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• How many users are sufficient to make a heavy load for web application

  - by galymzhan

  I have a web application, which has been suffering high load recent days. The application runs on single server which has 8-core Intel CPU and 4gb of RAM. Software: Drupal 5 (Apache 2, PHP5, MySQL5) running on Debian. After reaching 500 authenticated and 200 anonymous users (simultaneous), the application drastically decreases its performance up to total failure. The biggest load comes from authenticated users, who perform activities, causing insert/update/deletes on db. I think mysql is a bottleneck. Is it normal to slow down on such number of users? EDIT: I forgot to mention that I did some kind of profiling. I runned commands top, htop and they showed me that all memory was being used by MySQL! After some time MySQL starts to perform terribly slow, site goes down, and we have to restart/stop apache to reduce load. Administrators said that there was about 200 active mysql connections at that moment. The worst point is that we need to solve this ASAP, I can't do deep profiling analysis/code refactoring, so I'm considering 2 ways: my tables are MyIsam, I heard they use table-level locking which is very slow, is it right? could I change it to Innodb without worry? what if I take MySQL, and move it to dedicated machine with a lot of RAM?

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• Statistical analysis on large data set to be published on the web

  - by dassouki

  I have a non-computer related data logger, that collects data from the field. This data is stored as text files, and I manually lump the files together and organize them. The current format is through a csv file per year per logger. Each file is around 4,000,000 lines x 7 loggers x 5 years = a lot of data. some of the data is organized as bins item_type, item_class, item_dimension_class, and other data is more unique, such as item_weight, item_color, date_collected, and so on ... Currently, I do statistical analysis on the data using a python/numpy/matplotlib program I wrote. It works fine, but the problem is, I'm the only one who can use it, since it and the data live on my computer. I'd like to publish the data on the web using a postgres db; however, I need to find or implement a statistical tool that'll take a large postgres table, and return statistical results within an adequate time frame. I'm not familiar with python for the web; however, I'm proficient with PHP on the web side, and python on the offline side. users should be allowed to create their own histograms, data analysis. For example, a user can search for all items that are blue shipped between week x and week y, while another user can search for sort the weight distribution of all items by hour for all year long. I was thinking of creating and indexing my own statistical tools, or automate the process somehow to emulate most queries. This seemed inefficient. I'm looking forward to hearing your ideas Thanks

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• cannot modify header information puzzler

  - by outofmyleague_lucy

  hi im out of my league i have written what has now become identical code for two sites hosted on different servers. the first worked perfectly and i have therefore used it to compare the second to.from a login for i am directing to a loginaction.php when i include the db_connect.php ie <?php session_start(); include 'db_connect.php'; $user=$_POST['formUser'] $password=$_POST['formPassword']etc ? i am returning cannot modify header information header info already sent, but when i include the content of the db_connect ie <?php session_start(); $connect = mysql_connect("localhost", "user", "pass"); mysql_select_db(db_name, $connection); $user=$_POST['formUser'] $password=$_POST['formPassword']etc ? it works. any ideas? edit - requested error message Warning: Cannot modify header information - headers already sent by (output started at /home/avenncou/public_html/include/db_connect.php:4) in /home/avenncou/public_html/include/loginaction.php on line 14 at line 14 the is a header("Location: {$_SERVER["HTTP_REFERER"]}"); edit - requested db_connect.php <?php $connection = mysql_connect("localhost", "user", "pass");// or die ("Unable to connect!"); mysql_select_db("db", $connection);// or die ("Unable to select database!"); ?> that is all of it (dies commented out in case thats where the error was)!!!

