Database 设置默认数据库连接 Rails

我的Rails应用程序有自己的MySQL数据库(并且需要mysql2gem)，但是还需要连接某个特定模型的外部MongoDB数据库(因此我在Gemfile中包含了mongoid和bson_ext).现在，当我try 为新模型生成迁移时，它告诉我

$ rails g migration CreateLocations
       error  mongoid [not found]

当我生成位置模型时，它包含了Mongoid::Document，所以Rails显然认为它使用外部数据库作为我的主要数据存储.

数据库.yml:

development:
  adapter: mysql2
  encoding: utf8
  reconnect: false
  database: associalize_development
  pool: 5
  username: root
  password:
  socket: /tmp/mysql.sock

mongoid.yml:

development:
  host: pearl.mongohq.com
  port: 27019
  username: asfasdf
  password: sadfasdf
  database: app4574678

test:
  host: pearl.mongohq.com
  port: 27019
  username: asdfadhasdfa
  password: hadsadfas
  database: app4574678

production:
  host: pearl.mongohq.com
  port: 27019
  username: asdfdfsasda
  password: afdasdfdasdf
  database: app4574678

UPDATE

class ExternalMongoModel
  include Mongoid::Document

  field :title
  field :long_title
  field :deal_type
  field :merchandise_type
  field :market_id
  field :market_name
  field :market_location, type: Array
  field :featureType
  field :country_code
  field :subtitle
  field :offer_ends_at
  field :price
  field :value
  field :merchant_type
  field :content
  field :merchant

  index(
    [[:division_latlon, Mongo::GEO2D]], background: true
  )

end