Activeadmin gem error on heroku deploy during assets:precompile - ruby-on-rails-4

Question

I had my application running on local. During deployment, I got several issues:
First, I wrapped relational objects in order to deploy, otherwise deploy gave no "tags" table error
if ActiveRecord::Base.connection.table_exists? 'tags'
filter :tags_keyword, :as => :check_boxes, :collection => Tag.all.collect {|o| [o.keyword, o.keyword]}
end
Then I run migrations and seeded the database. But this time, I had no assets working which made me realize heroku is not compiling assets during deployment.
I updated .gitignore to oversight manifest files so heroku would compile assets beforehand. But that operation gave me the following error;
remote: -----> Preparing app for Rails asset pipeline
remote: Running: rake assets:precompile
remote: rake aborted!
remote: NoMethodError: undefined method `<<' for true:TrueClass
remote: /tmp/build_392c42fc00109fc0ac97d8dd67521707/vendor/bundle/ruby/2.2.0/gems/activeadmin-1.0.0.pre1/lib/active_admin/engine.rb:5:in `block (2 levels) in <class:Engine>'
remote: /tmp/build_392c42fc00109fc0ac97d8dd67521707/vendor/bundle/ruby/2.2.0/gems/activeadmin-1.0.0.pre1/lib/active_admin/engine.rb:4:in `each'
remote: /tmp/build_392c42fc00109fc0ac97d8dd67521707/vendor/bundle/ruby/2.2.0/gems/activeadmin-1.0.0.pre1/lib/active_admin/engine.rb:4:in `block in <class:Engine>'
remote: /tmp/build_392c42fc00109fc0ac97d8dd67521707/vendor/bundle/ruby/2.2.0/gems/railties-4.2.1/lib/rails/initializable.rb:30:in `instance_exec'
remote: /tmp/build_392c42fc00109fc0ac97d8dd67521707/vendor/bundle/ruby/2.2.0/gems/railties-4.2.1/lib/rails/initializable.rb:30:in `run'
remote: /tmp/build_392c42fc00109fc0ac97d8dd67521707/vendor/bundle/ruby/2.2.0/gems/railties-4.2.1/lib/rails/initializable.rb:55:in `block in run_initializers'
remote: /tmp/build_392c42fc00109fc0ac97d8dd67521707/vendor/bundle/ruby/2.2.0/gems/railties-4.2.1/lib/rails/initializable.rb:54:in `run_initializers'
remote: /tmp/build_392c42fc00109fc0ac97d8dd67521707/vendor/bundle/ruby/2.2.0/gems/railties-4.2.1/lib/rails/application.rb:352:in `initialize!'
remote: /tmp/build_392c42fc00109fc0ac97d8dd67521707/config/environment.rb:5:in `<top (required)>'
remote: /tmp/build_392c42fc00109fc0ac97d8dd67521707/vendor/bundle/ruby/2.2.0/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:274:in `require'
remote: /tmp/build_392c42fc00109fc0ac97d8dd67521707/vendor/bundle/ruby/2.2.0/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:274:in `block in require'
remote: /tmp/build_392c42fc00109fc0ac97d8dd67521707/vendor/bundle/ruby/2.2.0/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:240:in `load_dependency'
remote: /tmp/build_392c42fc00109fc0ac97d8dd67521707/vendor/bundle/ruby/2.2.0/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:274:in `require'
remote: /tmp/build_392c42fc00109fc0ac97d8dd67521707/vendor/bundle/ruby/2.2.0/gems/railties-4.2.1/lib/rails/application.rb:328:in `require_environment!
remote: /tmp/build_392c42fc00109fc0ac97d8dd67521707/vendor/bundle/ruby/2.2.0/gems/railties-4.2.1/lib/rails/application.rb:457:in `block in run_tasks_blocks'
remote: /tmp/build_392c42fc00109fc0ac97d8dd67521707/vendor/bundle/ruby/2.2.0/gems/sprockets-rails-2.3.2/lib/sprockets/rails/task.rb:64:in `block (2 levels) in define'
remote: Tasks: TOP => environment
remote: (See full trace by running task with --trace)
remote: !
remote: ! Precompiling assets failed.
remote: !
remote:
remote: ! Push rejected, failed to compile Ruby app
This is the engine.rb file that error logs which is under the AA gem folder
module ActiveAdmin
class Engine < ::Rails::Engine
initializer "active_admin.precompile", group: :all do |app|
ActiveAdmin.application.stylesheets.each do |path, _|
app.config.assets.precompile << path
end
ActiveAdmin.application.javascripts.each do |path|
app.config.assets.precompile << path
end
end
initializer 'active_admin.routes' do
require 'active_admin/helpers/routes/url_helpers'
end
end
end
I run on ubuntu 15.04, with ruby -v '2.2.2p95', rails -v '4.2.1' and kind of stuck about what to do

Answer 1

I found the issue is my not using rails_12_factor gem and playing too much with depreciated rails environment variables. After several trials I managed to roll-back to default settings and it worked!