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  • ActiveRecord and transactionsin between `before_save` and `save`

    - by JP
    I have some logic in before_save whereby (only) when some conditions are met I let the new row be created with special_number equal to the maximum special_number in the database + 1. (If the conditions aren't met then I do something different, so I can't use auto-increments) My worry is that two threads acting on this database at once might pick the same special_number if the second is executed while the first is saving. Is there way to lock the database between before_save and finishing the save, but only in some cases? I know all saves are sent in transactions, will this do the job for me? def before_save if things_are_just_right # -- Issue some kind of lock? # -- self.lock? I have no idea # Pick new special_number new_special = self.class.maximum('special_number') + 1 write_attribute('special_number',new_special) else # No need to lock in this case write_attribute('special_number',some_other_number) end end

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  • Polymorphic behavior not being implemented

    - by Garrett A. Hughes
    The last two lines of this code illustrate the problem: the compiler works when I use the reference to the object, but not when I assign the reference to an array element. The rest of the code is in the same package in separate files. BioStudent and ChemStudent are separate classes, as well as Student. package pkgPoly; public class Poly { public static void main(String[] arg) { Student[] stud = new Student[3]; // create a biology student BioStudent s1 = new BioStudent("Tom"); // create a chemistry student ChemStudent s2 = new ChemStudent("Dick"); // fill the student body with studs stud[0] = s0; stud[1] = s1; // compiler complains that it can't find symbol getMajor on next line System.out.println("major: " + stud[0].getMajor() ); // doesn't compile; System.out.println("major: " + s0.getMajor() ); // works: compiles and runs correctly } }

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  • app engine's back referencing is too slow. How can I make it faster?

    - by Ray Yun
    Google app engine has smart feature named back references and I usually iterate them where the traditional SQL's computed column need to be used. Just imagine that need to accumulate specific force's total hp. class Force(db.Model): hp = db.IntegerProperty() class UnitGroup(db.Model): force = db.ReferenceProperty(reference_class=Force,collection_name="groups") hp = db.IntegerProperty() class Unit(db.Model): group = db.ReferenceProperty(reference_class=UnitGroup,collection_name="units") hp = db.IntegerProperty() When I code like following, it was horribly slow (almost 3s) with 20 forces with single group - single unit. (I guess back-referencing force reload sub entities. Am I right?) def get_hp(self): hp = 0 for group in self.groups: group_hp = 0 for unit in group.units: group_hp += unit.hp hp += group_hp return hp How can I optimize this code? Please consider that there are more properties should be computed for each force/unit-groups and I don't want to save these collective properties to each entities. :)

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  • Sharing base object with inheritance

    - by max
    I have class Base. I'd like to extend its functionality in a class Derived. I was planning to write: class Derived(Base): def __init__(self, base_arg1, base_arg2, derived_arg1, derived_arg2): super().__init__(base_arg1, base_arg2) # ... def derived_method1(self): # ... Sometimes I already have a Base instance, and I want to create a Derived instance based on it, i.e., a Derived instance that shares the Base object (doesn't re-create it from scratch). I thought I could write a static method to do that: b = Base(arg1, arg2) # very large object, expensive to create or copy d = Derived.from_base(b, derived_arg1, derived_arg2) # reuses existing b object but it seems impossible. Either I'm missing a way to make this work, or (more likely) I'm missing a very big reason why it can't be allowed to work. Can someone explain which one it is? [Of course, if I used composition rather than inheritance, this would all be easy to do. But I was hoping to avoid the delegation of all the Base methods to Derived through __getattr__.]

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  • Global Import/using Aliasing in .NET

    - by Josh Stodola
    Using import aliasing in a single class, we can reference class library namespaces by assigning our own custom alias like this: ' VB Imports Db = Company.Lib.Data.Objects // C# using Db = Company.Lib.Data.Objects; And then we are able to reference the classes inside of Company.Lib.Data.Objects by using the Db alias that we assigned. Is it possible to do this at the global level so that the alias is applied to the entire solution instead of the given file? Currently, we are working with web applications, so I was hoping we could add something to web.config, but I am also interested in whether or not this is possible with windows forms, console apps, and/or class libraries.

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  • How to add button to UINavigationController.

