Rails - May 2011 - MyModel.descendants returns in a view after the first call?

[I posted this initially to the StackOverflow list.  There have been no
nibbles, so I''m coming here to consult the real experts! :)]

I want to display a selection list of MyModel subclasses in a view.
It''s
not working yet, so for sanity checking, I included this in my view:

<%= MyModel.descendants %>

The first time I render this page after re-starting the server, it shows
the list of descendants (there are six). All subsequent times, it shows
up as an empty list [].  When invoked from the console,
MyModel.descendants reliably returns all six subclasses.

FWIW, I have a require statement in my initializers:

Dir[Rails.root.join("app/models/my_models/**/*.rb").to_s].each {|f|
require f}
... and I''ve verified that they''re getting required.

What the @($%& is going on?

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I am guessing you are in development mode with class caching off :)
and that this behavior does not occur in test nor production mode ...

Try the code with either test or production if you haven''t already ...
I had gone to the place you are in as well, initially my workaround
was for my class to read directories and parse .rb files to find
sub-classes - LOL and bleyech!

If you are seeing that things work just fin in test and production
modes, you and I have the same issue - I just posted my solution to
this (my fix is not pretty) to the ruby talk mailing list - it works
consistently, maybe it will help you.

Let me know if you have trouble finding it.

Max

On 5/4/11, Fearless Fool <lists-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
wrote:
> [I posted this initially to the StackOverflow list.  There have been no
> nibbles, so I''m coming here to consult the real experts! :)]
>
> I want to display a selection list of MyModel subclasses in a view.
It''s
> not working yet, so for sanity checking, I included this in my view:
>
> <%= MyModel.descendants %>
>
> The first time I render this page after re-starting the server, it shows
> the list of descendants (there are six). All subsequent times, it shows
> up as an empty list [].  When invoked from the console,
> MyModel.descendants reliably returns all six subclasses.
>
> FWIW, I have a require statement in my initializers:
>
> Dir[Rails.root.join("app/models/my_models/**/*.rb").to_s].each
{|f|
> require f}
> ... and I''ve verified that they''re getting required.
>
> What the @($%& is going on?
>
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@Max:

Max Schubert wrote in post #996743:
> I am guessing you are in development mode with class caching off :)
> and that this behavior does not occur in test nor production mode ...
>
> Try the code with either test or production if you haven''t already
...
Thanks for the tip.  Rather than try to get my db migrated over to
production mode, I simply edited the line in
config/environments/development.rb to read:

     config.cache_classes = true

... and now it works.  BUT: this is a bug, right?

- ff

P.S.: Max: what''s your handle on ruby talk?  I poked around and
didn''t
see anything obvious

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Fearless,

On Thu, May 5, 2011 at 1:22 AM, Fearless Fool
<lists-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
wrote:
> Thanks for the tip.  Rather than try to get my db migrated over to
> production mode, I simply edited the line in
> config/environments/development.rb to read:
>
>     config.cache_classes = true
>
> ... and now it works.  BUT: this is a bug, right?
Well, not really a bug - it is ''behavior as designed'' but it
is also a
''fail'' for those of us using ''meta-programming as
designed LOL''
because meta-programming requires that classes be loaded and that the
callbacks associated with Object and Class get called!
> P.S.: Max: what''s your handle on ruby talk?  I poked around and
didn''t
> see anything obvious
I am using the Google group to respond, so no handle :).  Were you
able to find the code I used to work around the class caching issue?
It is not pretty but it works.

- Max

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Max Schubert wrote in post #996758:
> Fearless,
> Well, not really a bug - it is ''behavior as designed'' but
it is also a
> ''fail'' for those of us using ''meta-programming
as designed LOL''
> because meta-programming requires that classes be loaded and that the
> callbacks associated with Object and Class get called!
Now I''m intrigued, but that doesn''t explain to me why
MyModel.descendants returns a valid list of descendants on the first
call but an empty list thereafter.  Can you explain why this happens
(and why it''s considered behavior as designed)?
>> P.S.: I poked around and didn''t see anything obvious
>
> I am using the Google group to respond, so no handle :).  Were you
> able to find the code I used to work around the class caching issue?
I couldn''t find your code.  Without your patch, I can (a) restart the
server frequently to avoid the MyModel.descendants bug that I observed,
or (b) turn on class caching, which requires me to restart the server
between changes.  So your patch would be handy.

(...I''m still astonished this is how it''s designed to work!!)

