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• Grails XOM linkageerror - SAXParserException

  - by Stefan Kendall

  Possibly related: http://stackoverflow.com/questions/2762439/grails-attempting-to-include-htppbuilder-linkage-error I'm trying to include XOM in my grails project. How do I know which dependency library I need to exclude? I'm lost here. dependencies { build('xom:xom:1.1') { excludes "xml-apis" } } Error: java.lang.LinkageError: loader constraint violation: loader (instance of <bootloader>) previously initiated loading for a different type with name "org/xml/sax/SAXParseException" at java.lang.Class.getDeclaredMethods0(Native Method) at java.lang.Class.privateGetDeclaredMethods(Class.java:2427) at java.lang.Class.getDeclaredMethods(Class.java:1791) at java.security.AccessController.doPrivileged(Native Method) at org.codehaus.groovy.util.LazyReference.getLocked(LazyReference.java:33) at org.codehaus.groovy.util.LazyReference.get(LazyReference.java:20) at grails.util.PluginBuildSettings.getPluginInfos(PluginBuildSettings.groovy:124) at grails.util.PluginBuildSettings.getPluginInfos(PluginBuildSettings.groovy) at grails.util.PluginBuildSettings$getPluginInfos.callCurrent(Unknown Source) at grails.util.PluginBuildSettings.getPluginInfo(PluginBuildSettings.groovy:160) at grails.util.PluginBuildSettings$getPluginInfo.callCurrent(Unknown Source) at grails.util.PluginBuildSettings.getPluginInfoForSource(PluginBuildSettings.groovy:195) at org.codehaus.groovy.transform.ASTTransformationVisitor$3.call(ASTTransformationVisitor.java:303) at org.codehaus.groovy.control.CompilationUnit.applyToSourceUnits(CompilationUnit.java:820) at org.codehaus.groovy.control.CompilationUnit.doPhaseOperation(CompilationUnit.java:513) at org.codehaus.groovy.control.CompilationUnit.processPhaseOperations(CompilationUnit.java:489) at org.codehaus.groovy.control.CompilationUnit.compile(CompilationUnit.java:466) at _GrailsEvents_groovy.run(_GrailsEvents_groovy:54) at _GrailsEvents_groovy$run.call(Unknown Source) at _GrailsArgParsing_groovy$run.call(Unknown Source) at _GrailsArgParsing_groovy.run(_GrailsArgParsing_groovy:29) at _GrailsArgParsing_groovy$run.call(Unknown Source) at _GrailsInit_groovy$run.call(Unknown Source) at _GrailsInit_groovy.run(_GrailsInit_groovy:38) at _GrailsInit_groovy$run.call(Unknown Source) at Help_.run(Help_.groovy:27) at Help_$run.call(Unknown Source) at gant.Gant.processTargets(Gant.groovy:494) at gant.Gant.processTargets(Gant.groovy:480)

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• Deselecting a form input and waiting for a function before submitting

  - by blcArmadillo

  I have a form with an input field where a user enters a unique identifier. I then have some jQuery code that upon the user moving away from the input field (blur) goes out and fetches details about the part and populates some fields on the page. The problem is if the user clicks the submit button before moving out of the input field the jQuery code never has a chance to load in the data and populate the necessary fields. Whats the best way to go about doing this? I thought about maybe setting the focus to body and then having an infinite loop that keeps checking the page until all fields that should be filled in have been filled in but I feel like some sort of event based solution would be better than unpredictable infinite loops. Any ideas?

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• How Do I Convert text to a WAV file With Inaudible Waveform?

  - by Scott

  I am trying to create an audio watermarking system. I figure the best solution is to create an audio file (WAV) based on a unique string of text and then combine this with the original wav. The part that makes this tricky (for me anyway) is: How do I convert the text string to a wav? How do I ensure that the resulting WAV form is inaudible (or at least barely noticeable to the listener). I would prefer this be done server side (via PHP, etc) but if the processing load isn't too much then would be ok with something in Flash or Javascript. I'd be willing to pay someone to create me a workable solution (complete source code that functions as described). Thanks, Scott!