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• Access VBA question: Change the query being referenced by a function, depending on context

  - by Tara Amatista

  I have a custom function in Access2007 that hinges on grabbing data out of a specific query. It opens Outlook, creates a new email and populates the fields with specific addresses and data taken from the query ("DecisionEmail"). Now I want to make a different query ("RequestEmail") and have it populate the email with that data. So all I have to do is change this line: Set MailList = db.OpenRecordset("DecisionEmail") and that's where I get stumped. This is my desired result: If the user is on Form_Decision and clicks the button "Send email", "DecisionEmail" will get plugged into the function and that data will appear in the email. If the user on Form_SendRequest and clicks the button "Send email", "RequestEmail" will instead get plugged in. The reason that these are different queries is because they contain very different information that is smudged about in different ways. However, since it's just one little thing that needs to change in the function code, I don't think a brand new function is a good idea. My last resort would be to make a brand new function and use the Conditions field in the Macro interface to choose between them, but I have a feeling there's a more elegant solution possible. I have a vague notion of setting the query names as variables and using an If statement but I just don't have the mental vocabulary for thinking through this.

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• Formatting data for printing automatically

  - by 0bytes

  I have a requirement to retrieve data, format it to mimic an old request format we've used for years and then send it via IP address to any of a number of printers. Gathering the data and selecting the printer is no problem. I need to format the output for the printer and I'm just not sure what's best. The requirement is that the end users not have any interaction with the print request that's generated; only the intended recipients of the request job will know of the printed (& in a future release e-mailed) request. We also a requirement for a future update that we will have to incorporate into this solution an option to change the config in the DB so that each recipient site can choose printing or e-mail notification so I expect that I'll need to keep the control in a console app or webservice. I have the data and I can send it to the printers or generate an HTML-formatted e-mail easily; I just need to format it for the autoprinting. I don't think SSRS will help because I don't know of a way to designate a printer when running a report. Thanks for all help & suggestions.

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• Firing through HTTP a Perl script for sending signals to daemons

  - by Eric Fortis

  Hello guys, I'm using apache2 on Ubuntu. I have a Perl script which basically read the files names of a directory, then rewrites a text file, then sends a signal to a daemon. How can this be done, as secure as possible through a web-page? Actually I can run the code below, but not if I remove the comments. I'm looking for advise considering: Using HTTP Requests? How about Apache file permissions on the directory shown in code? Is htaccess enough to enable user/pass access to the cgi? Should I use a database instead of writing to a file and run a cron querying the db with permission granted to write and send the signal? Granting as less permissions as possible to the webserver. Should I set a VPN? #!/usr/bin/perl -wT use strict; use CGI; #@fileList = </home/user/*>; #read a directory listing my $query = CGI->new(); print $query->header( "text/html" ), $query->p( "FirstFileNameInArray" ), #$query->p( $fileList[0] ), #output the first file in directory $query->end_html;

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• Symfony 1.4 - Don't save a blank password on a executeUpdate action.

  - by Twelve47

  I have a form to edit a UserProfile which is stored in mysql db. Which includes the following custom configuration: public function configure() { $this->widgetSchema['password']=new sfWidgetFormInputPassword(); $this->validatorSchema['password']->setOption('required', false); // you don't need to specify a new password if you are editing a user. } When the user tries to save the executeUpdate method is called to commit the changes. If the password is left blank, the password field is set to '', but I want it to retain the old password instead of overwriting it. What is the best (/most in the symfony ethos) way of doing this? My solution was to override the setter method on the model (which i had done anyway for password encryption), and ignore blank values. public function setPassword( $password ) { if ($password=='') return false; // if password is blank don't save it. return $this->_set('password', UserProfile ::encryptPassword( $password )); } It seems to work fine like this, but is there a better way? If you're wondering I cannot use sfDoctrineGuard for this project as I am dealing with a legacy database, and cannot change the schema.

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• C# & SQL Server Authentication

  - by Peter

  Hello, I'm currently developing a C# app with an SQL Server DB back-end. I'm approaching the point of deployment and hitting a problem. The applicaiton will be deployed within an active directory network. As far as SQL authentication goes, I understand that I have 2 options - Windows Authenticaiton or Server Authenticaiton. If I use Server Authentication, I'm concerned that the username and password for the account will be stored in plain text in the app.config file, and therefore leave the database vulnerable. Using Windows Authenticaiton will avoid this issue, however it would mean giving every member of staff within our organisation read/write access to the database in order to run the app correctly. Whilst this is ok, it also means that they can easily connect to the database themselves via other means and directly alter the data outside of the app. I'm guessing there is someting really obvious I'm missing here, but I've been googling all evening to no avail. Any advice/guidance would be much appreciated! Peter Addition - my project is Windows Form based not ASP.NET - is encrypting the app.config file still the right answer? If it is, does anyone have any examples that are not ASP.NET based?

