First version

README

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+=Paperclip Meta
+
+Simple gem to let paperclip play nice with thumbnails width, height and size.
+
+Paperclip Meta will get image dimensions right after post_process_styles using paperclips own Geometry.from_file. This should make paperclip-meta storage independent.
+
+==Quick Start
+
+Create migration:
+
+  class AddMetaToAvatar < ActiveRecord::Migration
+    def self.up
+      add_column :users, :avatar_meta,    :text
+    end
+
+    def self.down
+      remove_column :users, :avatar_meta
+    end
+  end
+
+Rebuild all thumbnails to fill meta column if you already have some attachments.
+
+Now you can use meta-magic:
+
+  <%= image_tag @user.avatar.url, :size => @user.avatar.image_size %>
+  <%= image_tag @user.avatar.url(:medium), :size => @user.avatar.image_size(:medium) %>
+  <%= image_tag @user.avatar.url(:thumb), :size => @user.avatar.image_size(:thumb) %>
+
+==Internals
+
+Meta column is simple hash:
+
+  :style => {
+    :width => 100,
+    :height => 100,
+    :size => 42000
+  }
+
+This hash will be marshaled and base64 encoded before writing to model attribute.
+
+New methods provided:
+  - width
+  - height
+  - size
+
+You can pass thumbnail style to all this methods. If style not passed, default_style will be used.
+
+==TODO
+
+* It will be nice to write some tests. :D

Rakefile

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+require 'rake'
+require 'rake/testtask'
+require 'rake/rdoctask'
+
+desc "Build the gem into the current directory"
+task :gem => :gemspec do
+  `gem build #{spec.name}.gemspec`
+end

init.rb

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+require File.join(File.dirname(__FILE__), "lib", "paperclip_meta")

lib/paperclip_meta.rb

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+module Paperclip
+  class Attachment
+    alias original_post_process_styles post_process_styles
+
+    # If model has #{name}_meta column we getting sizes of processed
+    # thumbnails and saving it to #{name}_meta column.
+    def post_process_styles
+      original_post_process_styles
+
+      if instance.respond_to?(:"#{name}_meta=")
+        meta = {}
+
+        @queued_for_write.each do |style, file|
+          geo = Geometry.from_file file
+          meta[style] = {:width => geo.width.to_i, :height => geo.height.to_i, :size => File.size(file) }
+        end
+
+        instance_write(:meta, ActiveSupport::Base64.encode64(Marshal.dump(meta)))
+      end
+    end
+
+    # Meta access methods
+    [:width, :height, :size].each do |meth|
+      define_method(meth) do |*args|
+        style = args.first || default_style
+        meta_read(style, meth)
+      end
+    end
+
+    def image_size(style = default_style)
+      "#{width(style)}x#{height(style)}"
+    end
+
+    private
+    def meta_read(style, item)
+      if instance.respond_to?(:"#{name}_meta")
+        if meta = Marshal.load(ActiveSupport::Base64.decode64(instance_read(:meta)))
+          meta.key?(style) ? meta[style][item] : nil
+        end
+      end
+    end
+  end
+end

paperclip-meta.gemspec

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+include_files = ["README*", "LICENSE", "Rakefile", "init.rb", "{lib,test}/**/*"].map do |glob|
+  Dir[glob]
+end.flatten
+
+spec = Gem::Specification.new do |s|
+  s.name              = "paperclip-meta"
+  s.version           = "0.1"
+  s.author            = "Alexey Bondar"
+  s.email             = "y8@ya.ru"
+  s.homepage          = "http://github.com/y8/paperclip-meta"
+  s.description       = "Add width, height and size methods to paperclip thumbnails"
+  s.summary           = "Thumbnail dimensions for paperclip"
+  s.platform          = Gem::Platform::RUBY
+  s.files             = include_files
+  s.require_path      = "lib"
+  s.rubyforge_project = "paperclip-meta"
+  s.has_rdoc          = false
+  s.add_dependency 'paperclip'
+end