add rake task "db:truncate:x" and "db:truncate:all"

db_truncate_x.rb

namespace :db do
  def detect_env
    ENV['RAILS_ENV'] || 'development'
  end
  def truncate(table)
    begin
      case @config["adapter"]
      when "mysql", "mysql2"
        ActiveRecord::Base.connection.execute("TRUNCATE #{table}")
        puts "Table #{table} truncated!"
      when "sqlite", "sqlite3"
        ActiveRecord::Base.connection.execute("DELETE FROM #{table}")
        puts "Table #{table} deleted!"
        ActiveRecord::Base.connection.execute("DELETE FROM sqlite_sequence where name='#{table}'")
        puts "sqlite_sequence of #{table} deleted!"
        ActiveRecord::Base.connection.execute("VACUUM")
        puts "database vacuumed!"
      when "postgresql"
        # if you use postgresql 8.4 or later, you can use "TRUNCATE XXX RESTART IDENTITY"
#        ActiveRecord::Base.connection.execute("TRUNCATE #{table} RESTART IDENTITY")
#        puts "Table #{table} truncated!(with RESTART IDENTITY)"
        ActiveRecord::Base.connection.execute("TRUNCATE #{table}")
        puts "Table #{table} truncated!"
        ActiveRecord::Base.connection.execute("SELECT setval('#{table}_id_seq', 1, false)");
        puts "Sequence (#{table}_id_seq) is reset!"
      end
    rescue => ex
      puts "#{ex}"
    end
  end
  namespace :truncate do
    data_source = YAML.load_file(Rails.root.join('config', 'database.yml'))
    #    ActiveRecord::Base.configurations[detect_env]
    @config = data_source[detect_env]
    @connection = ActiveRecord::Base.establish_connection(@config)
    @tables = ActiveRecord::Base.connection.tables
    @tables.each do |table|
      unless ['schema_migrations'].include?(table)
        desc "Truncate #{table} table"
        task table => :environment do
          truncate(table)
        end
      end
    end
    desc "Truncate all the tables in the database"
    task :all => :environment do
      puts 'Notice: db:truncate:all is disabled because preventing you to truncate tables by mistakes'
#      @tables.each do |table|
#        unless ['schema_migrations'].include?(table)
#          truncate(table)
#        end
#      end
    end
  end
end

データベースのTRUNCATEができるtaskの追加
PostgresQLは検証済み
こちらの方のスクリプトを参考に
【Nothing stays the same • rake task for truncate db tables：http://blog.agustinvinao.com.ar/post/10528516587/rake-task-for-truncate-db-tables 】