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• Reduce durability in MySQL for performance

  - by Paul Prescod

  My site occasionally has fairly predictable bursts of traffic that increase the throughput by 100 times more than normal. For example, we are going to be featured on a television show, and I expect in the hour after the show, I'll get more than 100 times more traffic than normal. My understanding is that MySQL (InnoDB) generally keeps my data in a bunch of different places: RAM Buffers commitlog binary log actual tables All of the above places on my DB slave This is too much "durability" given that I'm on an EC2 node and most of the stuff goes across the same network pipe (file systems are network attached). Plus the drives are just slow. The data is not high value and I'd rather take a small chance of a few minutes of data loss rather than have a high probability of an outage when the crowd arrives. During these traffic bursts I would like to do all of that I/O only if I can afford it. I'd like to just keep as much in RAM as possible (I have a fair chunk of RAM compared to the data size that would be touched over an hour). If buffers get scarce, or the I/O channel is not too overloaded, then sure, I'd like things to go to the commitlog or binary log to be sent to the slave. If, and only if, the I/O channel is not overloaded, I'd like to write back to the actual tables. In other words, I'd like MySQL/InnoDB to use a "write back" cache algorithm rather than a "write through" cache algorithm. Can I convince it to do that? If this is not possible, I am interested in general MySQL write-performance optimization tips. Most of the docs are about optimizing read performance, but when I get a crowd of users, I am creating accounts for all of them, so that's a write-heavy workload.

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• using spring, hibernate and scala, is there a better way to load test data than dbunit?

  - by egervari

  Here are some things I really dislike about dbunit: 1) You cannot specify the exact ordering the inserts because dbunit likes to group your inserts by table name, and not by the order you define them in the XML file. This is a problem when you have records depending on other records in other tables, so you have to disable foreign key constraints during your tests... which actually sucks because these foreign key constraints will get fired in production while your tests won't be aware of them! 2) They seem hellbent on forcing you to use an xml namespace to define your xml... and I honestly can't be bothered to do this. I like the data.xml without any namespace. It works. But they are so hellbent on deprecating it. 3) Creating different xml files is hard on a per test basis, so it actually encourages creating data for your entire app. Unfortunately, this process is a little bloated too once the data grows in size and things get inter tangled. There has got to be a better way to split up your test data into chunks without having to copy/paste a lot of the test data across all of your tests. 4) Keeping track of id references in a big xml file is just impossible. If you have 130 domain classes, it just gets bewildering. This model simply does not scale. Is there something less bloated and better in the Spring/Hibernate space? db unit has worn out its welcome and I'm really looking for something better.

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• Database insertion not happening

  - by Pramod Setlur

  int i=0; ContentValues values = null; for ( SortedMap.Entry<Integer, String> entry : mapDefect.entrySet() ) { if( i++ < count ) continue; if( i < arrlst.size() ) { values = new ContentValues(); Log.d("MAP", "Id :"+entry.getKey()+"Des :"+entry.getValue()+"Co :"+ arrlst.get(i)); values.put( MARKER_COORD, arrlst.get( i ) ); values.put( MARKER_ID, entry.getKey() ); values.put( DEFECT_DESCRIPTION, entry.getValue() ); values.put( IMAGE_ID_F, imageID + 1 ); Log.d( "Err", "in insertNewDefectsDescription" ); long rowId = db.insert( TABLE_DEFECTS, null, values ); long rowId1 = rowId; i++; } } So, I want to access the arraylist only after the count variable, which is why the:- if(i++<count) continue; But, when I insert a break point at the 'insert' line, it is not working. The line is not executing at all, and hence the data is not being inserted into the database. What has gone wrong?? The 'values' are null.

