add validation for image urls

app/models/book.rb

@@ -2,4 +2,9 @@ class Book < ActiveRecord::Base
   attr_accessible :description, :image_url, :lent_to_user_id, :owner_id, :title
 
   validates_presence_of :description, :title
+
+  validates :image_url, allow_blank: true, format: {
+      with: %r{\.(gif|jpg|png)$}i,
+      message: 'must be a URL for GIF, JPG or PNG image.'
+  }
 end

test/fixtures/books.yml

@@ -1,15 +1,15 @@
 # Read about fixtures at http://api.rubyonrails.org/classes/ActiveRecord/Fixtures.html
 
 one:
-  title: MyString
-  description: MyText
-  image_url: MyString
+  title: Clean Code
+  description: A book about clean code.
+  image_url: image.png
   owner_id: 1
   lent_to_user_id: 1
 
 two:
-  title: MyString
-  description: MyText
-  image_url: MyString
+  title: Java for Beginners
+  description: Java book.
+  image_url:
   owner_id: 1
   lent_to_user_id: 1

test/unit/book_test.rb

@@ -4,6 +4,7 @@ class BookTest < ActiveSupport::TestCase
   test "should save book" do
     book = Book.new
     book.title = 'Clean Code'
+    book.image_url = 'cleancode.jpg'
     book.description = 'Clean Code book form Robert C. Martin'
     assert book.valid?
   end
@@ -19,4 +20,12 @@ class BookTest < ActiveSupport::TestCase
     book.title = 'Clean Code'
     assert book.invalid?
   end
+
+  test "should not save book with invalid image url" do
+    book = Book.new
+    book.title = 'Clean Code'
+    book.image_url = 'wrong_image'
+    book.description = 'Clean Code book form Robert C. Martin'
+    assert book.invalid?
+  end
 end