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On 5 May 2011, at 07:37, Fearless Fool
<lists-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org> wrote:
> Max Schubert wrote in post #996758:
>> Fearless,
>> Well, not really a bug - it is ''behavior as designed''
but it is also a
>> ''fail'' for those of us using
''meta-programming as designed LOL''
>> because meta-programming requires that classes be loaded and that the
>> callbacks associated with Object and Class get called!
> 
> Now I''m intrigued, but that doesn''t explain to me why
> MyModel.descendants returns a valid list of descendants on the first
> call but an empty list thereafter.  Can you explain why this happens
> (and why it''s considered behavior as designed)?
descendants returns all loaded subclasses, but when class caching is off,
classes are cleared out, so the freshly loaded copy of MyModel has no loaded
subclasses.

About the best thing you can do is (at the bottom of the base class) use
require_dependency to make all the subclasses be loaded when the base class is.

By design is perhaps not quite the right word - it''s something people
have long been aware, with no obvious solution, but it is obviously not
desirable behaviour.

Fred
> 
>>> P.S.: I poked around and didn''t see anything obvious
>> 
>> I am using the Google group to respond, so no handle :).  Were you
>> able to find the code I used to work around the class caching issue?
> 
> I couldn''t find your code.  Without your patch, I can (a) restart
the
> server frequently to avoid the MyModel.descendants bug that I observed,
> or (b) turn on class caching, which requires me to restart the server
> between changes.  So your patch would be handy.
> 
> (...I''m still astonished this is how it''s designed to
work!!)
> 
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Use require_dependency to load subclasses.

# product.rb
class Produtct < AR::Base
end
require_dependency ''book''
require_dependency ''movie''

# movie.rb

class Movie < Product
end

# book.rb

class Book < Product
end


This way it will work in development and production environment + code
reloading will work fine.

Another solution is to keep list of class names in database (aka
''dictionary'' table), retrive it and constantize those names.

Robert Pankowecki
http://robert.pankowecki.pl

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Frederick Cheung wrote in post #996771:
> descendants returns all loaded subclasses, but when class caching is
> off, classes are cleared out, so the freshly loaded copy of MyModel has
> no loaded subclasses.
Robert Pankowecki wrote in post #996778:
> Use require_dependency to load subclasses.
@ Fred: Ah!  That makes sense -- now I see why my initialize-time 
require worked only the first time.

@Robert & @Fred: require_dependency it shall be.

Thanks all!

- ff

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Fearless, here is the code pattern I chose.  it adds a bunch of custom code to

config/environments/development.rb

in order to not have to pollute your class heirarchies with
''development only hacks'' code ..

I put this below the configure do block in development.rb:

i personally prefer this because it is just generating special
behavior for the development environment instead of altering behavior
on all environments.

-= Max

if Rails.env.development?

 # Bootstrap classes being referenced as we use meta-programming that
 # does not get called in development because class caching
 # is off.  Merely accessing the classes by name triggers the
 # callbacks that otherwise get called immediately with class
 # caching on in non-development environments.
 def bootstrap_x_callbacks
   X1.nil?
   X2.nil?
   X3.nil?
   X4.nil?
   X5.nil?
   X6.nil?
 end

 # Run once on startup after Rails environment is all warmed up and
 # ready to rock.
 MyApp::Application.configure do
   config.after_initialize do
     bootstrap_x_callbacks
   end
 end

 # Schedule it to be called after every reload!
 ActionDispatch::Callbacks.after do
   bootstrap_x_callbacks
 end

end

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[@Max: our messages just crossed.  With all due respect, I think I like
my approach better!]

Okay.  I''m out of the weeds, but I''d like to know if
I''ve done it the
right way.

My code references subclasses of MeteredService several transactions
after metered_service.rb gets loaded (e.g. the result of posting a
form), at which point the sub-classes had been flushed and there was
nothing that forced a reload of metered_service.rb.  Ergo, adding
require_dependency statements at the bottom of metered_service.rb
didn''t
help.

My next gambit was to overload MeteredService.descendants:

class MeteredService
  ...
  def self.descendants
    require_dependency ''metered_services/pge_business''
    require_dependency ''metered_services/pge_residential''
    require_dependency ''metered_services/sce_residential''
    require_dependency ''metered_services/sdge_residential''
    require_dependency ''metered_services/socalgas_residential''
    require_dependency ''metered_services/srp_residential''
    super
  end
end

... but that had the problem that it threw "TypeError (superclass
mismatch for class PGEBusiness)" when it got called.  Perhaps it''s
in
the wrong context when it calls require_dependency?  I don''t know, and
I
couldn''t find any documentation that helped.