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• Importing data from third party datasource (open architecture design )

  - by mare

  How would you design an application (classes, interfaces in class library) in .NET when we have a fixed database design on our side and we need to support imports of data from third party data sources, which will most likely be in XML? For instance, let us say we have a Products table in our DB which has columns Id Title Description TaxLevel Price and on the other side we have for instance Products: ProductId ProdTitle Text BasicPrice Quantity. Currently I do it like this: Have the third party XML convert to classes and XSD's and then deserialize its contents into strong typed objects (what we get as a result of this process is classes like ThirdPartyProduct, ThirdPartyClassification, etc.). Then I have methods like this: InsertProduct(ThirdPartyProduct newproduct) I do not use interfaces at the moment but I would like them to. What I would like is implement something like public class Contoso_ProductSynchronization : ProductSynchronization InsertProduct(ContosoProduct p) where ProductSynchronization will be an interface or abstract class. There will most likely be many implementations of ProductSynchronization. I cannot hardcode the types - classes like ContosoProduct, NorthwindProduct might be created from the third party XML's (so preferably I would continue to use deserialization). Hopefully someone will understand what I'm trying to explain here. Just imagine you are the seller and you have numerous providers and each one uses their own proprietary XML format. I don't mind the development, which will of course be needed everytime new format appears, because it will only require 10-20 methods to be implemented, I just want the architecture to be open and support that.

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• How Best to Replace PL/SQL with C#?

  - by Mike

  Hi, I write a lot of one-off Oracle SQL queries/reports (in Toad), and sometimes they can get complex, involving lots of unions, joins, and subqueries, and/or requiring dynamic SQL, and/or procedural logic. PL/SQL is custom made for handling these situations, but as a language it does not compare to C#, and even if it did, it's tooling does not, and if even that did, forcing yet another language on the team is something to be avoided whenever possible. Experience has shown me that using SQL for the set based processing, coupled with C# for the procedural processing, is a powerful combination indeed, and far more readable, maintainable and enhanceable than PL/SQL. So, we end up with a number of small C# programs which typically would construct a SQL query string piece by piece and/or run several queries and process them as needed. This kind of code could easily be a disaster, but a good developer can make this work out quite well, and end up with very readable code. So, I don't think it's a bad way to code for smaller DB focused projects. My main question is, how best to create and package all these little C# programs that run ad hoc queries and reports against the database? Right now I create little report objects in a DLL, developed and tested with NUnit, but then I continue to use NUnit as the GUI to select and run them. NUnit just happens to provide a nice GUI for this kind of thing, even after testing has been completed. I'm also interested in suggestions for reporting apps generally. I feel like there is a missing piece or product. The perfect tool would allow writing and running C# inside of Toad, or SQL inside of Visual Studio, along with simple reporting facilities. All ideas will be appreciated, but let's make the assumption that PL/SQL is not the solution.

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• Missing tables on Scaffold Dynamic Data page

  - by Ben Amada

  I created a Linq-to-SQL DBML for the first time. I dragged and dropped all my tables over to the designer. The tables all appear in the designer.cs file. In Global.asax, I also have model.RegisterContext() with the ScaffoldAllTables = true option. The routes are also setup. I can pull up the Scaffolding page, but there's at least one table that is missing that I'm trying to get to show up. This missing table has a relationship with a child table that references it. The child table appears. When viewing data for the child table, the column that references the missing/parent table shows the numeric PK int value, rather than showing the "name". So instead of showing "Cars" it shows 1, and instead of showing "Planes" it shows 2, etc. There's another table in the DB that has the same type of structure as the missing table, and it is correctly appearing in the scaffolded tables. For this missing table, I've tried explicitly adding the ScaffoldTable attribute to no avail. Does anyone know what would cause a table like this to not appear in the list Scaffolded tables? Thanks very much.

