Add check for pinning when the answer is already pinned

lib/use_case/answer/pin.rb

@@ -8,6 +8,7 @@ module UseCase
 
       def call
         check_ownership!
+        check_unpinned!
 
         answer.pinned_at = Time.now.utc
         answer.save!
@@ -23,6 +24,10 @@ module UseCase
       def check_ownership!
         raise ::Errors::NotAuthorized unless answer.user == user
       end
+
+      def check_unpinned!
+        raise ::Errors::BadRequest if answer.pinned_at.present?
+      end
     end
   end
 end

spec/lib/use_case/answer/pin_spec.rb

@@ -18,6 +18,17 @@ describe UseCase::Answer::Pin do
           expect { subject }.to change { answer.pinned_at }.from(nil).to(Time.at(1603290950).utc)
         end
       end
+
+      context "answer is already pinned" do
+        before do
+          answer.update!(pinned_at: Time.at(1603290950).utc)
+        end
+
+        it "raises an error" do
+          expect { subject }.to raise_error(Errors::BadRequest)
+          expect(answer.reload.pinned_at).to eq(Time.at(1603290950).utc)
+        end
+      end
     end
 
     context "as other user" do