Rails 5.1 API CORS问题

我有一个生成json的rails 5.1.6 api . 我有一个单独的Vue.js客户端，旨在使用它 .

该模型是实体 . GET请求可以正常查看db中所有实体的索引 . 但是，当我尝试POST一个新实体时，我在控制台中收到以下错误：

来自origin>'http：// localhost：8080'的'http：// localhost：3000 / entities'对XMLHttpRequest的访问已被CORS策略阻止：对预检请求的响应未通过访问控制检查：否'访问-Control- Allow-Origin'标头出现在请求的资源上 .

在rails应用程序中，我安装了rack-cors gem并在 initializers/cors.rb 中：

Rails.application.config.middleware.insert_before 0, Rack::Cors do
  allow do
    origins 'localhost:3000'
    resource '*',
      headers: :any,
      methods: %i(get post put patch delete options head)
  end
end

在routes.rb中：

Rails.application.routes.draw do
  # For details on the DSL...
  post 'auth/login', to: 'authentication#authenticate'
  post 'signup', to: 'users#create'

  resources :entities do
    resources :transactions
  end
end

在客户端层：

src/services/Api.js:

import axios from 'axios'

export default() => {
  return axios.create({
    baseURL: 'http://localhost:3000'
  })
}



src/services/EntitiesService.js:

import Api from '@/services/Api'

export default {
  fetchEntities () {
    return Api().get('/entities')
  },

  addEntity (params) {
    return Api().post('entities', params)
  }
}

和

src/components/newEntity.vue:

<template>
  <div class="entities">
    <h1>Add Entity</h1>
      <div class="form">
        <div>
          <input type="text" name="title" placeholder="TITLE" v-model="title">
        </div>
        <div>
          <textarea rows="15" cols="15" placeholder="DESCRIPTION" v-model="description"></textarea>
        </div>
        <div>
          <button class="app_entity_btn" @click="addEntity">Add</button>
        </div>
      </div>
  </div>
</template>

<script>
import EntitiesService from '@/services/EntitiesService'
export default {
  name: 'NewEntity',
  data () {
    return {
      title: '',
      description: ''
    }
  },
  methods: {
    async addEntity () {
      await EntitiesService.addEntity({
        title: this.title,
        description: this.description
      })
      this.$router.push({ name: 'Entities' })
    }
  }
}
</script>
<style type="text/css">
.form input, .form textarea {
  width: 500px;
  padding: 10px;
  border: 1px solid #e0dede;
  outline: none;
  font-size: 12px;
}
.form div {
  margin: 20px;
}
.app_entity_btn {
  background: #4d7ef7;
  color: #fff;
  padding: 10px 80px;
  text-transform: uppercase;
  font-size: 12px;
  font-weight: bold;
  width: 520px;
  border: none;
  cursor: pointer;
}
</style>

这有什么不对？