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• datatables-multi-filter-select

  - by user1871603

  I am using the jquery plug-in datatables. I am using the feature, datatables-multi-filter-select on my website with php code. I want to move the drop down filter from the footer to the header like in the following example: http://www.datatables.net/extras/thirdparty/ColumnFilterWidgets/DataTables/extras/ColumnFilterWidgets/ Can anyone please update the following PHP code sample to accomplish this? Code: /** * Register necessary Plugin Filters */add_filter( 'tablepress_shortcode_table_default_shortcode_atts', 'tablepress_add_shortcode_parameters_multi_filter_select' );add_filter( 'tablepress_table_render_options', 'tablepress_set_table_foot_option', 10, 2 );add_filter( 'tablepress_table_js_options', 'tablepress_add_multi_filter_select_js_options', 10, 3 );add_filter( 'tablepress_datatables_command', 'tablepress_add_multi_filter_select_js_command', 10, 5 ); /** * Add "datatables_multi_filter_select" as a valid parameter to the [table /] Shortcode */function tablepress_add_shortcode_parameters_multi_filter_select( $default_atts ) { $default_atts['datatables_multi_filter_select'] = false; return $default_atts;} /** * Make sure that "table_foot" and "datatables_scrollX" are false, if "datatables_multi_filter_select" is true, * as the footer will be appended by the JS. Scrolling will not work with automatically added content */function tablepress_set_table_foot_option( $render_options, $table ) { if ( $render_options['datatables_multi_filter_select'] ) { $render_options['table_foot'] = false; $render_options['datatables_scrollX'] = false; } return $render_options;} /** * Pass "datatables_multi_filter_select" from Shortcode parameters to JavaScript arguments */function tablepress_add_multi_filter_select_js_options( $js_options, $table_id, $render_options ) { $js_options['datatables_multi_filter_select'] = $render_options['datatables_multi_filter_select']; // register the JS if ( $js_options['datatables_multi_filter_select'] ) { $suffix = ( defined( 'SCRIPT_DEBUG' ) && SCRIPT_DEBUG ) ? '' : '.min'; $js_multi_filter_select_url = plugins_url( "multi-filter-select{$suffix}.js", __FILE__ ); wp_enqueue_script( 'tablepress-multi_filter_select', $js_multi_filter_select_url, array( 'tablepress-datatables' ), '1.0', true ); } return $js_options;} /** * Evaluate "datatables_multi_filter_select" parameter and add corresponding JavaScript code, if needed */function tablepress_add_multi_filter_select_js_command( $command, $html_id, $parameters, $table_id, $js_options ) { if ( ! $js_options['datatables_multi_filter_select'] ) return $command; $name = str_replace( '-', '_', $html_id ); $datatables_name = "DT_{$name}"; $command = <<<JSvar {$name} = $('#{$html_id}'), {$datatables_name} = {$name}.dataTable({$parameters}), {$name}_tfoot, {$name}_selects, ths = '<tfoot>';{$name}.find('thead th').each( function( i ) { ths += '<th>' + datatables_fnCreateSelect( {$datatables_name}.fnGetColumnData(i) ) + '</th>';} );ths += '</tfoot>';{$name}_tfoot = {$name}.append(ths).find('tfoot');{$name}_selects = {$name}_tfoot.find('select');{$name}_tfoot.on( 'change', 'select', function() { {$datatables_name}.fnFilter( $(this).val(), {$name}_selects.index(this) );} );JS; return $command;} (function($) {/* * Function: fnGetColumnData * Purpose: Return an array of table values from a particular column. * Returns: array string: 1d data array * Inputs: object:oSettings - dataTable settings object. This is always the last argument past to the function * int:iColumn - the id of the column to extract the data from * bool:bUnique - optional - if set to false duplicated values are not filtered out * bool:bFiltered - optional - if set to false all the table data is used (not only the filtered) * bool:bIgnoreEmpty - optional - if set to false empty values are not filtered from the result array * Author: Benedikt Forchhammer <b.forchhammer /AT\ mind2.de> */$.fn.dataTableExt.oApi.fnGetColumnData = function ( oSettings, iColumn, bUnique, bFiltered, bIgnoreEmpty ) { // check that we have a column id if ( typeof iColumn == "undefined" ) return new Array(); // by default we only wany unique data if ( typeof bUnique == "undefined" ) bUnique = true; // by default we do want to only look at filtered data if ( typeof bFiltered == "undefined" ) bFiltered = true; // by default we do not wany to include empty values if ( typeof bIgnoreEmpty == "undefined" ) bIgnoreEmpty = true; // list of rows which we're going to loop through var aiRows; // use only filtered rows if (bFiltered == true) aiRows = oSettings.aiDisplay; // use all rows else aiRows = oSettings.aiDisplayMaster; // all row numbers // set up data array var asResultData = new Array(); for (var i=0,c=aiRows.length; i<c; i++) { iRow = aiRows[i]; var aData = this.fnGetData(iRow); var sValue = aData[iColumn]; // ignore empty values? if (bIgnoreEmpty == true && sValue.length == 0) continue; // ignore unique values? else if (bUnique == true && jQuery.inArray(sValue, asResultData) > -1) continue; // else push the value onto the result data array else asResultData.push(sValue); } return asResultData;}}(jQuery)); function datatables_fnCreateSelect( aData ) { var r = '<select><option value=""></option>', i, iLen = aData.length; for ( i=0 ; i<iLen ; i++ ) { r += '<option value="'+aData[i]+'">'+aData[i]+'</option>'; } return r + '</select>';}

