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

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

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+# 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
+```