Add support for migration dry runs.

lib/evolver.rb

@@ -37,9 +37,9 @@ def find(migration, session)
   # @return [ true ] True if all migrations succeeded.
   #
   # @since 0.0.0
-  def migrate
+  def migrate(options = {})
     load_migrations
-    Migrator.new(sessions).execute and true
+    Migrator.new(sessions, options).execute and true
   end
 
   # Load all the migrations in the application.

lib/evolver/migrator.rb

@@ -8,7 +8,11 @@ class Migrator
 
     TEMPLATE = File.join(File.dirname(__FILE__), "stats.txt.erb")
 
-    attr_reader :sessions
+    DEFAULT_OPTIONS = {
+      dry: false
+    }
+
+    attr_reader :sessions, :dry, :options
 
     # Execute the migrations. This grabs all pending migrations in order and
     # calls execute on them for each session that it has specified it runs on.
@@ -22,11 +26,13 @@ class Migrator
     #
     # @since 0.0.0
     def execute
+      log(:info, "DRY RUN: Migrations will only be logged and *NOT* executed.") if dry
+
       sessions.each_pair do |name, session|
         pending(name, session.with(safe: true, consistency: :strong)).each do |migration|
-          migration.execute
+          migration.execute unless dry
           log_execution(migration, name)
-          migration.mark_as_executed
+          migration.mark_as_executed unless dry
         end
       end
     end
@@ -52,10 +58,13 @@ def executed(session)
     #   Migrator.new([ session ])
     #
     # @param [ Array<Moped::Session> ] sessions The sessions to migrate on.
+    # @param [ Hash ] options Migration options.
     #
     # @since 0.0.0
-    def initialize(sessions)
+    def initialize(sessions, options = {})
+      @options  = DEFAULT_OPTIONS.merge(options)
       @sessions = sessions
+      @dry      = @options[:dry] || false
     end
 
     # Get the list of pending migrations - those that need to be executed

lib/evolver/tasks/db.rake

@@ -6,6 +6,13 @@ namespace :evolver do
     Evolver.migrate
   end
 
+  namespace :migrate do
+    desc "Dry run of any pending MongoDB data migrations on all sessions."
+    task dry: :environment do
+      Evolver.migrate(dry: true)
+    end
+  end
+
   desc "Revert the last run MongoDB data migration."
   task revert: :environment do
     Evolver.revert

spec/evolver/migrator_spec.rb

@@ -6,18 +6,35 @@
     Mongoid.default_session
   end
 
-  before(:all) do
+  before do
     session[:evolver_migrations].find.remove_all
   end
 
   describe "#execute" do
 
-    let(:migrator) do
-      described_class.new({ default: session })
+    context "dry run" do
+
+      let(:migrator) do
+        described_class.new({ default: session }, { dry: true })
+      end
+
+      before { migrator.execute }
+
+      let(:migrations) do
+        session[:evolver_migrations].find.sort(generated: 1).to_a
+      end
+
+      it "does not execute the migrations" do
+        migrations.count.should eq(0)
+      end
     end
 
     context "when no migrations have been run" do
 
+      let(:migrator) do
+        described_class.new({ default: session })
+      end
+
       before do
         session[:labels].insert({
           bands: [ "Depeche Mode", "Erasure" ],
@@ -28,11 +45,10 @@
       end
 
       after do
-        session[:evolver_migrations].find.remove_all
         session[:labels].find.remove_all
       end
 
-      let!(:migrations) do
+      let(:migrations) do
         session[:evolver_migrations].find.sort(generated: 1).to_a
       end