Heroku – How to run SOME initializers when doing a Rails assets:precompile

asset-pipelineherokuruby-on-rails-3ruby-on-rails-3.1ruby-on-rails-3.2

Background

I have an app that I recently updated to Rails 3.2.1 (from Rails 3.0.x) and have refactored the JS and CSS assets to make use of the new asset pipeline. The app is hosted on Heroku with the Celadon Cedar stack.

App Config

I keep application specific configuration in a YAML file called app_config.yml and load it into a global APP_CONFIG variable using an initializer:

# config/initializers/load_app_config.rb

app_config_contents = YAML.load_file("#{Rails.root.to_s}/config/app_config.yml")
app_config_contents["default"] ||= {}
APP_CONFIG = app_config_contents["default"].merge(
                       app_config_contents[Rails.env] || {} ).symbolize_keys

Asset Compilation on Heroku

Heroku has support for the Rails asset pipeline built into the Cedar stack. When you push an app to Heroku it automatically calls rake assets:precompile on the server as a step in the deploy process. However it does this in a sandboxed environment without database access or normal ENV vars.

If the application is allowed to initialize normally during asset precompilation an error is thrown trying to connect to the database. This is easily solved by adding the following to the application.rb file:

    # Do not load entire app when precompiling assets
    config.assets.initialize_on_precompile = false

My Problem

When initialize_on_precompile = false is set, none of the initializers in config/initializers/* are run. The problem I am running into is that I need the APP_CONFIG variable to be available during asset precompilation.

How can I get load_app_config.rb to be loaded during asset compilation without initializing the entire app? Can I do something with the group parameter passed to Rails::Application.initialize! ?

Best Answer

Rails lets you register initializers only in certain groups, but you need to use the Railtie API:

# in config/application.rb

module AssetsInitializers
  class Railtie < Rails::Railtie
    initializer "assets_initializers.initialize_rails",
                :group => :assets do |app|
      require "#{Rails.root}/config/initializers/load_config.rb"
    end
  end
end

You don't need to check if AppConfig is defined since this will only run in the assets group.

And you can (and should) continue to use initialize_on_precompile = false. The load_config.rb initializer will be run when initializing the app (since it's in config/initializers) and when pre-compiling without initializing (because of the above code).

Related Solutions

Ruby-on-rails – Rails: How to rename a database column in a Ruby on Rails migration

rename_column :table, :old_column, :new_column

You'll probably want to create a separate migration to do this. (Rename FixColumnName as you will.):

script/generate migration FixColumnName
# creates  db/migrate/xxxxxxxxxx_fix_column_name.rb

Then edit the migration to do your will:

# db/migrate/xxxxxxxxxx_fix_column_name.rb
class FixColumnName < ActiveRecord::Migration
  def self.up
    rename_column :table_name, :old_column, :new_column
  end

  def self.down
    # rename back if you need or do something else or do nothing
  end
end

For Rails 3.1 use:

While, the up and down methods still apply, Rails 3.1 receives a change method that "knows how to migrate your database and reverse it when the migration is rolled back without the need to write a separate down method".

See "Active Record Migrations" for more information.

rails g migration FixColumnName

class FixColumnName < ActiveRecord::Migration
  def change
    rename_column :table_name, :old_column, :new_column
  end
end

If you happen to have a whole bunch of columns to rename, or something that would have required repeating the table name over and over again:

rename_column :table_name, :old_column1, :new_column1
rename_column :table_name, :old_column2, :new_column2
...

You could use change_table to keep things a little neater:

class FixColumnNames < ActiveRecord::Migration
  def change
    change_table :table_name do |t|
      t.rename :old_column1, :new_column1
      t.rename :old_column2, :new_column2
      ...
    end
  end
end

Then just db:migrate as usual or however you go about your business.

For Rails 4:

While creating a Migration for renaming a column, Rails 4 generates a change method instead of up and down as mentioned in the above section. The generated change method is:

$ > rails g migration ChangeColumnName

which will create a migration file similar to:

class ChangeColumnName < ActiveRecord::Migration
  def change
    rename_column :table_name, :old_column, :new_column
  end
end

Ruby-on-rails – How to universally skip database touches when precompiling assets on Heroku

add this to config/application.rb

config.assets.initialize_on_precompile=false

took me a while to hunt this down... adding it to config/environments/*.rb did NOT work

UPDATE: It doesn't work with rails 4