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• Codeigniter Inserting Multidimensional Array as rows in MySQL

  - by RisingSun

  Please Refer to this question I asked Codeigniter Insert Multiple Rows in SQL To restate <tr> <td><input type="text" name="user[0][name]" value=""></td> <td><input type="text" name="user[0][address]" value=""><br></td> <td><input type="text" name="user[0][age]" value=""></td> <td><input type="text" name="user[0][email]" value=""></td> </tr> <tr> <td><input type="text" name="user[1][name]" value=""></td> <td><input type="text" name="user[1][address]" value=""><br></td> <td><input type="text" name="user[1][age]" value=""></td> <td><input type="text" name="user[1][email]" value=""></td> </tr> .......... Can Be Inserted into MySQL as this foreach($_POST['user'] as $user) { $this->db->insert('mytable', $user); } This results in multiple MySQL queries. Is it possible to optimise it further, so that the insert occurs in one query Something like this insert multiple rows via a php array into mysql but taking advantage of codeigniters simpler syntax. Thanks

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• Ajax Request using jQuery in Rails

  - by Steve

  Hi... I am sending an Ajax Request using jQuery. What happens is that I am getting an "405 Method Not Allowed" Error. I am just posting a form, which would get the detail from the form and insert it into the DB. Just the usual stuff.I am using WEBrick that comes as default with the rails package. Can somebody please tell me how to fix this. This is the code that triggers the Ajax Request $.post($(this).attr("action") + ".js",$(this).serialize(),null,"script"); Response Headers Cache-Control no-cache Allow GET, PUT, DELETE Content-Type text/html; charset=utf-8 Content-Length 9502 Server WEBrick/1.3.1 (Ruby/1.9.1/2009-12-07) Date Wed, 02 Jun 2010 20:41:33 GMT Connection Keep-Alive Request Headers Host localhost:3000 User-Agent Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.2.3) Gecko/20100401 Firefox/3.6.3 Accept application/json, text/javascript, */* Accept-Language en-us,en;q=0.5 Accept-Encoding gzip,deflate Accept-Charset ISO-8859-1,utf-8;q=0.7,*;q=0.7 Keep-Alive 115 Connection keep-alive Content-Type application/x-www-form-urlencoded; charset=UTF-8 X-Requested-With XMLHttpRequest Referer http://localhost:3000/viewspot/3 Content-Length 141 Pragma no-cache Cache-Control no-cache

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• Best practice how to store HTML in a database column

  - by tbrandao

  I have an application that modifies a table dynamically, think spreadsheet), then upon saving the form (which the table is part of) ,I store that changed table (with user modifications) in a database column named html_Spreadhseet,along with the rest of the form data. right now I'm just storing the html in a plain text format with basic escaping of characters... I'm aware that this could be stored as a separate file, the source table (html_workseeet) already is. But from a data handling perspective its easier to save the changed html table to and from a column so as to avoid having to come up with a file management strategy (which folder will this live in, now must include folder in backups, security issues now need to apply to files, how to sync db security with file system etc.), so to minimize these issues I'm only storing the ... part in the database column. My question is should I gzip the HTML , maybe use JSON, or some other format to easily store and retrieve the HTML from the database column, what is the best practice to store HTML content in a datbase? Or just store it as I currently am as an escaped text column?

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• How to use observer_field in RoR?

  - by Sindri Guðmundsson

  Hi, I have a single select_tag with categories gathered from array in controller. When the user selects a category I want the application to redirect to the selected category. I have the following code in my view which. (I've tried both using :method = :get and :post, only change is in development.log) <%=select_tag "cat_selected", options_for_select(@cats_for_mt)%><br> <%=observe_field 'cat_selected', :url => {:action => :viewflokkur}, :with => 'cat', :method => :get %> When I select one of the options the following gets logged to development.log. Processing CategoriesController#viewflokkur (for 127.0.0.1 at 2010-06-12 12:33:26) [GET] Parameters: {"cat"=>"Taugasjúkraþjálfun", "authenticity_token"=> "B2u5ULNr7IJ/ta0+hiAMBjmjEtTtc/yMAQQvSxFn2d0="} Rendering template within layouts/main Rendering categories/viewflokkur Completed in 20ms (View: 18, DB: 0) | 200 OK [http://localhost/categories/viewflokkur?cat=Taugasj%C3%BAkra%C3%BEj%C3%A1lfun&authenticity_token=B2u5ULNr7IJ%2Fta0%2BhiAMBjmjEtTtc%2FyMAQQvSxFn2d0%3D] According to this I should now be in "viewflokkur", but nothing changes in the browser window. Is there anything else I need to do, maybe in the controller? BR, Sindri

