# How can I prevent include one module multi-times? **URL:** \ **Category:** ruby-talk\ **Created:** [3 September 2008 02:59 UTC](https://rubytalk.org/t/how-can-i-prevent-include-one-module-multi-times/48821 "2008-09-03T02:59:06Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Zhao\_Yi](https://avatars.discourse-cdn.com/v4/letter/z/fbc32d/32.png) [@Zhao\_Yi](https://rubytalk.org/u/Zhao_Yi)\ **Post date:** [3 September 2008 02:59 UTC](https://rubytalk.org/t/how-can-i-prevent-include-one-module-multi-times/48821/1 "2008-09-03T02:59:06Z")
I am looking for a method to check whether this module has been included. Does Ruby have this feature? > **···** > > -- > Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/). ---
**Author:** ![\_Pena\_Botp1](https://avatars.discourse-cdn.com/v4/letter/_/94ad74/32.png) [@\_Pena\_Botp1](https://rubytalk.org/u/_Pena_Botp1)\ **Post date:** [3 September 2008 04:18 UTC](https://rubytalk.org/t/how-can-i-prevent-include-one-module-multi-times/48821/2 "2008-09-03T04:18:48Z")
# I am looking for a method to check whether this module has been # included. Does Ruby have this feature? you can query the class/module thru #included\_modules eg, > class C > end =\> nil > C.included\_modules =\> [Kernel] > module M > end =\> nil > class C > include M > end =\> C > C.included\_modules =\> [M, Kernel] or if you want to query the whole program space, search the ObjectSpace eg, > ObjectSpace.each\_object(Module).select{|m| m.class.name == "Module"} =\> [IRB, Exception2MessageMapper, Marshal, ObjectSpace, GC, Math, Process::Sys, Process::GID, Process::UID, Process, Signal, File::Constants, FileTest, Errno, Precision, Enumerable, Comparable, Kernel, Readline, RubyToken, IRB::Notifier, M, IRB::MethodExtender, IRB::ContextExtender, IRB::ExtendCommandBundle] kind regards -botp > **···** > > From: Zhao Yi [mailto:youhaodeyi@gmail.com] ---
**Author:** ![Roger\_Pack4](https://avatars.discourse-cdn.com/v4/letter/r/bb73d2/32.png) [@Roger\_Pack4](https://rubytalk.org/u/Roger_Pack4)\ **Post date:** [3 September 2008 05:33 UTC](https://rubytalk.org/t/how-can-i-prevent-include-one-module-multi-times/48821/3 "2008-09-03T05:33:35Z")
Zhao Yi wrote: > I am looking for a method to check whether this module has been > included. Does Ruby have this feature? Might try parsing Object.constants for the module name. > **···** > > -- > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C). ---
**Author:** ![TPReal](https://avatars.discourse-cdn.com/v4/letter/t/e68b1a/32.png) [@TPReal](https://rubytalk.org/u/TPReal)\ **Post date:** [3 September 2008 06:59 UTC](https://rubytalk.org/t/how-can-i-prevent-include-one-module-multi-times/48821/4 "2008-09-03T06:59:44Z")
Zhao Yi wrote: > I am looking for a method to check whether this module has been > included. Does Ruby have this feature? What's your question? In the subject you ask 'How can I prevent include one module multi-times?' and this is checked automatically without any special handling. > **···** > > -- > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C). ---
**Author:** ![James\_Coglan2](https://avatars.discourse-cdn.com/v4/letter/j/c67d28/32.png) [@James\_Coglan2](https://rubytalk.org/u/James_Coglan2)\ **Post date:** [3 September 2008 07:14 UTC](https://rubytalk.org/t/how-can-i-prevent-include-one-module-multi-times/48821/5 "2008-09-03T07:14:16Z")
That's partially correct, but you'll find that the module's included() hook gets called repeatedly: module M   def self.included(base)     puts "included M"   end end class C   # prints "included M" 3 times   include M   include M   include M end To check whether M is already mixed into C, the expression 'M \> C' returns true if C includes M. You could put this check inside M.included and throw an exception if it's true. > **···** > > 2008/9/3 Thomas B. \ > > > Zhao Yi wrote: > > \> I am looking for a method to check whether this module has been > > \> included. Does Ruby have this feature? > > > > What's your question? In the subject you ask 'How can I prevent include > > one module multi-times?' and this is checked automatically without any > > special handling. ---
