How to use a method's string argument as a method name variable

Question

In my rails app I am using carrierwave for images. Carrierwave creates different versions of an image and the url can be obtained like this: picture.large.url, picture.small.url, picture.thumb.url, etc.

I would like to create a method that can accept a string argument that can then be used for the image url. Something like this:

def profile_url (version)
  picture.version.url
end

So then I can write @user.profile_url('thumb') and it should give me the url in thumb size.

I get an undefined method 'version' error. Is this possible?

You can pass the version as an argument to url, i.e. picture.url('thumb') — Stefan
– Stefan, Commented May 22, 2016 at 8:22
@Stefan If you are certain of this, then I think you should offer it as an answer. Although the proposed answer works, yours removes the verbosity and complexity of an additional layer over the library and is therefore better IMO. PS, to the OP, although you wanted to specify a string, I suggest you reconsider and use a symbol instead. It's more idiomatic, easier to type, and simpler (no case or encoding). — Keith Bennett
– Keith Bennett, Commented May 22, 2016 at 9:25

tadman · Accepted Answer · 2016-05-22 00:36:44Z

6

Normally you can do it this way:

def profile_url(version)
  version = version.to_sym

  case version
  when :large, :small, :thumb
    picture.send(version).url
  end
end

The reason for the to_sym call here is so you can call this profile_url('thumb') or profile_url(:thumb) and both will work.

answered May 22, 2016 at 0:36

tadman

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5 Comments

Keith Bennett Over a year ago

Do you really want this to fail silently on a bad arg? I think raising an exception would be appropriate. This would also simplify the method tremendously: picture.send(version.to_sym).url.

tadman Over a year ago

@KeithBennett Depends on how important this is. Sometimes failing silently is good enough. It can be made more robust by triggering an exception.

Keith Bennett Over a year ago

True, but the 1-liner I provided does what your method does but will raise an exception if an unsupported name is used (if there is no method by that name).

tadman Over a year ago

@KeithBennett I mean this could be made to trigger a custom exception rather than a generic method not found version.

Keith Bennett Over a year ago

Could be a custom exception, or maybe in IllegalArgumentError? I wrote some code in a gem to handle this exact case, check out gist.github.com/keithrbennett/81bcfc12d913c05c7f299bd6e6fb351b and github.com/keithrbennett/trick_bag/blob/master/lib/trick_bag/…. I use a custom InvalidValueError but should probably just change that to IllegalArgumentError.

Stefan · Accepted Answer · 2016-05-22 12:08:10Z

1

According to the documentation for CarrierWave's url, you can pass the version as an argument:

When given a version name as a parameter, will return the url for that version [...]
my_uploader.url           #=> /path/to/my/uploader.gif
my_uploader.url(:thumb)   #=> /path/to/my/thumb_uploader.gif

Your code can be written as:

class User < ActiveRecord::Base
  mount_uploader :picture, PictureUploader

  def profile_url(version)
    picture.url(version)
  end
end

And called via:

@user.profile_url('thumb')
# or
@user.profile_url(:thumb)

You could also invoke the method directly:

@user.picture.url('thumb')
# or
@user.picture.url(:thumb)

answered May 22, 2016 at 12:08

Stefan

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