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• Why put a DAO layer over a persistence layer (like JDO or Hibernate)

  - by Todd Owen

  Data Access Objects (DAOs) are a common design pattern, and recommended by Sun. But the earliest examples of Java DAOs interacted directly with relational databases -- they were, in essence, doing object-relational mapping (ORM). Nowadays, I see DAOs on top of mature ORM frameworks like JDO and Hibernate, and I wonder if that is really a good idea. I am developing a web service using JDO as the persistence layer, and am considering whether or not to introduce DAOs. I foresee a problem when dealing with a particular class which contains a map of other objects: public class Book { // Book description in various languages, indexed by ISO language codes private Map<String,BookDescription> descriptions; } JDO is clever enough to map this to a foreign key constraint between the "BOOKS" and "BOOKDESCRIPTIONS" tables. It transparently loads the BookDescription objects (using lazy loading, I believe), and persists them when the Book object is persisted. If I was to introduce a "data access layer" and write a class like BookDao, and encapsulate all the JDO code within this, then wouldn't this JDO's transparent loading of the child objects be circumventing the data access layer? For consistency, shouldn't all the BookDescription objects be loaded and persisted via some BookDescriptionDao object (or BookDao.loadDescription method)? Yet refactoring in that way would make manipulating the model needlessly complicated. So my question is, what's wrong with calling JDO (or Hibernate, or whatever ORM you fancy) directly in the business layer? Its syntax is already quite concise, and it is datastore-agnostic. What is the advantage, if any, of encapsulating it in Data Access Objects?

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• Url shortening algorithm

  - by Bozho

  Now, this is not strictly about URL shortening, but my purpose is such anyway, so let's view it like that. Of course the steps to URL shortening are: Take the full URL Generate a unique short string to be the key for the URL Store the URL and the key in a database (a key-value store would be a perfect match here) Now, about the 2nd point. Here's what I've come up with: ByteArrayOutputStream baos = new ByteArrayOutputStream(); DataOutputStream dos = new DataOutputStream(baos); UUID uuid = UUID.randomUUID(); dos.writeLong(uuid.getMostSignificantBits()); String encoded = new String(Base64.encodeBase64(baos.toByteArray()), "ISO-8859-1"); String shortUrl = StringUtils.left(6); // returns the leftmost 6 characters // check if exists in database, repeat until it does not I wonder if this is good enough. Is it?

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• SQLite multi process access

  - by Nicolas

  Hello, We are using SQLite in a multi processes and multi threaded application. The SQLite database files are encrypted using the embedded SQLite encryption. The FAQ states that SQLite should be able to manage multi process accesses using locks mechanism. We are experiencing a strange problem: When many threads are accessing the same database file, sometime constrains violations occur, more specifically - a field with a unique constrain is getting duplicate values after calling "insert or replace" statement. It happens quite often now, that we are using the encryption. Before we started using SQLite encryption we did not notice such a behavior. Are there any specific known issues with this?

