rake 数据库适配器与database.yml不一致

In database.yml（rails 生成的默认文件）：

default: &default
    adapter: sqlite3
    pool: 5
    timeout: 5000

development:
  <<: *default
  database: db/development.sqlite3

test:
  <<: *default
  database: db/test.sqlite3

production:
  <<: *default
  database: db/production.sqlite3

跑步时rake about，我有这个错误：

Gem::LoadError: Specified 'postgresql' for database adapter, but the gem is not loaded.    
Add `gem 'pg'` to your Gemfile (and ensure its version is at the minimum required by    
ActiveRecord).

如果我添加pg宝石，那么rake about给出这个输出：

About your application's environment
Ruby version              2.1.0-p0 (x86_64-darwin13.0)
RubyGems version          2.2.2
Rack version              1.5
Rails version             4.1.0
JavaScript Runtime        Node.js (V8)
Active Record version     4.1.0
Action Pack version       4.1.0
Action View version       4.1.0
Action Mailer version     4.1.0
Active Support version    4.1.0
Middleware                Rack::Sendfile, ActionDispatch::Static, Rack::Lock, #   <ActiveSupport::Cache::Strategy::LocalCache::Middleware:0x007f8043154a30>, Rack::Runtime, Rack::MethodOverride, ActionDispatch::RequestId, Rails::Rack::Logger, ActionDispatch::ShowExceptions, ActionDispatch::DebugExceptions, ActionDispatch::RemoteIp, ActionDispatch::Reloader, ActionDispatch::Callbacks, ActiveRecord::Migration::CheckPending, ActiveRecord::ConnectionAdapters::ConnectionManagement, ActiveRecord::QueryCache, ActionDispatch::Cookies, ActionDispatch::Session::CookieStore, ActionDispatch::Flash, ActionDispatch::ParamsParser, Rack::Head, Rack::ConditionalGet, Rack::ETag
Environment               development
Database adapter          postgresql
Database schema version   0

知道为什么会发生这种情况吗？我想使用 sqlite3 适配器。

我发现了问题。显然，Rake 正在寻找环境变量 DATABASE_URL（我已将其设置为 postgres），并且该变量优先于 database.yml 文件。删除环境变量后，一切正常。