application loading for rake tasks when no db exists

Since rails 5, the whole of the application is loaded (including all initializers and models) when a rake task is to be executed. In case no database exists, e.g. when issuing a db:create, this runs into errors in case calls to ActiveRecord methods require the existence of a database. Using the null-db gem, we can mimick the existence of a db in such cases.

Rails seems to reset the database connection to the one defined in the config/database.yml by itself.

Gemfile

@@ -288,6 +288,9 @@ platforms :mri, :mingw, :x64_mingw do
   group :postgres do
     gem 'pg', '~> 1.1.0'
   end
+
+  # Support application loading when no database exists yet.
+  gem 'activerecord-nulldb-adapter', '~> 0.3.9'
 end
 
 group :opf_plugins do

Gemfile.lock

@@ -257,6 +257,8 @@ GEM
       arel (>= 9.0)
     activerecord-import (0.28.1)
       activerecord (>= 3.2)
+    activerecord-nulldb-adapter (0.3.9)
+      activerecord (>= 2.0.0)
     activerecord-session_store (1.1.1)
       actionpack (>= 4.0)
       activerecord (>= 4.0)
@@ -895,6 +897,7 @@ DEPENDENCIES
   actionpack-xml_parser (~> 2.0.0)
   activemodel-serializers-xml (~> 1.0.1)
   activerecord-import (~> 0.28.1)
+  activerecord-nulldb-adapter (~> 0.3.9)
   activerecord-session_store (~> 1.1.0)
   acts_as_list (~> 0.9.9)
   acts_as_tree (~> 2.8.0)

config/initializers/05-null_db_fallback.rb

@@ -0,0 +1,36 @@
+#-- encoding: UTF-8
+
+#-- copyright
+# OpenProject is a project management system.
+# Copyright (C) 2012-2018 the OpenProject Foundation (OPF)
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2017 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See docs/COPYRIGHT.rdoc for more details.
+#++
+
+# Since Rails 5, rake commands like db:create load the whole application.
+# As initializers and other parts of the boot sequence rely on calls accessing
+# the DB, the null db gem is used to fake the existence of a database in cases where
+# the db has not been created yet.
+
+OpenProject::NullDbFallback.fallback

lib/open_project/null_db_fallback.rb

@@ -0,0 +1,68 @@
+#-- encoding: UTF-8
+
+#-- copyright
+# OpenProject is a project management system.
+# Copyright (C) 2012-2018 the OpenProject Foundation (OPF)
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2017 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See docs/COPYRIGHT.rdoc for more details.
+#++
+
+module OpenProject
+  module NullDbFallback
+    class << self
+      def fallback
+        ActiveRecord::Base.connection
+      rescue ActiveRecord::NoDatabaseError
+        applied!
+        ActiveRecord::Base.establish_connection adapter: :nulldb
+      end
+
+      def reset
+        return unless applied?
+
+        ActiveRecord::Base.establish_connection(database_config)
+      end
+
+      private
+
+      attr_accessor :applied
+
+      def applied!
+        self.applied = true
+      end
+
+      def unapplied!
+        self.applied = false
+      end
+
+      def applied?
+        !!applied
+      end
+
+      def database_config
+        YAML.load_file(File.join(Rails.root, "config", "database.yml"))[Rails.env]
+      end
+    end
+  end
+end