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Sem Ptiri

2011-Jan-17 10:02 UTC

Dynamic class creation at runtime

I have a scenario where I need to dynamically create classes from
existing application (external to app) databases, to pull data for
reports.

# Table definitions
APP_TABLES     = %w(#list of tables for a given app - I''m sure I could
query this on a simple connect. But let''s stick to this for now.)

APP_TABLES.each do |table_name|
  # camelize for class creation
  class_name = table_name.camelize

  klass = Class.new(App) # uses App class to establish_connection to
externalDB

klass = class_eval do
      #unsure about this. I need to set table_name to same as class name
      ActiveRecord::Base.set_table_name("#{class_name}")
      ActiveRecord::Base.const_set(constant_name, klass)
end # class_eval

# After creating the class - I want to establish an instance - but it
says that the class does not exist - more specifically constant doesn''t
exist.
eval("#{table_name} = #{class_name}.new")




I would love to hear thoughts on this, especially if I''m approaching
this incorrectly.

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Frederick Cheung

2011-Jan-17 11:45 UTC

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Re: Dynamic class creation at runtime

On Jan 17, 10:02 am, Sem Ptiri
<li...-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
wrote:>   klass = Class.new(App) # uses App class to establish_connection to
> externalDB
>
> klass = class_eval do
>       #unsure about this. I need to set table_name to same as class name
>       ActiveRecord::Base.set_table_name("#{class_name}")
>       ActiveRecord::Base.const_set(constant_name, klass)
> end # class_eval
This sounds fishy, you''re calling set_table_name on AR::Base, not your
class. just klass.set_table_name ''blah'' should be enough
>
> # After creating the class - I want to establish an instance - but it
> says that the class does not exist - more specifically constant
doesn''t
> exist.
> eval("#{table_name} = #{class_name}.new")
>
> I would love to hear thoughts on this, especially if I''m
approaching
> this incorrectly.
>
You haven''t created a constant called class_name -
ActiveRecord::Base.const_set(constant_name, klass)  would have created
ActiveRecord::Base::Foo.
You probably wanted Object.const_set class_name, klass

Fred
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Sem Ptiri

2011-Jan-17 12:44 UTC

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Re: Dynamic class creation at runtime

Thanks for the reply.

Frederick Cheung wrote in post #975422:>> klass = class_eval do
>>    #unsure about this. I need to set table_name to same as class name
>>    ActiveRecord::Base.set_table_name("#{class_name}")
>>    ActiveRecord::Base.const_set(constant_name, klass)
>> end # class_eval
>
> This sounds fishy, you''re calling set_table_name on AR::Base, not
your
> class. just klass.set_table_name ''blah'' should be enough
True. I receive an unknown method error though.
>> eval("#{table_name} = #{class_name}.new")
> You haven''t created a constant called class_name -
> You probably wanted Object.const_set class_name, klass
Sorry - I seem to have missed posting the
  class_name = table_name.camelize
line in there (have large sections of comments that I didn''t want to 
clutter the post).

I''ve changed the above code to:

APP_TABLES.each do |table_name|
  class_name = table_name.camelize
    klass = Class.new(App)
    constant_name = "#{class_name}"

    klass = class_eval do
      set_table_name("#{class_name}") or puts "Class set table
name
failed"
      Object.const_set(constant_name, klass) or puts "Class name 
constant set failed"
    end # class_eval

but get an unknown method error on the set_table_name call.

The App class (below) inherits from ActiveRecord::Base which in turn 
should have that method.

class App < ActiveRecord::Base
  abstract_class = true

  def initialize
    establish_connection :app_development
  end

  # to avoid trying to connect to the Spydirdb table by default
  def self.columns() @columns ||= []; end
end

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Frederick Cheung

2011-Jan-17 14:08 UTC

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Re: Dynamic class creation at runtime