**Author:** ![Sebastian\_Hungereck1](https://avatars.discourse-cdn.com/v4/letter/s/3ab097/32.png) [@Sebastian\_Hungereck1](https://rubytalk.org/u/Sebastian_Hungereck1)\ **Post date:** [3 September 2008 08:28 UTC](https://rubytalk.org/t/how-can-i-prevent-include-one-module-multi-times/48821/6 "2008-09-03T08:28:36Z")
Roger Pack wrote: > Zhao Yi wrote: > \> I am looking for a method to check whether this module has been > \> included. Does Ruby have this feature? > > Might try parsing Object.constants for the module name. Why? That only tells you whether that module exists (and it only tells you that when the module is in the top-level namespace). It doesn't tell you whether that module has been included into anything. > **···** > > -- > Jabber: sepp2k@jabber.org > ICQ: 205544826 ---
**Author:** ![James\_Coglan2](https://avatars.discourse-cdn.com/v4/letter/j/c67d28/32.png) [@James\_Coglan2](https://rubytalk.org/u/James_Coglan2)\ **Post date:** [3 September 2008 07:15 UTC](https://rubytalk.org/t/how-can-i-prevent-include-one-module-multi-times/48821/7 "2008-09-03T07:15:27Z")
I should have mentioned that including a module multiple times will not have any nasty side effects on the inheritance tree of the including class -- this is checked for you. > **···** > > 2008/9/3 James Coglan \ > > > 2008/9/3 Thomas B. \ > > > > > Zhao Yi wrote: > > > \> I am looking for a method to check whether this module has been > > > \> included. Does Ruby have this feature? > > > > > > What's your question? In the subject you ask 'How can I prevent include > > > one module multi-times?' and this is checked automatically without any > > > special handling. > > > > That's partially correct, but you'll find that the module's included() hook > > gets called repeatedly: > > > > module M > >   def self.included(base) > >     puts "included M" > >   end > > end > > > > class C > >   # prints "included M" 3 times > >   include M > >   include M > >   include M > > end > > > > To check whether M is already mixed into C, the expression 'M \> C' returns > > true if C includes M. You could put this check inside M.included and throw > > an exception if it's true. ---
**Author:** ![James\_Coglan2](https://avatars.discourse-cdn.com/v4/letter/j/c67d28/32.png) [@James\_Coglan2](https://rubytalk.org/u/James_Coglan2)\ **Post date:** [3 September 2008 08:35 UTC](https://rubytalk.org/t/how-can-i-prevent-include-one-module-multi-times/48821/8 "2008-09-03T08:35:23Z")
Ah, I think I misunderstood the question. I wrote this a while ago to find all the descendants of a module: class Module   def descendants     classes =     ObjectSpace.each\_object do |klass|       next unless Module === klass       classes \<\< klass if self \> klass     end     classes   end end So, a module has been included if mod.descendants.size.nonzero? > **···** > > 2008/9/3 Sebastian Hungerecker \ > > > Roger Pack wrote: > > \> Zhao Yi wrote: > > \> \> I am looking for a method to check whether this module has been > > \> \> included. Does Ruby have this feature? > > \> > > \> Might try parsing Object.constants for the module name. > > > > Why? That only tells you whether that module exists (and it only tells you > > that when the module is in the top-level namespace). It doesn't tell you > > whether that module has been included into anything. ---
**Author:** ![Roger\_Pack4](https://avatars.discourse-cdn.com/v4/letter/r/bb73d2/32.png) [@Roger\_Pack4](https://rubytalk.org/u/Roger_Pack4)\ **Post date:** [4 September 2008 18:04 UTC](https://rubytalk.org/t/how-can-i-prevent-include-one-module-multi-times/48821/9 "2008-09-04T18:04:04Z")
> Ah, I think I misunderstood the question. I wrote this a while ago to > find > all the descendants of a module: ah--if the problem is determining if a descendant exists, you could write an included(instance) method within the included Module to track. Cheers. -=R > **···** > > -- > Posted via [http://www.ruby-forum.com/\](http://www.ruby-forum.com/%5C).