Notes/tech/coding/rails.md

Notes/tech/coding/rails.md

@@ -100,6 +100,49 @@ expect(items).to match_array([
 ```
 Source: [benpickles](https://www.benpickles.com/articles/73-testing-an-array-of-objects-with-rspec-have_attributes)
 
+### Custom matcher
+
+```ruby
+RSpec::Matchers.define :have_errors_on do |attribute|
+  chain :with_message do |message|
+    @message = message
+  end
+
+  match do |model|
+    model.valid?
+
+    @has_errors = model.errors.key?(attribute)
+
+    if @message
+      @has_errors && model.errors[attribute].include?(@message)
+    else
+      @has_errors
+    end
+  end
+
+ failure_message_for_should do |model|
+     if @message
+       "Validation errors #{model.errors[attribute].inspect} should include #{@message.inspect}"
+     else
+       "#{model.class} should have errors on attribute #{attribute.inspect}"
+     end
+   end
+
+   failure_message_for_should_not do |model|
+     "#{model.class} should not have an error on attribute #{attribute.inspect}"
+   end
+ end
+
+
+# usage
+describe User do
+  before { subject.email = "foobar" }
+
+  it { should have_errors_on(:email).with_message("Email has an invalid format") }
+
+end
+```
+
 ## Data class
 
 Similiar to the [value-alike objects](https://docs.ruby-lang.org/en/master/Data.html) in ruby(3.2), here an example for rails: