python - Better way to set default value in django model field -

-

i implementing field in django model calls following function default value

def get_default_value():     = mymodel.objects.aggregate(max_id=max('id'))     return get_unique_value_based_on_num(a['max_id'] or 0)  class mymodel:     default_value_field = charfield(default=get_default_value)

although may wrong on this, fear implementation may result in race condition.

is there better way ? may use f object or else ?

to avoid race conditions, best letting database handle integrity of table, 1 of things databases made for.

to so, catch integrityerror raised saving model instance , try again different value when fails.

from django.db import integrityerror, models, transaction   def get_default_value():     = mymodel.objects.aggregate(max_id=max('id'))     return get_unique_value_based_on_num(a['max_id'] or 0)  class mymodel(models.model):     # have unicity enforced @ database level unique=true.     default_value_field = models.charfield(max_length=200, unique=true)      def save(self):         if not self.default_value_field:             max_tries = 100  # choose sensible value!             in range(max_tries):                 try:                     self.default_value_field = get_default_value()                      # atomic block rollback transaction in case of integrityerror.                     transaction.atomic():                         super(mymodel, self).save()                         break                 except integrityerror:                     # default_value_field not unique, try again new value.                     continue             else:                 # max tries reached, raise.                 raise integrityerror('could not save model because etc...')         else:             super(mymodel, self).save(*args, **kwargs)

Comments

Post a Comment

Popular posts from this blog

java - Custom OutputStreamAppender not run: LOGBACK: No context given for <MYAPPENDER> -

-
i wish write custom appender on basis of outputstreamappender . wrote following class package tests; import java.io.ioexception; import java.io.outputstream; import ch.qos.logback.core.outputstreamappender; public class myappender<e> extends outputstreamappender<e> { public myappender() { system.out.println("myappender created"); setoutputstream(new outputstream() { @override public void write(int b) throws ioexception { throw new unsupportedoperationexception(); } }); } } then wrote following runner: package tests; import org.slf4j.loggerfactory; import ch.qos.logback.classic.logger; public class runner { private static final logger log = (logger) loggerfactory.getlogger(runner.class); public static void main(string[] args) { //log.addappender(new myappender<iloggingevent>()); log.info("hello world"); } } finall...
Read more

java - UML - How would you draw a try catch in a sequence diagram? -

-
Image
is there standard way of drawing try catch sequence diagrams? i've made attempt based on this feel end result doesn't feel right. code diagram based around: public static void save () { try { filehandle filehandle = gdx.files.external(file); filehandle.writestring(boolean.tostring(constantshandler.soundenabled)+"\n", false); (int = 0; < 5; i++) { filehandle.writestring(integer.tostring(constantshandler.highscores[i])+"\n", true); } } catch (throwable e) { } } my attempt note: know still need add loop. that's ok. fragments meant show conditional control flow in sequence diagram. superstructures puts limitations on use of fragments (see pp. 467 of ss2.1.1). should use critical region try part above. , option catch . however, keep telling can take quite freedom in using uml. it's language , such changes in life time. long reader ge...
Read more

c++ - No viable overloaded operator for references a map -

-
i trying use map can make tag names reference number. when try , use it, in code error (6 in total every time reference map): src/main.cpp:25:45: error: no viable overloaded operator[] type 'std::map<std::string, std::string>' const char* idcs = node.child_value(tagmap[3]); this code: #include "pugi/pugixml.hpp" #include <iostream> #include <string> #include <map> int main() { pugi::xml_document doca, docb; std::map<std::string, pugi::xml_node> mapa, mapb; std::map<std::string, std::string> tagmap {{"1", "data"}, {"2", "entry"}, {"3", "id"}, {"4", "content"}}; if (!doca.load_file("a.xml") || !docb.load_file("b.xml")) { std::cout << "can't find input files"; return 1; } (auto& node: doca.child(tagmap[1]).children(tagmap[2])) { co...
Read more