Add Check Specific Arguments To Received Method as an RSpec til

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1156 TILs and counting..._
+_1157 TILs and counting..._
 
 ---
 
@@ -47,6 +47,7 @@ _1156 TILs and counting..._
 * [React Native](#react-native)
 * [React Testing Library](#react-testing-library)
 * [ReasonML](#reasonml)
+* [RSpec](#rspec)
 * [Ruby](#ruby)
 * [sed](#sed)
 * [Shell](#shell)
@@ -870,6 +871,10 @@ _1156 TILs and counting..._
 - [Using Optional Labeled Function Arguments](reason/using-optional-labeled-function-arguments.md)
 - [Wrapping A Component For Use In JavaScript](reason/wrapping-a-component-for-use-in-javascript.md)
 
+### RSpec
+
+- [Check Specific Arguments To Received Method](rspec/check-specific-arguments-to-received-method.md)
+
 ### Ruby
 
 - [A Basic Case Statement](ruby/a-basic-case-statement.md)

rspec/check-specific-arguments-to-received-method.md

@@ -0,0 +1,40 @@
+# Check Specific Arguments To Received Method
+
+Let's say we have a method receiving a big hash of arguments. A hash like this:
+
+```ruby
+{
+  name: 'Taco Tray',
+  product_id: 'taco123',
+  price: 4500,
+  description: 'A big tray of tacos',
+  discounts: {
+    coupon: 'DISCOUNT_TACOS'
+  }
+}
+```
+
+In an RSpec test we want to check one of those hash values in a certain
+scenario. It can be tedious to type out and check the entire hash. Instead, we
+want the test to surgically check just one part of the hash.
+
+We can do this with RSpec's dynamic matcher syntax. The [`hash_including`
+argument
+matcher](https://rspec.info/documentation/3.4/rspec-mocks/RSpec/Mocks/ArgumentMatchers.html#hash_including-instance_method)
+can be nested within the `#with` part of `expect().to receive().with()`.
+
+```ruby
+expect(TacoTruck)
+  .to receive(:take_order)
+  .with(
+    hash_including(
+      discounts: { coupon: 'DISCOUNT_TACOS'}
+    )
+  )
+```
+
+This will assert about the `discounts` portion of the hash that `#take_order`
+gets called with. The rest of the hash will be ignored.
+
+Without `hash_including`, the `with` call would result in a failure when trying
+to match against the entire hash.