Add category spec and uniqueness check.

app/models/category.rb

@@ -1,6 +1,7 @@
 class Category < ActiveRecord::Base
   has_and_belongs_to_many :photos
   validates_presence_of :name, :slug
+  validates_uniqueness_of :name, :slug
 
   has_slug :slug, from: :name
 end

spec/factories/categories.rb

@@ -0,0 +1,7 @@
+# Read about factories at https://github.com/thoughtbot/factory_girl
+
+FactoryGirl.define do
+  factory :category do
+    name "Test Category"
+  end
+end

spec/models/category_spec.rb

@@ -0,0 +1,15 @@
+require 'spec_helper'
+
+describe Category do
+  it { should have_and_belong_to_many(:photos) }
+  it { should validate_presence_of(:name) }
+  it { should validate_presence_of(:slug) }
+  it { should validate_uniqueness_of(:name) }
+  it { should validate_uniqueness_of(:slug) }
+
+  let(:category) { create(:category, name: "A Test Category") }
+
+  it "should have a slug generated from name" do
+    expect(category.slug).to eql("a-test-category")
+  end
+end