New IssueObserver class and spec.

Handles emails for new issues and reassigned issues.
Need to add creating a Note on Issue close.

app/models/issue_observer.rb

+class IssueObserver < ActiveRecord::Observer
+  cattr_accessor :current_user
+
+  def after_create(issue)
+    Notify.new_issue_email(issue.id) if issue.assignee != current_user
+  end
+
+  def after_change(issue)
+    if issue.assignee_id_changed?
+      recipient_ids = [issue.assignee_id, issue.assignee_id_was].keep_if {|id| id != current_user.id }
+
+      recipient_ids.each do |recipient_id|
+        Notify.reassigned_issue_email(recipient_id, issue.id, issue.assignee_id_was)
+      end
+    end
+  end
+end

spec/models/issue_observer_spec.rb

+require 'spec_helper'
+
+describe IssueObserver do
+  let(:some_user) { Factory.new(:user, :id => 1) }
+  let(:assignee) { Factory.new(:user, :id => 2) }
+  let(:issue)    { Factory.new(:issue, :id => 42, :assignee => assignee) }
+
+  before(:each) { subject.stub(:current_user).and_return(some_user) }
+
+  subject { IssueObserver.instance }
+
+  context 'when an issue is created' do
+
+    it 'sends an email to the assignee' do
+      Notify.should_receive(:new_issue_email).with(issue.id)
+
+      subject.after_create(issue)
+    end
+
+    it 'does not send an email to the assignee if assignee created the issue' do
+      subject.stub(:current_user).and_return(assignee)
+      Notify.should_not_receive(:new_issue_email)
+
+      subject.after_create(issue)
+    end
+  end
+
+  context 'when an issue is modified' do
+    it 'but not reassigned, does not send a reassigned email' do
+      issue.stub(:assignee_id_changed?).and_return(false)
+      Notify.should_not_receive(:reassigned_issue_email)
+
+      subject.after_change(issue)
+    end
+
+    context 'and is reassigned' do
+      let(:previous_assignee) { Factory.new(:user, :id => 3) }
+
+      before(:each) do
+        issue.stub(:assignee_id_changed?).and_return(true)
+        issue.stub(:assignee_id_was).and_return(previous_assignee.id)
+      end
+
+      it 'sends a reassigned email to the previous and current assignees' do
+        Notify.should_receive(:reassigned_issue_email).with(assignee.id, issue.id, previous_assignee.id)
+        Notify.should_receive(:reassigned_issue_email).with(previous_assignee.id, issue.id, previous_assignee.id)
+
+        subject.after_change(issue)
+      end
+
+      context 'does not send an email to the user who made the reassignment' do
+        it 'if the user is the assignee' do
+          subject.stub(:current_user).and_return(assignee)
+          Notify.should_not_receive(:reassigned_issue_email).with(assignee.id, issue.id, previous_assignee.id)
+
+          subject.after_change(issue)
+        end
+        it 'if the user is the previous assignee' do
+          subject.stub(:current_user).and_return(previous_assignee)
+          Notify.should_not_receive(:reassigned_issue_email).with(previous_assignee.id, issue.id, previous_assignee.id)
+
+          subject.after_change(issue)
+        end
+      end
+    end
+  end
+end