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• mongoose updating a field in a MongoDB not working

  - by Masiar

  I have this code var UserSchema = new Schema({ Username: {type: String, index: true}, Password: String, Email: String, Points: {type: Number, default: 0} }); [...] var User = db.model('User'); /* * Function to save the points in the user's account */ function savePoints(name, points){ if(name != "unregistered user"){ User.find({Username: name}, function(err, users){ var oldPoints = users[0].Points; var newPoints = oldPoints + points; User.update({name: name}, { $inc: {Points: newPoints}}, function(err){ if(err){ console.log("some error happened when update"); } else{ console.log("update successfull! with name = " + name); User.find({Username: name}, function(err, users) { console.log("updated : " + users[0].Points); }); } }); }); } } savePoints("Masiar", 666); I would like to update my user (by finding it with its name) by updating his/her points. I'm sure oldPoints and points contain a value, but still my user keep being at zero points. The console prints "update successful". What am I doing wrong? Sorry for the stupid / noob question. Masiar

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• str_replace() and strpos() for arrays?

  - by Josh

  I'm working with an array of data that I've changed the names of some array keys, but I want the data to stay the same basically... Basically I want to be able to keep the data that's in the array stored in the DB, but I want to update the array key names associated with it. Previously the array would have looked like this: $var_opts['services'] = array('foo-1', 'foo-2', 'foo-3', 'foo-4'); Now the array keys are no longer prefixed with "foo", but rather with "bar" instead. So how can I update the array variable to get rid of the "foos" and replace with "bars" instead? Like so: $var_opts['services'] = array('bar-1', 'bar-2', 'bar-3', 'bar-4'); I'm already using if(isset($var_opts['services']['foo-1'])) { unset($var_opts['services']['foo-1']); } to get rid of the foos... I just need to figure out how to replace each foo with a bar. I thought I would use str_replace on the whole array, but to my dismay it only works on strings (go figure, heh) and not arrays.

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• WPF C# Client/Server announcement system

  - by manemawanna

  I'm currently in the process of creating an announcement system at my place of work. The role of this system will be to replace all users email due to people misusing it and generally abusing the facility. The system will consist of: Web Portal: Will allow staff to enter any important announcements (this will be restricted via AD). SQL Server 2k5 DB: Will hold the announcements along with records of staff members and if they've read the announcements etc. Front End: Created in WPF & C# which is nearly complete, it will display the announcements to the users. Web Page: Client will contact every so often, which will return an xml file for the client to read. However my boss has now shifted the goal posts and would like the announcements to appear to the user once they are written to the database, rather than waiting on the client to contact the webpage. So now I'm a bit unsure as to how to go about this. I have one idea where I would create a small server application to monitor for new announcements then contact the clients to inform them to approach the website for the information they need. But I'm just looking to see if theres a better or more efficient way to do this or if someone else has a more appropriate idea or suggestion.

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• How to add an array value to the middle of an associative array?

  - by Citizen

  Lets say I have this array: $array = array('a'=>1,'z'=>2,'d'=>4); Later in the script, I want to add the value 'c'=>3 before 'z'. How can I do this? EDIT: Yes, the order is important. When I run a foreach() through the array, I do NOT want this newly added value added to the end of the array. I am getting this array from a mysql_fetch_assoc() EDIT 2: The keys I used above are placeholders. Using ksort() will not achieve what I want. EDIT 3: http://www.php.net/manual/en/function.array-splice.php#88896 accomplishes what I'm looking for but I'm looking for something simpler. EDIT 4: Thanks for the downvotes. I gave feedback to your answers and you couldn't help, so you downvoted and requested to close the question because you didn't know the answer. Thanks. EDIT 5: Take a sample db table with about 30 columns. I get this data using mysql_fetch_assoc(). In this new array, after column 'pizza' and 'drink', I want to add a new column 'full_dinner' that combines the values of 'pizza' and 'drink' so that when I run a foreach() on the said array, 'full_dinner' comes directly after 'drink'

