Rails - Aug 2006 - Conditional extend of active record

Hi list,

I''m trying to figure out the best way to add some functionality to 
active record. I have a table of files, and need to write a set of core 
functions that do different things depending on the type of file eg:

class file < ActiveRecord::Base

  def resize
     ....does stuff
  end

end

the problem is the "....does stuff". For example, if the file is an 
image it will be resized using rmagick, if it''s a video it will be 
resized and re-encoded with an external encoder. I''d like to
encapsulate
the specific "resize" logic somewhere and avoid long winded case/if 
statements.

def resize
  if type == image
    ..resize in rmagick
  elseif type == video
    ..send to encoder
  else
     ..do some default resize action
  end
end

It would be nice to be able to have something like this:
def resize
  ExtendedClass.resize
end
where ExtendedClass represents a more specific version of the resize 
function depending on the file type.

I think what I''m looking for is a mixture of extending (via 
file.extend(Image) or file.extend(Video) for example), but that is 
getting a bit messy to manage.
Any ideas? What options are available to extend or override methods in a 
base class, depending on the values found in the base.

sorry for the vague explanation - it''s the end of a long day!
thanks,
Dan




-- 
Posted via http://www.ruby-forum.com/.

On 8/15/06, Daniel Greig <dgreig@magian.com>
wrote:
> Hi list,
>
> I''m trying to figure out the best way to add some functionality to
> active record. I have a table of files, and need to write a set of core
> functions that do different things depending on the type of file eg:
>
> class file < ActiveRecord::Base
>
>   def resize
>      ....does stuff
>   end
>
> end
>
> the problem is the "....does stuff". For example, if the file is
an
> image it will be resized using rmagick, if it''s a video it will be
> resized and re-encoded with an external encoder. I''d like to
encapsulate
> the specific "resize" logic somewhere and avoid long winded
case/if
> statements.
>
Somewhere in the db, you will probably already store the type of file
(image, video, etc.). In which case it is a relatively small step to
make all your file ARs inherit from a base class and add the specific
resize stuff to the subclasses:

class GenericFile < ActiveRecord::Base
  # common stuff, associations, etc.

  def resize
   # do nothing - or print a message or something
  end
end

class Image < GenericFile
  def resize
   # RMagick
  end
end

class Video < GenericFile
  def resize
    # video resizing
  end
end

In your db table, add a "type" field - AR will automatically store the
type of model object and return instances of the correct subclass when
queried.

Max

> I think what I''m looking for is a mixture of extending (via
> file.extend(Image) or file.extend(Video) for example), but that is
> getting a bit messy to manage.
> Any ideas? What options are available to extend or override methods in a
> base class, depending on the values found in the base.
>
Somewhere in the db, you will probably already store the type of file
(image, video, etc.). In which case it is a relatively small step to
make all your file ARs inherit from a base class and add the specific
resize stuff to the subclasses:

class GenericFile < ActiveRecord::Base
 # common stuff, associations, etc.

 def resize
  # do nothing - or print a message or something
 end
end

class Image < GenericFile
 def resize
  # RMagick
 end
end

class Video < GenericFile
 def resize
   # video resizing
 end
end

In your db table, add a "type" field - AR will automatically store the
type of model object and return instances of the correct subclass when
queried.

you should possibly look at the file_column plugin and extend it to fit your
neds... it already does some neat things with rmagick''able files, so it
could be of use to you. dont forget to contribute your work back if it can
be of use for other ppl... :)

2006/8/15, Max Muermann <ruby@muermann.org>:
>
> > I think what I''m looking for is a mixture of extending (via
> > file.extend(Image) or file.extend(Video) for example), but that is
> > getting a bit messy to manage.
> > Any ideas? What options are available to extend or override methods in
a
> > base class, depending on the values found in the base.
> >
>
> Somewhere in the db, you will probably already store the type of file
> (image, video, etc.). In which case it is a relatively small step to
> make all your file ARs inherit from a base class and add the specific
> resize stuff to the subclasses:
>
> class GenericFile < ActiveRecord::Base
> # common stuff, associations, etc.
>
> def resize
>   # do nothing - or print a message or something
> end
> end
>
> class Image < GenericFile
> def resize
>   # RMagick
> end
> end
>
> class Video < GenericFile
> def resize
>    # video resizing
> end
> end
>
> In your db table, add a "type" field - AR will automatically
store the
> type of model object and return instances of the correct subclass when
> queried.
> _______________________________________________
> Rails mailing list
> Rails@lists.rubyonrails.org
> http://lists.rubyonrails.org/mailman/listinfo/rails
>


-- 
Michael Siebert <info@siebert-wd.de>

www.stellar-legends.de - Weltraum-Browsergame im Alpha-Stadium
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Max Muermann wrote:
> Somewhere in the db, you will probably already store the type of file
> (image, video, etc.). In which case it is a relatively small step to
> make all your file ARs inherit from a base class and add the specific
> resize stuff to the subclasses:
> 
> class GenericFile < ActiveRecord::Base
>  # common stuff, associations, etc.
> 
>  def resize
>   # do nothing - or print a message or something
>  end
> end
> 
> class Image < GenericFile
>  def resize
>   # RMagick
>  end
> end
> 
> class Video < GenericFile
>  def resize
>    # video resizing
>  end
> end
> 
> In your db table, add a "type" field - AR will automatically
store the
> type of model object and return instances of the correct subclass when
> queried.
I''m not sure why, but I didn''t remember a thing about Single
table
inheritance until you reminded me - thanks!

One more question - does anyone know if the type casting can be done on 
a new record, or is it only done when you retrieve an existing record. 
Using the above code snippet from Max I''ve been able to get the various
resize functions happening after retrieving a record, but if I call it 
on a new record (even after the type has been set and saved) it''s 
executing the generic resize.

Thanks,
Dan





-- 
Posted via http://www.ruby-forum.com/.

>
> One more question - does anyone know if the type casting can be done on
> a new record, or is it only done when you retrieve an existing record.
> Using the above code snippet from Max I''ve been able to get the
various
> resize functions happening after retrieving a record, but if I call it
> on a new record (even after the type has been set and saved) it''s
> executing the generic resize.
>
You don''t want to set the type manually, rather instantiate objects of
the right type in the first place:

new_file = Image.new
... do stuff, do NOT set type ...
new_file.save

another_new_file = Video.new
...
etc.

AR takes care of the rest.

Max