For Rails apps, only run migrations and restart after deploy if migrations pending

lib/heroku_san/deploy/rails.rb

@@ -6,9 +6,12 @@ class Rails < Base
       def deploy
         # TODO: Add announce/logger
         super
-        @stage.run('rake db:migrate')
-        @stage.restart
+        if @stage.has_pending_migrations
+          @stage.run('rake db:migrate')
+          @stage.restart
+        end
       end
     end
   end
-end
+end
+

lib/heroku_san/stage.rb

@@ -129,5 +129,45 @@ def logs(tail = false)
     def revision
       git_named_rev(git_revision(repo))
     end
+
+    # Will return true if not using Rails or the remote DB isn't postgresql.
+    # Otherwise returns true if the remote DB has pending migrations and false if not.
+    def has_pending_migrations
+      return @has_pending_migrations if @has_pending_migrations != nil
+      return true unless defined?(::Rails)
+      database_url = long_config['DATABASE_URL']
+      return true unless database_url =~ /\Apostgres:\/\//
+      @has_pending_migrations = (local_migrations - remote_migrations(database_url)).present?
+    end
+
+  private
+
+    # returns an array of the schema_migrations run on the server
+    def remote_migrations(database_url)
+      database_uri = URI.parse(database_url)
+      begin
+        ENV['PGPASSWORD'] = database_uri.password
+        ENV['PGSSLMODE']  = 'require'
+        psql_cmd = "psql -U #{database_uri.user} -h #{database_uri.host} -p #{database_uri.port || 5432} #{database_uri.path[1..-1]}"
+        psql_output = `#{psql_cmd} -Atc "SELECT * FROM schema_migrations;" 2>&1`
+        if psql_output =~ /ERROR:  relation "schema_migrations" does not exist/
+          puts "'schema_migrations' table doesn't exist remotely, assuming no remote migrations"
+          return []
+        end
+        raise 'Error getting remote migrations' unless $?.success?
+        psql_output.split.map(&:to_i)
+      ensure
+        ENV.delete('PGPASSWORD')
+        ENV.delete('PGSSLMODE')
+      end
+    end
+
+    # returns an array of schema migration versions in the local source directory
+    def local_migrations
+      Rake::Task[:environment].invoke
+      ActiveRecord::Base.establish_connection
+      ActiveRecord::Migrator.migrations(ActiveRecord::Migrator.migrations_paths).map(&:version)
+    end
+
   end
 end

spec/heroku_san/deploy/rails_spec.rb

@@ -8,10 +8,20 @@ module Deploy
       it "calls push, rake db:migrate & restart" do
         subject = described_class.new(stage, {})
         stage.should_receive(:push) { "pushed" } # "mock" super
+        stage.should_receive(:has_pending_migrations) { true }
         stage.should_receive(:run).with('rake db:migrate') { "migrated" }
         stage.should_receive(:restart) { "restarted" }
         subject.deploy
       end
+
+      it "skips rake db:migrate & restart if there are no pending migrations" do
+        subject = described_class.new(stage, {})
+        stage.should_receive(:push) { "pushed" } # "mock" super
+        stage.should_receive(:has_pending_migrations) { false }
+        stage.should_not_receive(:run).with('rake db:migrate')
+        stage.should_not_receive(:restart)
+        subject.deploy
+      end
     end
   end
 end