add module_release spec

app/models/training/module.rb

@@ -60,7 +60,6 @@ def debug_summary
       SUMMARY
     end
 
-    # TODO: add unit test and new model spec
     # @return [DateTime, nil]
     def first_published_at
       ModuleRelease.find_by(module_position: position)&.first_published_at

spec/models/module_release_spec.rb

@@ -0,0 +1,10 @@
+require 'rails_helper'
+
+RSpec.describe ModuleRelease, type: :model do
+  describe '.ordered' do
+    include_context 'with module releases'
+    it 'returns module releases in order of module position' do
+      expect(described_class.ordered.map(&:module_position)).to eq([1, 2, 3])
+    end
+  end
+end