    - by Aeolai
    I have the following code with adds a navigation controller to a modal view. The nav bar and view all appear ok, but the right button does not. What am I doing wrong? UpgradesViewController* upgradesViewController = [[UpgradesViewController alloc] initWithNibName:@"UpgradesView" bundle:nil]; upgradesViewController.title = @"Upgrades"; UINavigationController *navController = [[UINavigationController alloc] initWithRootViewController:upgradesViewController]; navController.modalTransitionStyle = UIModalTransitionStyleCoverVertical; navController.navigationBar.barStyle = UIBarStyleBlack; UIBarButtonItem* doneButton = [[UIBarButtonItem alloc] initWithTitle:@"Show" style:UIBarButtonItemStylePlain target: self action:nil]; navController.navigationItem.rightBarButtonItem = doneButton; [self presentModalViewController:navController animated:YES]; [navController release]; [upgradesViewController release];

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  • Django: optimizing queries

    - by Josh
    I want to list the number of items for each list. How can I find this number in a single query, rather than a query for each list? Here is a simplified version of my current template code: {% for list in lists %} <li> {{ listname }}: {% with list.num_items as item_count %} {{ item_count }} item{{ item_count|pluralize }} {% endwith %} </li> {% endfor %} lists is passed as: List.objects.filter(user=user) and num_items is a property of the List model: def _get_num_items(self): return self.item_set.filter(archived=False).count() num_items = property(_get_num_items) This queries SELECT COUNT(*) FROM "my_app_item" WHERE... n times, where n is the number of lists. Is it possible to make a single query here?

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  • Singelton restricted to instance of dll

    - by codeySmurf
    If I create a singleton class in the context of a dll, the singleton class is instantiated once and used by all instances of the dll. I am using a dll as a plug-in for an application. Now the following thing came to my mind: If I use a singleton Class, it will be shared across multiple instances of the plug-in. However, this makes it difficult to manage the lifetime of the singleton class efficiently. The only way I could think of would be to use a reference count and to make the singleton delete its self when the reference count is 0. Does anyone have any better ideas on that? Is there any good way to restrict the singleton object to one instance of the dll? Language is c++

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  • Is it possible for a parent to use a child's constant or static variables from inside a static metho

    - by ryeguy
    Below is an example of what I'm trying to do. The parent can't access any of the child's variables. It doesn't matter what technique I use (static or constants), I just need some kind of functionality like this. class ParentClass { public static function staticFunc() { //both of these will throw a (static|const) not defined error echo self::$myStatic; echo self::MY_CONSTANT; } } class ChildClass extends ParentClass { const MY_CONSTANT = 1; public static $myStatic = 2; } ChildClass::staticFunc();

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  • How is does this module code work?

    - by phsr
    I'm new to ruby and I am trying to figure out how the following code works The following code is inside a class in a module. The method is called later with the following code: @something ||= Module::Class.config class << self def config &block options = OpenStruct.new yield options if block_given? init_client! Client.new(options) end def init_client!(client) base_eigenclass = class << Base; self; end base_eigenclass.send :define_method, :client do @client = client end client end end The class has some constants in it, and when the classes initialize is called, the instance member are set to option.variable || VARIABLE_CONSTANT. I understand that if there is no value for option.variable then VARIABLE_CONSTANT is used, but I don't understand that calling Module::Class.config do |options| #some block end set the @client until config is called again with options The code definitely works, but I want to understand how it does

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  • How to link .lib library in linux

    - by giga
    I'm pretty new to c programming and want to port a windows c application to linux. My code uses a .lib file va_g729.lib - is it possible to use the same library in Linux and compile it with gcc? All my .c and .h files along with the one .lib files are in the same directory. What I'm doing now is executing this command in the directory: gcc *.c -lm and I get following errors: lbcodec2.c:(.text+0xa6b): undefined reference to `va_g729a_init_encoder' lbcodec2.c:(.text+0xa83): undefined reference to `va_g729a_encoder' collect2: error: ld returned 1 exit status Is it possible to link the .lib file and compile with gcc? Thanks.

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  • Program not running from expected directory?