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• How to enforce foreign keys using Xerial SQLite JDBC?

  - by Space_C0wb0y

  According to their release notes, the Xerial SQLite JDBC driver supports foreign keys since version 3.6.20.1. I have tried some time now to get a foreign key constraint to be enforced, but to no avail. Here is what I came up with: public static void main(String[] args) throws ClassNotFoundException, SQLException { Class.forName("org.sqlite.JDBC"); SQLiteConfig config = new SQLiteConfig(); config.enforceForeignKeys(true); Connection connection = DriverManager.getConnection("jdbc:sqlite::memory:", config.toProperties()); connection.createStatement().executeUpdate( "CREATE TABLE artist(" + "artistid INTEGER PRIMARY KEY, " + "artistname TEXT);"); connection.createStatement().executeUpdate( "CREATE TABLE track("+ "trackid INTEGER," + "trackname TEXT," + "trackartist INTEGER," + "FOREIGN KEY(trackartist) REFERENCES artist(artistid)" + ");"); connection.createStatement().executeUpdate( "INSERT INTO track VALUES(14, 'Mr. Bojangles', 3)"); } The table definitions are taken directly from the sample in the SQLite documentation. This is supposed to fail, but it doesn't. I also checked, and it really inserts the tuple (no ignore or something like that). Does anyone have any experience with that, or knows how to make it work?

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• Google Local Search API

  - by Gublooo

  hey guys couple of quick questions 1) In the local search results - we can get a lot of parameters like street title, address, city, state, lat, long , url etc - In order for me to uniquely identify this record - can I consider URL to be unique to this address or concatenation of latitude and longitude Ref: http://code.google.com/apis/ajaxsearch/documentation/reference.html#_class_GlocalResult 2) In terms of usage, depending upon what user enters, I'm displaying a list of local business for the user to choose. Now when a user selects a particular business address - is it legal for me to store that business address along with lat and longitude information in my database for future look ups. I've seen a lot of blogs talking about storing the lat/long info but just want to be sure that i'm not violating and google rules. Thanks

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• which collection should I use

  - by Masna

  Hello, I have a number of custom objects of type X. X has a number of parameters and must be unique in the collection. (I created my own equals method based on the custom parameters to examine this) In each object of type x, I have a list of objects y. I want to add/remove/modify easily an object y. For example: To write the add method, it would be something like add(objTypeX, objTypeY) I would check or the collections already has a objTypeX. If so: i would add the objTypeY to the already existing objTypeX else: i would create objTypeX and add objTypeY to this object. To modify an objTypeY, it would be something like(objTypeX, objTypeY, newobjTypeY) I would get objTypeX out of the collections and modify objTypeY to newobjTypeY Which collections should I use? I tried with hashset but i can get a specific object out of the list, without run down the list till I find that object. I develop this in vb.net 3.5

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• On Edit Grid view Multiple row edit validation

  - by franics

  I am using a telerik radgrid so there are around 5 columns each column edittemplate contains a control along with a required field validator and also a property called AllowMultiRowEdit is set to true so i am able to multiple rows in edit mode. If any of the values are cleared the for multiple rows I want only those to be validated on update of that particular row. So i implemented the Grids item data bound event find each and every validator along with the update button and set a unique validation group. The above implementation most times but fails at some time. Is there any other way of going about this ?

