我在用 grape 写 api，用了 activerecord，没用 rails

访问 http://localhost:9292/api/v1/users/1 抛异常： ActiveRecord::ConnectionNotEstablished at /api/v1/users/1 No connection pool for User

rake db:create

可以在 mysql 中创建数据库

rake db:migrate

可以在 mysql 中创建表

说明 rake 执行时能连接 mysql 数据库的，高手指教下查询时为何异常？

config/database.yml 代码：

development:
  adapter: mysql2
  database: grape
  encoding: utf8
  host: localhost
  port: 3306
  username: steve
  password: steve
  pool: 5
  timeout: 5000

config.ru 代码

require File.expand_path('../app/api/rest', __FILE__)
require 'active_record'
use ActiveRecord::ConnectionAdapters::ConnectionManagement
run Rest::API

app/api/rest.rb 代码

require 'grape'
require_relative '../models/user'

module Rest
    class API < Grape::API
        version 'v1', using: :path
        format :json
        prefix :api

        resource :users do
            desc '获取一个用户'
            params do 
                requires :id, type: Integer, desc: '用户id'
            end
            route_param :id do
                get do 
                    User.find(params[:id])
                end
            end
        end
    end
end

app/models/user.rb 代码

require 'active_record'

class User < ActiveRecord::Base
    attr_accessor :name
end

Gemfile

source 'https://ruby.taobao.org'
gem 'grape'
gem 'activerecord'
gem 'active_record_migrations'
gem 'mysql2'

应用启动命令：

rackup config.ru