Allow policies and scopes to be injected into controllers.

In controller specs instead of relying on Pundit to instantiate the correct
policy object allow it to be injected into the controller. More often than not
a double is used in controller specs which means the policy cannot be
inferred. This also allows us to double the policy to ensure that on a unit
level the rights methods are being called on callaborators.

class PostsController < ApplicationController
  attr_writer :post
  helper_method :post

  def create
    authorize post

    post.save
    respond_with post
  end

  private

  def post
    @post ||= Post.new post_attributes
  end
end

describe PagesController do
  let(:policy) { double 'SomePolicy', create?: true }

  before do
    controller.policy = policy
  end

  it 'delegates authorization to policy' do
    expect(policy).to have_received(:create?)
  end
end

Add spec for injecting policy.

Use `or` instead of ternary operator.

Allow policy_scope to be injected for controller tests.

lib/pundit.rb

@@ -63,12 +63,14 @@ module Pundit
 
   def policy_scope(scope)
     @_policy_scoped = true
-    Pundit.policy_scope!(pundit_user, scope)
+    @policy_scope or Pundit.policy_scope!(pundit_user, scope)
   end
+  attr_writer :policy_scope
 
   def policy(record)
-    Pundit.policy!(pundit_user, record)
+    @policy or Pundit.policy!(pundit_user, record)
   end
+  attr_writer :policy
 
   def pundit_user
     current_user

spec/pundit_spec.rb

@@ -242,6 +242,13 @@ describe Pundit do
     it "throws an exception if the given policy can't be found" do
       expect { controller.policy(article) }.to raise_error(Pundit::NotDefinedError)
     end
+
+    it "allows policy to be injected" do
+      new_policy = OpenStruct.new
+      controller.policy = new_policy
+
+      controller.policy(post).should == new_policy
+    end
   end
 
   describe ".policy_scope" do
@@ -252,5 +259,12 @@ describe Pundit do
     it "throws an exception if the given policy can't be found" do
       expect { controller.policy_scope(Article) }.to raise_error(Pundit::NotDefinedError)
     end
+
+    it "allows policy_scope to be injected" do
+      new_scope = OpenStruct.new
+      controller.policy_scope = new_scope
+
+      controller.policy_scope(post).should == new_scope
+    end
   end
 end