Move signup lookup into it's own Interactor

Trying to stay away from direct Repository access in the controller but
for now making this a static method instead of a full object as it's so simple.

app/controllers/signups_controller.rb

@@ -18,7 +18,7 @@ def create
   #
   # Run +command+ on the given signup
   def update
-    signup = Repository.for(Signup).find params[:id].to_i
+    signup = FindSignup.by_id(params[:id].to_i)
     action = UpdateSignup.new current_user, signup
     action.run params[:command]
 

app/interactors/find_signup.rb

@@ -0,0 +1,8 @@
+require 'repository'
+require 'models/signup'
+
+class FindSignup
+  def self.by_id(id)
+    Repository.for(Signup).find(id)
+  end
+end

test/integration/signups_controller_test.rb

@@ -41,7 +41,7 @@ class SignupsControllerTest < ActionController::TestCase
         raid = Raid.new id: 7
         signup = Signup.new raid: raid
 
-        Repository.for(Signup).expects(:find).with(14).returns(signup)
+        FindSignup.expects(:by_id).with(14).returns(signup)
         UpdateSignup.any_instance.expects(:run).with("accept")
 
         put :update, :id => 14, :command => :accept

test/unit/interactors/find_signup.rb

@@ -0,0 +1,16 @@
+require 'unit/test_helper'
+require 'models/signup'
+require 'interactors/find_signup'
+
+describe FindSignup do
+
+  describe ".by_id" do
+    it "returns the signup with the given id" do
+      s = Signup.new id: 4
+      Repository.for(Signup).save(s)
+
+      FindSignup.by_id(4).must_equal s
+    end
+  end
+
+end