On Jan 17, 12:44 pm, Sem Ptiri
<li...-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
wrote:>
>     klass = class_eval do
>       set_table_name("#{class_name}") or puts "Class set
table name
> failed"
>       Object.const_set(constant_name, klass) or puts "Class name
> constant set failed"
>     end # class_eval
>
> but get an unknown method error on the set_table_name call.
you don''t need the class_eval . just klass.set_table_name should work
(I don''t think the method is protected, if it is you might have to use
send)>
> The App class (below) inherits from ActiveRecord::Base which in turn
> should have that method.
>
> class App < ActiveRecord::Base
>   abstract_class = true
>this should be self.abstract_class = true (right now you''re just
setting a local variable)
>   def initialize
>     establish_connection :app_development
>   end
>This is dodgy. First of all you''re overriding initialize, but without
calling through to the super class, so your activerecord objects won''t
get initialized properly. You''ve also change the signature of
initialize. Also, do you really want to be reconnecting to the
database everytime a new instance is created? lastly this
establish_connection is a class method
>   # to avoid trying to connect to the Spydirdb table by default
>   def self.columns() @columns ||= []; end
> end
>
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Sem Ptiri

2011-Jan-17 14:20 UTC

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Re: Dynamic class creation at runtime

Frederick Cheung wrote in post #975445:> On Jan 17, 12:44pm, Sem Ptiri
<li...-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org> wrote:
>> class App < ActiveRecord::Base
>>  abstract_class = true
>>
> this should be self.abstract_class = true (right now you''re just
> setting a local variable)
Point taken.
>>  def initialize
>>   establish_connection :app_development
>>  end
>>
> This is dodgy. First of all you''re overriding initialize, but
without
> calling through to the super class, so your activerecord objects
won''t
> get initialized properly.Taken from online suggestions for handling multiple DBs.
> You''ve also change the signature of
> initialize. Also, do you really want to be reconnecting to the
> database everytime a new instance is created? lastly this
> establish_connection is a class method
Thanks. I''ll fix all that and get back to you.

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Sem Ptiri

2011-Jan-18 06:31 UTC

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Re: Dynamic class creation at runtime

# APP_TABLES contain a list of table names - would be better to 
dynamically read them after connecting

APP_TABLES.each do |table_name|
  class_name = table_name.camelize
  klass = Class.new(App)
  constant_name = "#{class_name}"

  # even though klass is created with App as superclass (inheriting from 
ActiveRecord::Base, the set_table_name seems to be unavailable - what am 
I missing here?
  klass.set_table_name ("#{class_name}") or puts "Class set table
name
failed"
  klass.const_set(constant_name, klass) or puts "Class name constant set 
failed"
  eval("#{table_name} = #{class_name}.new") or puts "Class
instantiation
failed"
end



How can I determine if the klass has actually been created?
How long will the dynamic classes exist in memory?
For the duration of this rake task execution?
Is there a way to list classes created at runtime?

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Frederick Cheung

2011-Jan-18 07:24 UTC

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Re: Dynamic class creation at runtime

On Jan 18, 6:31 am, Sem Ptiri
<li...-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
wrote:> # APP_TABLES contain a list of table names - would be better to
> dynamically read them after connecting
>
> APP_TABLES.each do |table_name|
>   class_name = table_name.camelize
>   klass = Class.new(App)
>   constant_name = "#{class_name}"
>
>   # even though klass is created with App as superclass (inheriting from
> ActiveRecord::Base, the set_table_name seems to be unavailable - what am
> I missing here?
what happens when you try?
>   klass.set_table_name ("#{class_name}") or puts "Class set
table name
> failed"
>   klass.const_set(constant_name, klass) or puts "Class name constant
set
> failed"
>   eval("#{table_name} = #{class_name}.new") or puts "Class
instantiation
> failed"
> end
>
> How can I determine if the klass has actually been created?
I  don''t think Class.new can actually fail
> How long will the dynamic classes exist in memory?
> For the duration of this rake task execution?
> Is there a way to list classes created at runtime?
I think that klass exists much like any variable - it may be garbage
collected if nothing references it (classes might be special though) .
You can step through objects of any class (including Class) via
objectspace, although you probably won''t be able to distinguish
classes created like this from other classes.

Fred>
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Sem Ptiri

2011-Jan-18 07:55 UTC

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Re: Dynamic class creation at runtime

Thanks for your repeated answers.