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• Modules vs. Classes and their influence on descendants of ActiveRecord::Base

  - by Chris

  Here's a Ruby OO head scratcher for ya, brought about by this Rails scenario: class Product < ActiveRecord::Base has_many(:prices) # define private helper methods end module PrintProduct attr_accessor(:isbn) # override methods in ActiveRecord::Base end class Book < Product include PrintProduct end Product is the base class of all products. Books are kept in the products table via STI. The PrintProduct module brings some common behavior and state to descendants of Product. Book is used inside fields_for blocks in views. This works for me, but I found some odd behavior: After form submission, inside my controller, if I call a method on a book that is defined in PrintProduct, and that method calls a helper method defined in Product, which in turn calls the prices method defined by has_many, I'll get an error complaining that Book#prices is not found. Why is that? Book is a direct descendant of Product! More interesting is the following.. As I developed this hierarchy PrintProduct started to become more of an abstract ActiveRecord::Base, so I thought it prudent to redefine everything as such: class Product < ActiveRecord::Base end class PrintProduct < Product end class Book < PrintProduct end All method definitions, etc. are the same. In this case, however, my web form won't load because the attributes defined by attr_accessor (which are "virtual attributes" referenced by the form but not persisted in the DB) aren't found. I'll get an error saying that there is no method Book#isbn. Why is that?? I can't see a reason why the attr_accessor attributes are not found inside my form's fields_for block when PrintProduct is a class, but they are found when PrintProduct is a Module. Any insight would be appreciated. I'm dying to know why these errors are occurring!

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• What is the best / proper idiom in django for modifying a field during a .save() where you need to o

  - by MDBGuy

  Hi, say I've got: class LogModel(models.Model): message = models.CharField(max_length=512) class Assignment(models.Model): someperson = models.ForeignKey(SomeOtherModel) def save(self, *args, **kwargs): super(Assignment, self).save() old_person = #????? LogModel(message="%s is no longer assigned to %s"%(old_person, self).save() LogModel(message="%s is now assigned to %s"%(self.someperson, self).save() My goal is to save to LogModel some messages about who Assignment was assigned to. Notice that I need to know the old, presave value of this field. I have seen code that suggests, before super().save(), retrieve the instance from the database via primary key and grab the old value from there. This could work, but is a bit messy. In addition, I plan to eventually split this code out of the .save() method via signals - namely pre_save() and post_save(). Trying to use the above logic (Retrieve from the db in pre_save, make the log entry in post_save) seemingly fails here, as pre_save and post_save are two seperate methods. Perhaps in pre_save I can retrieve the old value and stick it on the model as an attribute? I was wondering if there was a common idiom for this. Thanks.

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• how can i set up a uniqueness constraint in mysql for columns that can be null?

  - by user299689

  I know that in MySQL, UNIQUE constraits don't treat NULL values as equal. So if I have a unique constraint on ColumnX, then two separate rows can have values of NULL for ColumnX and this wouldn't violate the constraint. How can I work around this? I can't just set the value to an arbitrary constant that I can flag, because ColumnX in my case is actually a foreign key to another table. What are my options here? Please note that this table also has an "id" column that is its primary key. Since I'm using Ruby on Rails, it's important to keep this id column as the primary key. Note 2: In reality, my unique key encompasses many columns, and some of them have to be null, because they are foreign keys, and only one of them should be non-null. What I'm actually trying to do is to "simulate" a polymorphic relationship in a way that keep referential integrity in the db, but using the technique outlined in the first part of the question asked here: http://stackoverflow.com/questions/922184/why-can-you-not-have-a-foreign-key-in-a-polymorphic-association

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