    - by twpc
    I am working on a web-based program, using Java. I am not sure exactly how to phrase this, but I expect the program to be running from within the c:/Resin/webapps/apps directory. However, when I reference a file in the program like this: "../files/randomfile.pdf", it cannot find that file. It works when I reference it like this: "c:/Resin/webapps/files/randomfile.pdf". How to I change the "running location"? (And what is the technical term for this?)

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  • Subtype polymorphism and arrays

    - by user133466
    Computer[] labComputers = new Computer[10]; with public class Computer { ... void toString(){ // print computer specs } } public class Notebook extends Computer{ ... void toString(){ // print computer specs + laptop color } } each subscripted variable labComputers[i] can reference either a Computer object or a Notebook object because Notebook is a subclass of Computer. For the method call labComputers[i].toString(), polymorphism ensures that the correct toString method is called. I wonder what if we do Notebook[] labComputers = new Notebook[10]; what kind or error would I get if I reference with Computer object and a Notebook object

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  • What are the disadvantages of using a StringBuilder?

    - by stickman
    I know that StringBuilder is more efficient than a normal string when processing code which modifies the string value a lot because although strings act like value types, they are actually reference, which makes them immutable so every time we change it, we need to create a new reference in memory. My question is that, why doesn't .NET just use stringBuilder by default? There must be some disadvantages of it over just using String. Can anyone tell me what they are? The only thing I can think of is perhaps it is a heavier object and it takes more time to instantiate so if you aren't changing the string too much, this would override the benefits of StringBuilder

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  • TypeError: init_animals() takes 1 positional arguments but 2 were given

    - by libra
    I know this title look familiar to some old questions, but i've looked at every single one of them, none of them solves. And here is my codes: class Island (object):E,W,R,P def __init__(self, x, y): self.init_animals(y) def init_animals(y): pass isle = Island(x,y) However, i got the following error: Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 3, in __init__ TypeError: init_animals() takes 1 positional arguments but 2 were given Please tell me if i got any mistakes, im so confused by this. Best regards

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  • Why won't my movie play using MPMoviePlayerController?

    - by Nosrettap
    I'm trying to get a basic movie to play in an iPhone app; however, I can't seem to get it to work. Here's my entire code: #import <MediaPlayer/MediaPlayer.h> #import "ViewController.h" @implementation ViewController - (IBAction)playMoviePressed:(id)sender { NSURL *url = [NSURL URLWithString:@"http://www.ebookfrenzy.com/ios_book/movie/movie.mov"]; MPMoviePlayerController *moviePlayer = [[MPMoviePlayerController alloc] initWithContentURL:url]; [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(moviePlayBackDidFinish:) name:MPMoviePlayerPlaybackDidFinishNotification object:moviePlayer]; moviePlayer.controlStyle = MPMovieControlStyleDefault; moviePlayer.shouldAutoplay = YES; [self.view addSubview:moviePlayer.view]; [moviePlayer setFullscreen:YES animated:YES]; [moviePlayer prepareToPlay]; [moviePlayer play]; } @end I have one button on screen that calls playMoviePressed when it is tapped. I know that this method is getting called. I have also tried this with .mov and .mp4 local files that I have dragged into xcode.

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  • Help moving multiple images at once with touches

    - by daveMac
    Here is my problem: I am trying to move a puzzle piece around the screen and then connect to the other piece if they are in close proximity. I have achieved this, though it is perhaps a little odd the way I did. My problem though is that once they have connected, I can't figure out how to move them as one image, instead of two separate entities. I would really appreciate any help or suggestions. Here is a sample of what I have been doing: (void)touchesMovedNSSet *)touches withEventUIEvent *)event{ UITouch *touch = [touches anyObject]; [self dispatchTouchEvent:[touch view] toPosition:[touch locationInView:self.view]]; } -(void)dispatchTouchEventUIView *)theView toPositionCGPoint)position{ if ((CGRectContainsPoint([picture frame], position))) { picture.center = position; } if (CGRectContainsPoint([picture2 frame], position)) { picture2.center = position; } }

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  • How should I use try...except while defining a function?

    - by SpawnCxy
    Hi all, I find I've been confused by the problem that when I needn't to use try..except.For last few days it was used in almost every function I defined which I think maybe a bad practice.For example: class mongodb(object): def getRecords(self,tname,conditions=''): try: col = eval("self.db.%s" %tname) recs = col.find(condition) return recs except Exception,e: #here make some error log with e.message What I thought is ,exceptions may be raised everywhere and I have to use try to get them. And my question is,is it a good practice to use it everywhere when defining functions?If not are there any principles for it?Help would be appreciated! Regards

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  • Passing class method as selector problem.