So now I overload MeteredService.descendants, calling
''require'' instead
of ''require_dependency''.  This seems to work just fine:

class MeteredService
  ...
if Rails.env.development?
  def self.descendants
    require ''metered_services/pge_business''
    require ''metered_services/pge_residential''
    require ''metered_services/sce_residential''
    require ''metered_services/sdge_residential''
    require ''metered_services/socalgas_residential''
    require ''metered_services/srp_residential''
    super
  end
end
end

Yes, I will generalize this with a method that reads the contents of the
metered_services directory so I don''t have to manually add each one.

Let me know if I''m missing anything important.

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Fearless Fool wrote in post #996867:
> [@Max: our messages just crossed.  With all due respect, I think I like
> my approach better!]
> <snip>
> So now I overload MeteredService.descendants, calling
''require'' instead
> of ''require_dependency''.  This seems to work just fine:
> <snip>
> Let me know if I''m missing anything important.
Aaaaannnnnnd, there just one little problem: it doesn''t work.

So my next approach will be some magick with const_defined? -- this time 
for sure!!!  ;)

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Fearless Fool wrote in post #996869:
> So my next approach will be some magick with const_defined? -- this time
> for sure!!!  ;)
I tried that and now I''m really confused.  @Fred: would you expect it
to
be the case that:

  class MeteredService < ActiveRecord::Base ; end
  class PGEBusiness < MeteredService ; end

  Object.const_defined?("PGEBusiness") => true
yet
  MeteredService.descendants => []
?

That is, the system still knows about the class PGEBusiness, but it
doesn''t show up as a descendant of MeteredService?  That is, in fact,
what I''m seeing.

More to the point, how can I tell when I need to reload the subclasses?
(And if require_dependencies is the answer, how to I avoid the
superclass mismatch error that I''m seeing?)

(Feh.  Maybe I should just start running in production mode.)

TIA.

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Resistance is futile.

Until someone chimes in with a better idea, I''m adopting Max''s
approach
(above).

- ff

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@Max:

Fearless Fool wrote in post #996874:
> Until someone chimes in with a better idea, I''m adopting
Max''s approach
> (above).
I thought you might want to know: I''ve implemented your approach and
I''m
STILL seeing the problem: the descendants are defined the first time, 
but gone thereafter.  I put a Rails.logger.debug() call in 
bootstrap_x_callbacks and I can see it getting called on every reload. 
But still no joy.

Maybe I should take up macramé instead.

- ff

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On Thu, May 5, 2011 at 2:17 PM, Fearless Fool
<lists-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
wrote:
> @Max:
>
> Fearless Fool wrote in post #996874:
>> Until someone chimes in with a better idea, I''m adopting
Max''s approach
>> (above).
>
> I thought you might want to know: I''ve implemented your approach
and I''m
> STILL seeing the problem: the descendants are defined the first time,
> but gone thereafter.  I put a Rails.logger.debug() call in
> bootstrap_x_callbacks and I can see it getting called on every reload.
> But still no joy.
>
> Maybe I should take up macramé instead.
Could you respond with your development.rb file (with names sanitized
as needed?) .. what version of Rails are you using?  We have been
using 3.0.5, just upgrading to 3.0.7 this week ....

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On 5 May 2011, at 18:51, Fearless Fool
<lists-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org> wrote:
> Fearless Fool wrote in post #996869:
>> So my next approach will be some magick with const_defined? -- this
time
>> for sure!!!  ;)
> 
> I tried that and now I''m really confused.  @Fred: would you expect
it to
> be the case that:
> 
>  class MeteredService < ActiveRecord::Base ; end
>  class PGEBusiness < MeteredService ; end
> 
>  Object.const_defined?("PGEBusiness") => true
> yet
>  MeteredService.descendants => []
> ?
> 
> That is, the system still knows about the class PGEBusiness, but it
> doesn''t show up as a descendant of MeteredService?  That is, in
fact,
> what I''m seeing.
> 
Have you been mixing in calls to require with calls to
require_dependency/letting rails load things automatically? That can make a
giant mess of things where some classes are reloaded and others aren''t,
so for example PGEService could be a subclass of the previous request''s
MeteredService

Fred

> More to the point, how can I tell when I need to reload the subclasses?
> (And if require_dependencies is the answer, how to I avoid the
> superclass mismatch error that I''m seeing?)
> 
> (Feh.  Maybe I should just start running in production mode.)
> 
> TIA.
> 
> -- 
> Posted via http://www.ruby-forum.com/.
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# product.rb
class Produtct < AR::Base
end
require_dependency ''book''
require_dependency ''movie''

# movie.rb
class Movie < Product
end

# book.rb
class Book < Product
end

This works fine for me in development and production.

