Conditional extend of active record

Rails
1

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

2

On 8/15/06, Daniel G. [email protected] wrote:

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

3

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.

4

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… :slight_smile:

2006/8/15, Max M. [email protected]:

make all your file ARs inherit from a base class and add the specific
class Image < GenericFile

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
[email protected]
http://lists.rubyonrails.org/mailman/listinfo/rails

–
Michael S. [email protected]

www.stellar-legends.de - Weltraum-Browsergame im Alpha-Stadium

5

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

6

Max M. 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