Add use case for pinning answers

lib/use_case/answer/pin.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+module UseCase
+  module Answer
+    class Pin < UseCase::Base
+      option :user, type: Types.Instance(::User)
+      option :answer, type: Types.Instance(::Answer)
+
+      def call
+        check_ownership!
+
+        answer.pinned_at = Time.now.utc
+        answer.save!
+
+        {
+          status:   200,
+          resource: answer
+        }
+      end
+
+      private
+
+      def check_ownership!
+        raise ::Errors::NotAuthorized unless answer.user == user
+      end
+    end
+  end
+end

spec/lib/use_case/answer/pin_spec.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+describe UseCase::Answer::Pin do
+  include ActiveSupport::Testing::TimeHelpers
+
+  subject { UseCase::Answer::Pin.call(user:, answer:) }
+
+  context "answer exists" do
+    let(:answer) { FactoryBot.create(:answer, user: FactoryBot.create(:user)) }
+
+    context "as answer owner" do
+      let(:user) { answer.user }
+
+      it "pins the answer" do
+        travel_to(Time.at(1603290950).utc) do
+          expect { subject }.to change { answer.pinned_at }.from(nil).to(Time.at(1603290950).utc)
+        end
+      end
+    end
+
+    context "as other user" do
+      let(:user) { FactoryBot.create(:user) }
+
+      it "does not pin the answer" do
+        expect { subject }.to raise_error(Errors::NotAuthorized)
+        expect(answer.reload.pinned_at).to eq(nil)
+      end
+    end
+  end
+end