1. Development:
a) Request comes
b) you call Product.dependencies()
c) reading product.rb calls require_dependecy and reads movie.rb and
book.rb
d) Product.dependencies returns => [Movie, Book]
e) request is finished. Product, Movie, Book constanst are forgotten.

2. Development:
a) Request comes
b) you call Movie.something()
c) Movie inherits from Product
d) autoloading reads product.rb
e) reading product.rb calls require_dependecy and reads movie.rb and
book.rb
f) Movie.something() returns something
g) request finished. Product, Movie, Book constanst are forgotten.

3. Production

a) Loading rails application leads to require all files in app/**/*.rb
for faster forking application.
b) movie.rb or product.rb is read first.
c) require_dependency behaves like require.
d) Everything works still fine.

Robert Pankowecki

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Max Schubert wrote in post #996908:
> Could you respond with your development.rb file (with names sanitized
> as needed?) .. what version of Rails are you using?  We have been
> using 3.0.5, just upgrading to 3.0.7 this week ....
Sure.  Here it is.  Thanks.

===== versions (I''ll move to 3.0.7 when Heroku does):
% rake about
About your application''s environment
Ruby version              1.9.2 (x86_64-darwin10.6.0)
RubyGems version          1.3.7
Rack version              1.2
Rails version             3.0.5
Active Record version     3.0.5
Action Pack version       3.0.5
Active Resource version   3.0.5
Action Mailer version     3.0.5
Active Support version    3.0.5
Environment               development

===== development.rb (it''s still vanilla):
MyApp::Application.configure do
  config.cache_classes = false
  config.whiny_nils = true
  config.consider_all_requests_local       = true
  config.action_view.debug_rjs             = true
  config.action_controller.perform_caching = false
  config.action_mailer.raise_delivery_errors = false
  config.action_mailer.default_url_options = { :host =>
''localhost:3000''
}
  config.active_support.deprecation = :log
  config.action_dispatch.best_standards_support = :builtin
end

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@ Fred, @Robert Pankowecki:

Frederick Cheung wrote in post #996972:
> Have you been mixing in calls to require with calls to
> require_dependency/letting rails load things automatically? That can
> make a giant mess of things where some classes are reloaded and others
> aren''t, so for example PGEService could be a subclass of the
previous
> request''s MeteredService
Fred:

Your explanation makes sense.  I haven''t *intentionally* been mixing
calls -- I swore that off back in college after mixing kahlua and
tequila.  What I *have* tried:

[1] adding "require_dependency PGEBusiness'' at the end of its
superclass
(MeteredService), as suggested by Fred and Robert.  It didn''t fix the
problem, I THINK because I have a form that passes back "PGEBusiness"
several transactions after MeteredService was loaded, by which point the
subclasses has been flushed.

[2] overloading MeteredService.descendents method as described above:

class MeteredService
  def self.descendants
    require_dependency ''metered_services/pge_business''
    ...
    super
  end
end

... which resulted in the "TypeError (superclass mismatch for class
PGEBusiness)" error I''ve seen.

[3] Same as [2], but using ''require'' instead of
''require_dependency''.
Um, I think that also got the TypeError message.  That was several
commits ago -- I can go back and verify that was the problem.

So I''m still open to suggestions...

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@Robert, @Fred, @Max

GOT IT!  You all had a piece of the puzzle.

(Robert: As an aside, you mean "you call Product.descendants()" -- not
dependencies() -- but that''s a trifle.)

I realized that my reason that the Fred/Robert solution didn''t work was
correct: a request passes in the string "PGEBusiness", and the 
controller code does something like:

file: metered_services_controller.rb
  def create
    ...
    class_name = params["metered_service"]   # e.g.
"PGEBusiness"
    @metered_service = Object.const_get(class_name).new( ... )
    ...
  end

Notice there''s nothing there that forces a load of MeteredService, so 
the calls to require_dependency() at the bottom of metered_service.rb 
aren''t invoked.  But I can force a load of metered_service.rb by a call
to descendants():

  def create
    ...
    MeteredService.descendants()
    class_name = params["metered_service"]   # e.g.
"PGEBusiness"
    @metered_service = Object.const_get(class_name).new( ... )
    ...
  end

And that works.  *phew*  Thanks for your patience.

- ff

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Hello everyone!

During I probing Rails 3.1.rc1 noticed along the majors many little changes and
deprecations.
Most of them (changes and deprecations) easy fixable except ONE:

If we have:
	Dog as ActiveRecord class

	puppy = Dog.new
	puppy.new_record?
		=> true
	dumped_puppy = Marshal.load(Marshal.dump(puppy))
	dumped_puppy.new_record?
		=> false

	Why?

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