一些更新

以下仅针对 ruby -v 2.12 and rails -v 4.1.5
1.
In Rails 4.1+, they deprecated db:test:prepare with that message. You can now just use:
ActiveRecord::Migration.maintain_test_schema!
in spec_helper.rb (or similar files if you're not using RSpec). That will automatically keep your test database in sync with your schema. Because of this 'automatic' method, db:test:prepare is no longer needed in most cases.
If you need to do it manually for some reason, you can still use
rake db:schema:load RAILS_ENV=test

reference: http://stackoverflow.com/questions/23351783/when-i-ran-bundle-exec-rake-testprepare-it-errored-out-but-bundle-exec-rake
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2. Section 10.2.2
User.all(limit: 6) 更改为 User.limit(6)

reference: http://stackoverflow.com/questions/23749612/wrong-number-of-arguments-1-for-0-hartl-chapter-10-bundle-exec-rake-dbpopul
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