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• Dynamic Selectors with Jquery with php while loop

  - by Anders Kitson

  I have a while loop which creates a list of anchor tags each with a unique class name counting from 1 to however many items there are. I would like to change a css attriubute on a specific anchor tag and class when it is clicked so lets say the background color is changed. Here is my code while($row = mysql_fetch_array($results)){ $title = $row['title']; $i++; echo "<a class='$i'>$title</a> } I would like my jquery to look something like this, it is obviously going to be more complicated than this I am just confused as where to start. $(document).ready(function() { $('a .1 .2 .3 .4 and so on').click(function() { $('a ./*whichever class was clicked*/').css('background':'red'); }); });

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• Entity Relationship Multiple 1:1's

  - by Evan

  I have an application where I have a generic object (table) called Hull. Each hull in the table is unique. I have another object that has three hulls, but they are specifically the Port_Hull, Center_Hull and Starboard_Hull. Rather than create a One to Many relationship, I was trying to create a one to one relationship for each one, but this results in numerous errors unless I make the relationship from Hull to Vessel one to many (which it is not). Any idea how I go about this, or should I abandon the concept and make the vessel to hull relationship one to many and deal with lists that always have three entries? p.s. Using uniqueidentifiers as many users can be adding records while disconnected. Hull Table HullID uniqueidentifier (primary key) plus bunch of hull data fields Vessel Table VesselID uniqueidentifier (primary key) MainHullID uniqueidentifier (tried as key and non-key) PortHullID uniqueidentifier StarboardHullID uniqueidentifier plus bunch of Vessel data fields

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• polymorphic hql

  - by Berryl

  I have a base type where "business id" must be unique for a given subclass, but it is possible for there to be different subclasses with the same business id. If there is a base type with a requested id but of the wrong subclass I want to return null, using a named query. The code below does this, but I am wondering if I can avoid the try/catch with a better HQL. Can I? Cheers, Berryl current hql <query name="FindActivitySubjectByBusinessId"> <![CDATA[ from ActivitySubject act where act.BusinessId = :businessId ]]> </query> current fetch code public ActivitySubject FindByBusinessId<T>(string businessId) where T : ActivitySubject { Check.RequireStringValue(businessId, "businessId"); try { return _session.GetNamedQuery("FindActivitySubjectByBusinessId") .SetString("businessId", businessId) .UniqueResult<T>(); } catch (InvalidCastException e) { // an Activity Subject was found with the requested id but the wrong type return null; } }

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• alignment and granularity of mmap

  - by OwnWaterloo

  I am confused by the specification of mmap. Let pa be the return address of mmap (the same as the specification) pa = mmap(addr, len, prot, flags, fildes, off); In my opinion after the function call succeed the following range is valid [ pa, pa+len ) My question is whether the range of the following is still valid? [ round_down(pa, pagesize) , round_up(pa+len, pagesize) ) [ base, base + size ] for short That is to say: is the base always aligned on the page boundary? is the size always a multiple of pagesize (the granularity is pagesize in other words) Thanks for your help. I think it is implied in this paragraph : The off argument is constrained to be aligned and sized according to the value returned by sysconf() when passed _SC_PAGESIZE or _SC_PAGE_SIZE. When MAP_FIXED is specified, the application shall ensure that the argument addr also meets these constraints. The implementation performs mapping operations over whole pages. Thus, while the argument len need not meet a size or alignment constraint, the implementation shall include, in any mapping operation, any partial page specified by the range [pa,pa+len). But I'm not sure and I do not have much experience on POSIX. Please show me some more explicit and more definitive evidence Or show me at least one system which supports POSIX and has different behavior Thanks agian.

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• Can ActionScript tell when a SWF was published?

  - by spiralganglion

  I'd like to write a little class that adds a Day/Month box showing the date a SWF was published from Flash. The company I work for regularly produces many, many SWFs and many versions of each, iterating over the course of months. A version-tracking system we've been using to communicate with our clients is a Day/Month date-box that gives the date the SWF was published. Up until now, we've been filling in the publish date by hand. If there's any way I can do this programatically with ActionScript that'd be fantastic. Any insight? Basically, all I need is the call that gives me the publish date, or even.. anything about the circumstances under which a SWF was published that I could use to roll into some form of.. automated version identification, unique to this SWF. So, can ActionScript tell when a SWF was published?

