqlite/sql compact edition work well for message passing? So if I have modules containing data such as: \SubsystemA\SubSubSysB\ModuleB\ModuleDataC, \SubSystemB\SubSubSystemC\ModuleA\ModuleDataX Using traditional message passing/routing, you have to go through intermediate modules in order to pass a message to ModuleB to request say ModuleDataC. Instead of doing that, if we we simply store "\SubsystemA\SubSubSysB\ModuleB\ModuleDataC" in a sqlite database, getting that data is as simple as a sql query and needs no routing and passing stuff around. Has anyone done this before? Even if you haven't, do you foresee any issues & performance impact? The only concern I have right now would be the passing of custom types, e.g. if ModuleDataC is a custom data structure or a pointer, I'll need some way of storing the data structure into the DB or storing the pointer into the DB. Thanks, JW EDIT One usage case I haven't thought about is when you want to send a message from ModuleA to ModuleB to get ModuleB to do something rather than just getting/setting data. Is it possible to do this using an embedded DB? I believe callback from the DB would be needed, how feasible is this?

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  • wav stopped working with AudioServices on my iPhone app?

    - by user157733
    I have a wav file that was working fine. It is played using the AudioServices methods. Suddenly it stopped working. The weird thing is if i change he wav file to a different one that works. Any idea what is going on? The non working sound is slightly longer (still <10seconds) but it was originally working so I just can't figure it out. Any suggestions of what to try would be most appreciated. Thanks :-)

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  • Java memory usage increases when App is used, but doesnt decrease when not being used.

    - by gren
    I have a java application that uses a lot of memory when used, but when the program is not being used, the memory usage doesnt go down. Is there a way to force Java to release this memory? Because this memory is not needed at that time, I can understand to reserve a small amount of memory, but Java just reserves all the memory it ever uses. It also reuses this memory later but there must be a way to force Java to release it when its not needed. System.gc is not working.

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  • How do you force a Linux process (Java webstart App) to stop locking a Filesystem (CD-ROM) WITHOUT k

    - by Blake
    In Linux (CentOS 5.4), how do you force a process to stop locking a file system without killing the process? I am trying to get my Java Webstart Application, running locally, to eject a CD. I do not have this problem if I am just browsing through the files using a JFileChooser, but once I read the contents of a file, I can no longer eject the CD...even after removing ALL references to any files. Hitting the eject button will give the error (Title - "Cannot Eject Volume"): "An application is preventing the volume 'volume name' from being ejected" Thus, my goal is to tell the process to stop targeting the CD-ROM in order to free it up. Thank you for any help or direction!! Attempted Fix: -running the commands: sudo umount -l /media/Volume_Name //-l Lazy Unmount forces the unmount sudo eject Problem: When a new CD is inserted, it is no longer mounted automatically probably because the process is still "targeting" it.

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  • Do I need to create icons other than RGBA for my Windows app?

    - by user843732
    I'm curious if anyone still creates icons other than RGB/A (or 32-bit with alpha-channel) for your Windows apps? PS. I know that suggested model is to create 256-color versions of icons, but I always struggle with those. If I automatically convert them in an icon editor they totally decimate the look of my original icons with jagged edges and circa-Windows95 look. So if those are still required, does anyone know the stats on how many people are still using color settings other then 32-bit TrueColor?

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