    - by Anoide
    Hi, I want to build a selector from a class method. I'm doing it this way: NavigationTreeActionHandler* handler=[NavigationTreeActionHandler self]; NavigationTreeNode* bombsNode=new NavigationTreeNode("Bombs","bigbomb.tif" ,handler,@selector(BigBombButtonPressed:)); I need to pass to NavigationTreeNode the target and the selector to the target method. I try to get the target using the self property of the class object (Don't know if htis is the correct way to do it). Then I get the selector for the class method I want to call on the class. Everything compiles ok but it fails when I use: [[handler class] instanceMethodSignatureForSelector:selector]; I get a nil and don't really know why... could anybody help please? Thanks in advance, Olson.

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  • How does this module code work?

    - by phsr
    I'm new to ruby and I am trying to figure out how the following code works The following code is inside a class in a module. The method is called later with the following code: @something ||= Module::Class.config class << self def config &block options = OpenStruct.new yield options if block_given? init_client! Client.new(options) end def init_client!(client) base_eigenclass = class << Base; self; end base_eigenclass.send :define_method, :client do @client = client end client end end The class has some constants in it, and when the classes initialize is called, the instance member are set to option.variable || VARIABLE_CONSTANT. I understand that if there is no value for option.variable then VARIABLE_CONSTANT is used, but I don't understand that calling Module::Class.config do |options| #some block end set the @client until config is called again with options The code definitely works, but I want to understand how it does

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  • Is it thread safe to read a form controls value (but not change it) without using Invoke/BeginInvoke from another thread

    - by goku_da_master
    I know you can read a gui control from a worker thread without using Invoke/BeginInvoke because my app is doing it now. The cross thread exception error is not being thrown and my System.Timers.Timer thread is able to read gui control values just fine (unlike this guy: can a worker thread read a control in the GUI?) Question 1: Given the cardinal rule of threads, should I be using Invoke/BeginInvoke to read form control values? And does this make it more thread-safe? The background to this question stems from a problem my app is having. It seems to randomly corrupt form controls another thread is referencing. (see question 2) Question 2: I have a second thread that needs to update form control values so I Invoke/BeginInvoke to update those values. Well this same thread needs a reference to those controls so it can update them. It holds a list of these controls (say DataGridViewRow objects). Sometimes (not always), the DataGridViewRow reference gets "corrupt". What I mean by corrupt, is the reference is still valid, but some of the DataGridViewRow properties are null (ex: row.Cells). Is this caused by question 1 or can you give me any tips on why this might be happening? Here's some code (the last line has the problem): public partial class MyForm : Form { void Timer_Elapsed(object sender) { // we're on a new thread (this function gets called every few seconds) UpdateUiHelper updateUiHelper = new UpdateUiHelper(this); foreach (DataGridViewRow row in dataGridView1.Rows) { object[] values = GetValuesFromDb(); updateUiHelper.UpdateRowValues(row, values[0]); } // .. do other work here updateUiHelper.UpdateUi(); } } public class UpdateUiHelper { private readonly Form _form; private Dictionary<DataGridViewRow, object> _rows; private delegate void RowDelegate(DataGridViewRow row); private readonly object _lockObject = new object(); public UpdateUiHelper(Form form) { _form = form; _rows = new Dictionary<DataGridViewRow, object>(); } public void UpdateRowValues(DataGridViewRow row, object value) { if (_rows.ContainsKey(row)) _rows[row] = value; else { lock (_lockObject) { _rows.Add(row, value); } } } public void UpdateUi() { foreach (DataGridViewRow row in _rows.Keys) { SetRowValueThreadSafe(row); } } private void SetRowValueThreadSafe(DataGridViewRow row) { if (_form.InvokeRequired) { _form.Invoke(new RowDelegate(SetRowValueThreadSafe), new object[] { row }); return; } // now we're on the UI thread object newValue = _rows[row]; row.Cells[0].Value = newValue; // randomly errors here with NullReferenceException, but row is never null! }

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