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• keep duplicate number records only - perl

  - by manu

  Hello I have one text string which is having some duplicate characters (FFGGHHJKL), these can be made unique by using the positive lookahead [perl script for the same$ perl -pe 's/(.)(?=.*?\1)//g']. (FFEEDDCCGG OUTPUT == FEDCG) My question is how to make it work on the numbers (Ex. 212 212 43 43 5689 6689 5689 71 81 === output should be 212 43 5689 6689 71 81) ? Also if we want to have only duplicate records to be given as the output from a file having n rows (212 212 43 43 5689 6689 5689 71 81 \n 66 66 67 68 69 69 69 71 71 52 ..\n .. .. \n... OUTPUT == 212 212 43 43 5689 5689 \n 66 66 69 69 69 71 71) then what should be done ? Thanks and regards -manu

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• Zip up web page groups to view in browsers

  - by Arlen Beiler

  There should be a standard for saving viewing bunches of webpages as a website. For instance, say I have a whole bunch of pages, such as I get from the WordPress plugin "Really Static" which saves an entire site, and I have all the links start with a slash. Now, I can't really use those links if I am reading it from the file system. If there would be a standard where we could zip up files, give them a unique extension (like "hzip" for html zip), and open the file with any browser, which would display it as though the root of that file were the root of the pages. "http://example.com" The links would all work right. This would facilitate sharing and copying groups of webpages.

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• Fluent Nhibernate Mapping Single class on two database tables

  - by nabeelfarid

  Hi guys, I am having problems with Mapping. I have two tables in my database as follows: Employee and EmployeeManagers Employee EmployeeId int Name nvarchar EmployeeManagers EmployeeIdFk int ManagerIdFk int So the employee can have 0 or more Managers. A manager itself is also an Employee. I have the following class to represent the Employee and Managers public class Employee { public virtual int Id { get; set; } public virtual string Name { get; set; } public virtual IList<Employee> Managers { get; protected set; } public Employee() { Managers = new List<Employee>(); } } I don't have any class to represent Manager because I think there is no need for it, as Manager itself is an Employee. I am using autoMapping and I just can't figure out how to map this class to these two tables. I am implementing IAutoMappingOverride for overriding automappings for Employee but I am not sure what to do in it. public class NodeMap : IAutoMappingOverride { public void Override(AutoMapping<Node> mapping) { //mapping.HasMany(x => x.ValidParents).Cascade.All().Table("EmployeeManager"); //mapping.HasManyToMany(x => x.ValidParents).Cascade.All().Table("EmployeeManager"); } } I also want to make sure that an employee can not be assigned the same manager twice. This is something I can verify in my application but I would like to put constraint on the EmployeeManager table (e.g. a composite key) so a same manager can not be assigned to an employee more than once. Could anyone out there help me with this please? Awaiting Nabeel

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• Mysql stored procedures

  - by Richard M

  Hello, I have a unique issue that I need advice on. I've been writing asp.net apps with SQL Server back ends for the past 10 years. During that time, I have also written some PHP apps, but not many. I'm going to be porting some of my asp.net apps to PHP and have run into a bit of an issue. In the Asp.net world, it's generally understood that when accessing any databases, using views or stored procedures is the preferred way of doing so. I've been reading some PHP/MySQL books and I'm beginning to get the impression that utilizing stored procedures in mysql is not advisable. I hesitate in using that word, advisable, but that's just the felling I get. So, the advice I'm looking for is basically, am I right or wrong? Do PHP developers use stored procedures at all? Or, is it something that is shunned? Thanks in advance.

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• HowTo init Django model, before using it?

  - by mosg

  Hi. I'm new to python and django. Apps | Versions: Python 2.6.2 Django (working with PostgreSQL) Question: I wrote a simple model: class OperationType(models.Model): eid = models.IntegerField(unique=True) name = models.CharField(max_length=64) def __unicode__(self): tpl = 'eid="', str(self.eid), '" name="', self.name, '"' return ''.join(tpl) Now I need to initialize it, for example with this data: 0, "None" 1, "Add" 2, "Edit" 3, "Delete" But I need to initialize this data not with admin web panel, but after class model created in the same code. How to do this? Thanks for help!

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• Does HashMap provide a one to one correspondence?

  - by Roman

  I found the following statement: Map is an object that stores key/volume pairs. Given a key, you can find its value. Keys must be unique, but values may be duplicated. For example I have just one key/value pair (3,4). And now I put a new pair (3,5). It will remove the old pair. Right? But if I put (2,4) instead of (3,4) I will add a new pair of key/value to the HashMap. Right?

