API exposes the latest submission for a task

To be able to show the user the status of their task, we need to be able
to get the most recent submission. This makes it available as part of
the API.

app/serializable/serializable_task.rb

@@ -1,6 +1,7 @@
 class SerializableTask < JSONAPI::Serializable::Resource
   type 'tasks'
 
+  has_one :latest_submission
   has_many :submissions
   belongs_to :framework
 

spec/requests/v1/tasks_spec.rb

@@ -61,7 +61,7 @@
       expect(json['data'][2]).to have_attribute(:status).with_value('in_progress')
     end
 
-    it 'can optionally return included models' do
+    it 'can optionally include submissions' do
       task = FactoryBot.create(:task)
       submission = FactoryBot.create(:submission, task: task, aasm_state: 'pending')
 
@@ -75,6 +75,22 @@
       expect(json['included'][0])
         .to have_attribute(:status).with_value('pending')
     end
+
+    it 'can optionally include the latest_submission' do
+      task = FactoryBot.create(:task)
+      submission = FactoryBot.create(:submission, task: task, aasm_state: 'pending')
+
+      get '/v1/tasks?include=latest_submission'
+
+      expect(response).to be_successful
+      expect(json['data'][0]).to have_id(task.id)
+      expect(json['data'][0])
+        .to have_relationship(:latest_submission)
+        .with_data('id' => submission.id, 'type' => 'submissions')
+
+      expect(json['included'][0])
+        .to have_attribute(:status).with_value('pending')
+    end
   end
 
   describe 'GET /tasks?filter[status]=' do