add article for instance double

_config.yml

@@ -33,6 +33,9 @@ nav_external_links:
   - title: Expect vs should
     url: "#expect-vs-should"
     hide_icon: true
+  - title: Instance double over double
+    url: "#instance-double-over-double"
+    hide_icon: true
   - title: Factories, not fixtures
     url: "#factories-not-fixtures"
     hide_icon: true

_includes/instance_double_over_double.html

@@ -0,0 +1,45 @@
+<article>
+  <h2 id="instance-double-over-double">
+    <a href="#instance-double-over-double">
+      Instance double over double
+    </a>
+  </h2>
+
+  <p>Instance double ensures the object responds to methods being called, which is not true for double.</p>
+
+  <div class="example">
+{% highlight ruby %}
+class User
+  def full_name
+    "#{first_name} #{last_name}"
+  end
+end
+{% endhighlight %}
+  </div>
+
+  <div class="bad">
+{% highlight ruby %}
+it "passes" do
+  user = double(:user, name: "Ayrton Senna")
+  puts user.name
+end
+{% endhighlight %}
+  </div>
+
+  <div class="good">
+{% highlight ruby %}
+it "fails" do
+  user = instance_double(User, name: "Ayrton Senna")
+  puts user.name
+end
+{% endhighlight %}
+  </div>
+
+  <p>The above test will fail with the following message since the <code>name</code> method really doesn't exist.</p>
+
+  <div class="output">
+{% highlight text %}
+the User class does not implement the instance method: name. Perhaps you meant to use `class_double` instead?
+{% endhighlight %}
+  </div>
+</article>

_sass/custom/custom.scss

@@ -31,10 +31,10 @@
 }
 
 .output {
-  color: #bbb;
+  color: #333;
 
   &::before {
-    content: "FORMATTED OUTPUT";
+    content: "OUTPUT";
   }
 }
 

index.html

@@ -32,6 +32,7 @@ <h2>
 {% include group_expectations.html %}
 {% include all_possible_cases.html %}
 {% include expect_vs_should.html %}
+{% include instance_double_over_double.html %}
 {% include factories_not_fixtures.html %}
 {% include lets_not.html %}
 {% include avoid_hooks.html %}