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• What are people's opinions vis-a-vis my choice of authorization plugins?

  - by brad

  I'm slowly but surely putting together my first rails app (first web-app of any kind in fact - I'm not really a programmer) and it's time to set up a user registration/login system. The nature of my app is such that each user will be completely separated from each other user (except for admin roles). When users log in they will have their own unique index page looking at only their data which they and no-one else can ever see or edit. However, I may later want to add a role for a user to be able to view and edit several other user's data (e.g. a group of users may want to allow their secretary to access and edit their data but their secretary would not need any data of their own). My plan is to use authlogic to create the login system and declarative authorization to control permissions but before I embark on this fairly major and crucial task I thought I would canvas a few opinions as to whether this combo was appropriate for the tasks I envisage or whether there would be a better/simpler/faster/cheaper/awesomer option.

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• How to represent and insert into an ordered list in SQL?

  - by Travis

  I want to represent the list "hi", "hello", "goodbye", "good day", "howdy" (with that order), in a SQL table: pk | i | val ------------ 1 | 0 | hi 0 | 2 | hello 2 | 3 | goodbye 3 | 4 | good day 5 | 6 | howdy 'pk' is the primary key column. Disregard its values. 'i' is the "index" that defines that order of the values in the 'val' column. It is only used to establish the order and the values are otherwise unimportant. The problem I'm having is with inserting values into the list while maintaining the order. For example, if I want to insert "hey" and I want it to appear between "hello" and "goodbye", then I have to shift the 'i' values of "goodbye" and "good day" (but preferably not "howdy") to make room for the new entry. So, is there a standard SQL pattern to do the shift operation, but only shift the elements that are necessary? (Note that a simple "UPDATE table SET i=i+1 WHERE i=3" doesn't work, because it violates the uniqueness constraint on 'i', and also it updates the "howdy" row unnecessarily.) Or, is there a better way to represent the ordered list? I suppose you could make 'i' a floating point value and choose values between, but then you have to have a separate rebalancing operation when no such value exists. Or, is there some standard algorithm for generating string values between arbitrary other strings, if I were to make 'i' a varchar? Or should I just represent it as a linked list? I was avoiding that because I'd like to also be able to do a SELECT .. ORDER BY to get all the elements in order.

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• Creating self-referential tables with polymorphism in SQLALchemy

  - by Jace

  I'm trying to create a db structure in which I have many types of content entities, of which one, a Comment, can be attached to any other. Consider the following: from datetime import datetime from sqlalchemy import create_engine from sqlalchemy import Column, ForeignKey from sqlalchemy import Unicode, Integer, DateTime from sqlalchemy.orm import relation, backref from sqlalchemy.ext.declarative import declarative_base Base = declarative_base() class Entity(Base): __tablename__ = 'entities' id = Column(Integer, primary_key=True) created_at = Column(DateTime, default=datetime.utcnow, nullable=False) edited_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False) type = Column(Unicode(20), nullable=False) __mapper_args__ = {'polymorphic_on': type} # <...insert some models based on Entity...> class Comment(Entity): __tablename__ = 'comments' __mapper_args__ = {'polymorphic_identity': u'comment'} id = Column(None, ForeignKey('entities.id'), primary_key=True) _idref = relation(Entity, foreign_keys=id, primaryjoin=id == Entity.id) attached_to_id = Column(Integer, ForeignKey('entities.id'), nullable=False) #attached_to = relation(Entity, remote_side=[Entity.id]) attached_to = relation(Entity, foreign_keys=attached_to_id, primaryjoin=attached_to_id == Entity.id, backref=backref('comments', cascade="all, delete-orphan")) text = Column(Unicode(255), nullable=False) engine = create_engine('sqlite://', echo=True) Base.metadata.bind = engine Base.metadata.create_all(engine) This seems about right, except SQLAlchemy doesn't like having two foreign keys pointing to the same parent. It says ArgumentError: Can't determine join between 'entities' and 'comments'; tables have more than one foreign key constraint relationship between them. Please specify the 'onclause' of this join explicitly. How do I specify onclause?

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