diff --git a/README.md b/README.md
index b617baf..6a4c235 100644
--- a/README.md
+++ b/README.md
@@ -1,12 +1,12 @@
-# Promocode
+# Promocode Generator
 
-TODO: Write a gem description
+Generates save and unique (when used with AR) codes which can be handed to customers for promotional campaigns.
 
 ## Installation
 
 Add this line to your application's Gemfile:
 
-    gem 'promocode'
+    gem 'promocode_generator'
 
 And then execute:
 
@@ -14,11 +14,26 @@ And then execute:
 
 Or install it yourself as:
 
-    $ gem install promocode
+    $ gem install promocode_generator
 
 ## Usage
 
-TODO: Write usage instructions here
+Without Rails:
+  
+  Use PromocodeGenerator.generate(length) to produce a promotional code of the desired length.
+
+With Rails:
+
+  In your model, use the promocode_attribute method to automatically generate a promotional code of length 8 in the before_save hook:
+  
+  ```ruby
+  class Campaign < ActiveRecord::Base
+    promocode_attribute :code
+  end
+  ```
+  
+  Basic uniqueness validation comes by default so that codes are generated until a non-existing code is found in the db. Provide the :reject_if option to customize:
+  `promocode_attribute :code, :reject_if => Proc.new { |code| ModelA.where(:code => code).any? }`
 
 ## Contributing
 
diff --git a/Rakefile b/Rakefile
index 2995527..1b41e31 100644
--- a/Rakefile
+++ b/Rakefile
@@ -1 +1,5 @@
 require "bundler/gem_tasks"
+require 'rspec/core/rake_task'
+
+RSpec::Core::RakeTask.new(:spec)
+task default: :spec
diff --git a/lib/promocode/version.rb b/lib/promocode/version.rb
deleted file mode 100644
index 03dba59..0000000
--- a/lib/promocode/version.rb
+++ /dev/null
@@ -1,3 +0,0 @@
-module Promocode
-  VERSION = "0.0.1"
-end
diff --git a/lib/promocode_generator.rb b/lib/promocode_generator.rb
index 364951e..70192e5 100644
--- a/lib/promocode_generator.rb
+++ b/lib/promocode_generator.rb
@@ -1,5 +1,11 @@
-require "promocode/version"
+require "promocode_generator/version"
+require "promocode_generator/model_additions"
 
-module Promocode
-  # Your code goes here...
+module PromocodeGenerator
+  # Generates an 8 character random code of uppercase letters and numbers
+  # It explicitly excludes the letters I, L, O and the number 0 to avoid
+  # confusion
+  def self.generate(length = 8)
+    [('A'..'Z').to_a - ['I', 'L', 'O'], ('1'..'9').to_a].flatten.shuffle[0,length].join
+  end
 end
diff --git a/lib/promocode_generator/model_additions.rb b/lib/promocode_generator/model_additions.rb
new file mode 100644
index 0000000..347f13a
--- /dev/null
+++ b/lib/promocode_generator/model_additions.rb
@@ -0,0 +1,14 @@
+module PromocodeGenerator
+  module ModelAdditions
+    def promocode_attribute(attribute, options = {})
+      options[:reject_if] ||= Proc.new { |code| self.where(attribute => code).any? }
+
+      before_save do
+        # Make sure not to generate the same code twice!
+        begin
+          self.code = PromocodeGenerator.generate
+        end while options[:reject_if].call(code)
+      end 
+    end
+  end
+end
diff --git a/lib/promocode_generator/version.rb b/lib/promocode_generator/version.rb
new file mode 100644
index 0000000..dd97fb6
--- /dev/null
+++ b/lib/promocode_generator/version.rb
@@ -0,0 +1,3 @@
+module PromocodeGenerator
+  VERSION = "0.0.1"
+end
diff --git a/promocode_generator.gemspec b/promocode_generator.gemspec
index e952b39..31b62a4 100644
--- a/promocode_generator.gemspec
+++ b/promocode_generator.gemspec
@@ -21,4 +21,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "bundler", "~> 1.3"
   spec.add_development_dependency "rake"
   spec.add_development_dependency "rspec"
+  spec.add_development_dependency "supermodel"
 end
diff --git a/spec/promocode_generator/model_additions_spec.rb b/spec/promocode_generator/model_additions_spec.rb
new file mode 100644
index 0000000..1d955d4
--- /dev/null
+++ b/spec/promocode_generator/model_additions_spec.rb
@@ -0,0 +1,40 @@
+require 'spec_helper'
+
+# Dummy reject condition that can be stubbed in the tests below
+module RejectCondition
+  def self.check(code)
+    false 
+  end
+end
+
+class Campaign < SuperModel::Base
+  include ActiveModel::Validations::Callbacks
+  extend PromocodeGenerator::ModelAdditions
+  promocode_attribute :code, :reject_if => Proc.new { |code| RejectCondition.check(code) }
+end
+
+describe PromocodeGenerator::ModelAdditions do
+  let(:campaign) { Campaign.new(:name => "foo") }
+
+  context "when the model is saved" do
+    it "generates a code" do
+      expect(PromocodeGenerator).to receive(:generate).once.and_return('ABCDEFG')
+      campaign.save!
+    end
+
+    it "assigns the code to the model" do
+      campaign.save!
+      expect(campaign.code).not_to be_nil
+    end
+  end
+
+  context "when the reject_if block returns false" do
+    before { RejectCondition.stub(:check).and_return(true, false) }
+    after { RejectCondition.unstub(:check) }
+
+    it "generates a new code" do
+      expect(PromocodeGenerator).to receive(:generate).exactly(2).times
+      campaign.save!
+    end
+  end
+end
diff --git a/spec/promocode_generator_spec.rb b/spec/promocode_generator_spec.rb
new file mode 100644
index 0000000..7448448
--- /dev/null
+++ b/spec/promocode_generator_spec.rb
@@ -0,0 +1,13 @@
+require 'spec_helper'
+
+describe PromocodeGenerator do
+  describe ".generate" do
+    it "generates a code of the given length" do
+      expect(PromocodeGenerator.generate(10).length).to eq(10)
+    end
+
+    it "generates a code with default length" do
+      expect(PromocodeGenerator.generate.length).to be > 0
+    end
+  end
+end
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
index c6664f1..1d935e5 100644
--- a/spec/spec_helper.rb
+++ b/spec/spec_helper.rb
@@ -1 +1,2 @@
-require 'promocode'
+require 'promocode_generator'
+require 'supermodel'