Add Expect A Method To Be Called And Actually Call It as a ruby til

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_827 TILs and counting..._
+_828 TILs and counting..._
 
 ---
 
@@ -637,6 +637,7 @@ _827 TILs and counting..._
 - [Editing Code In Pry](ruby/editing-code-in-pry.md)
 - [Encode A String As URL-Safe Base64](ruby/encode-a-string-as-url-safe-base64.md)
 - [Evaluating One-Off Commands](ruby/evaluating-one-off-commands.md)
+- [Expect A Method To Be Called And Actually Call It](ruby/expect-a-method-to-be-called-and-actually-call-it.md)
 - [FactoryGirl Sequences](ruby/factory-girl-sequences.md)
 - [Fail](ruby/fail.md)
 - [Finding The Source of Ruby Methods](ruby/finding-the-source-of-ruby-methods.md)

ruby/expect-a-method-to-be-called-and-actually-call-it.md

@@ -0,0 +1,42 @@
+# Expect A Method To Be Called And Actually Call It
+
+You can assert that a method is called without actually executing it. This is
+often what `expect(...).to receive(:method_name)`  is used for. If you do want
+that method called, RSpec can accommodate you.
+
+Let's say we have the following two classes:
+
+```ruby
+class Greeting
+  def self.say_hello
+    raise "Don't actually execute this"
+    puts "Hello"
+  end
+end
+
+class GreetingService
+  def self.run
+    Greeting.say_hello
+  end
+end
+```
+
+We can assert that `say_hello` gets called without actually raising the
+exception (first `it` block). If we tack on `and_call_original` then RSpec will
+make the assertion and execute the method (second `it` block).
+
+```ruby
+describe "expect and call original" do
+  it "expect the message is received" do
+    expect(Greeting).to receive(:say_hello)
+    GreetingService.run
+    # passes
+  end
+
+  it "expect and call original" do
+    expect(Greeting).to receive(:say_hello).and_call_original
+    GreetingService.run
+    # fails, RuntimeError
+  end
+end
+```