Frederick Cheung wrote in post #975619:> what happens when you try?
Class set table name failed

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Frederick Cheung

2011-Jan-18 11:28 UTC

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Re: Dynamic class creation at runtime

On Jan 18, 7:55 am, Sem Ptiri
<li...-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
wrote:> Thanks for your repeated answers.
>
> Frederick Cheung wrote in post #975619:
>
> > what happens when you try?
>
> Class set table name failed
>That''s just what you''re printing out - the return value of
set_table_name isn''t documented, so i wouldn''t assume nil or
false to
mean failure

Fred
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Sem Ptiri

2011-Jan-19 08:05 UTC

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Re: Dynamic class creation at runtime

Frederick Cheung wrote in post #975672:
> That''s just what you''re printing out - the return value
of
> set_table_name isn''t documented, so i wouldn''t assume nil
or false to
> mean failure
OK, so I''m down to:

class ExternalApp < ActiveRecord::Base
  self.abstract_class = true
  establish_connection :external_app
  def self.columns() @columns ||= []; end
end

#App tables has a list of tables in external app, until I get this bit 
working
APP_TABLES.each do |table_name|
  constant_name = class_name = table_name.camelize
  klass = Class.new(ExternalApp)
  klass.set_table_name ("#{class_name}")
  klass.const_set(constant_name, klass)
  eval("#{table_name} = #{class_name}.new") or puts "Class
instantiation
failed"
end


On running this rake task I get the following error:

lib/rspec/core/backward_compatibility.rb:20:in `const_missing'': 
uninitialized constant Services

Is const_set where the klass name is ''set in stone''?

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Frederick Cheung

2011-Jan-19 08:12 UTC

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Re: Re: Dynamic class creation at runtime

On 19 Jan 2011, at 08:05, Sem Ptiri
<lists-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org> wrote:
> #App tables has a list of tables in external app, until I get this bit 
> working
> APP_TABLES.each do |table_name|
>  constant_name = class_name = table_name.camelize
>  klass = Class.new(ExternalApp)
>  klass.set_table_name ("#{class_name}")
>  klass.const_set(constant_name, klass)
>  eval("#{table_name} = #{class_name}.new") or puts "Class
instantiation
> failed"
> end
> 
> 
> On running this rake task I get the following error:
> 
> lib/rspec/core/backward_compatibility.rb:20:in `const_missing'': 
> uninitialized constant Services
> 
> Is const_set where the klass name is ''set in stone''?
It is, but you''re using it back to front: klass.const_set sets a
constant on klass, whereas you want to set a top level constant ie
Object.const_set

Fred. > 
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Sem Ptiri

2011-Jan-19 08:20 UTC

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Re: Re: Dynamic class creation at runtime

Frederick Cheung wrote in post #975964:> It is, but you''re using it back to front: klass.const_set sets a
> constant on klass, whereas you want to set a top level constant ie
> Object.const_set
That makes sense, thought it might be a scope problem.

Thank you.

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Sem Ptiri

2011-Jan-19 08:35 UTC

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Re: Dynamic class creation at runtime

Ok, so this seems to work

1) Create dynamic classes from a list (some rework on getting that from 
tables automatically needed)
2) Instantiate those objects

A minor error pops up now.

The tables exist, as do the fields.

However, with:

service = Services.new(:portID => 333, :method => "PUT")

But I get the error:

unknown attribute: portID

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Sem Ptiri

2011-Jan-19 08:59 UTC

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Re: Re: Dynamic class creation at runtime

Frederick Cheung wrote in post #975964:> It is, but you''re using it back to front: klass.const_set sets a
> constant on klass, whereas you want to set a top level constant ie
> Object.const_set
You are my personal hero, today. Thank you so much for helping out on 
this thread. It works.

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Frederick Cheung

2011-Jan-19 11:01 UTC

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Re: Dynamic class creation at runtime

On Jan 19, 8:35 am, Sem Ptiri
<li...-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org> wrote:
> The tables exist, as do the fields.
>
> However, with:
>
> service = Services.new(:portID => 333, :method => "PUT")
>
> But I get the error:
>
> unknown attribute: portID
>
Probably the result of you suppressing the columns method. If AR
things there are no columns, it will think there are no attributes

Fred> --
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Sem Ptiri

2011-Jan-19 11:37 UTC

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Re: Dynamic class creation at runtime

Frederick Cheung wrote in post #975987:> Probably the result of you suppressing the columns method. If AR
> things there are no columns, it will think there are no attributes
From the superclass. Right. Thanks.

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Sem Ptiri

2011-Jan-19 12:15 UTC

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Re: Dynamic class creation at runtime

Everything''s perfect now. Thanks again for the repeated replies.

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Rails - Jan 2011 - Dynamic class creation at runtime

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