rails database.yml not accepting ERB

Question

In my database.yml, I have:

staging:
  adapter: <%= ENV['DATABASE_ADAPTER'] %>
  encoding: <%= ENV['DATABASE_ENCODING'] %>
  database: <%= ENV['DATABASE'] %>
  host: <%= ENV['DATABASE_HOST'] %>
  port: <%= ENV['DATABASE_PORT'].to_i %>
  pool: <%= ENV['DATABASE_POOL'].to_i %>
  username: <%= ENV['DATABASE_USERNAME'] %>
  password: <%= ENV['DATABASE_PASSWORD'] %>

However, it does not read the ERB part when actually booting puma:

/usr/local/lib/ruby/gems/2.1.0/gems/activesupport-4.2.1/lib/active_support/dependencies.rb:274:in 
`require': Could not load 'active_record/connection_adapters/<%= ENV['DATABASE_ADAPTER'] %>_adapter'. 
Make sure that the adapter in config/database.yml is valid. If you use an adapter other than 'mysql', 'mysql2', 'postgresql' or 'sqlite3'
add the necessary adapter gem to the Gemfile. (LoadError)

which makes no sense, since in the Rails code to load the database configuration:

  def database_configuration
    yaml = Pathname.new(paths["config/database"].existent.first || "")

    config = if yaml.exist?
      require "yaml"
      require "erb"
      YAML.load(ERB.new(yaml.read).result) || {}
    elsif ENV['DATABASE_URL']
      # Value from ENV['DATABASE_URL'] is set to default database connection
      # by Active Record.
      {}
    else
      raise "Could not load database configuration. No such file - #{yaml}"
    end

    config
  rescue Psych::SyntaxError => e
    raise "YAML syntax error occurred while parsing #{paths["config/database"].first}. " 
          "Please note that YAML must be consistently indented using spaces. Tabs are not allowed. " 
          "Error: #{e.message}"
  rescue => e
    raise e, "Cannot load `Rails.application.database_configuration`:
#{e.message}", e.backtrace
  end

(taken from the Rails 4.2 stable code, I'm running 4.2.1)

I'm absolutely baffled why this isn't working, any ideas?

Answer 1

I just experienced the same thing, and came across your post. I had been following a tutorial that had me create a puma.conf file that contained the code below:

ActiveRecord::Base.establish_connection( YAML.load_file( "#{app_dir}/config/database.yml" )[rails_env])

I modified to the following, and everything worked as expected:

require 'erb'
ActiveRecord::Base.establish_connection( YAML.load( ERB.new( File.read( "#{app_dir}/config/database.yml" )).result)[rails_env])