因为我花了很多时间尝试与所有的替代品在这和其他职位，没有成功，这里的解决方案，我的作品

数据库.yml

external_database:
  adapter: postgresql
  database: <%= ENV['DATABASE_NAME'] %>
  username: <%= ENV['DATABASE_USERNAME'] %>
  password: <%= ENV['DATABASE_PASSWORD'] %>
  host: <%= ENV['DATABASE_HOST'] %>
  port: <%= ENV['DATABASE_PORT'] || 5432 %>

型号/您的型号.rb

class YourModel < ActiveRecord::Base
  establish_connection :external_database
end

您的迁移

class YourMigration < ActiveRecord::Migration
  def self.up
    YourModel.connection.enable_extension "plpgsql"
    YourModel.connection.create_table :your_table_name, id: :uuid do |t|
      t.text :title
      ...
    end
  end

  def self.down
    YourModel.connection.drop_table :your_table_name
  end
end

**解释：**因为ActiveRecord：：Base.establish_connection关闭了连接，所以我直接使用来自模型的连接。另外，因为我使用UUID和uuid_generate_v4（）方法作为默认值，所以我使用相同的连接启用了扩展

=========================更新===========
我找到一篇很好的文章谈论它：http://www.thegreatcodeadventure.com/managing-multiple-databases-in-a-single-rails-application/
=========================更新===========
我只是将establish_connection从迁移类中取出。

ActiveRecord::Base.establish_connection "users_#{Rails.env}"
class AddDeviseToUsers < ActiveRecord::Migration
  def self.up

    change_table(:users) do |t|
      ## Database authenticatable
      t.string :email,              :null => false, :default => ""
      t.string :encrypted_password, :null => false, :default => ""

      ## Recoverable
      t.string   :reset_password_token
      t.datetime :reset_password_sent_at

      ## Rememberable
      t.datetime :remember_created_at

      ## Trackable
      t.integer  :sign_in_count, :default => 0, :null => false
      t.datetime :current_sign_in_at
      t.datetime :last_sign_in_at
      t.string   :current_sign_in_ip
      t.string   :last_sign_in_ip

      ## Confirmable
      # t.string   :confirmation_token
      # t.datetime :confirmed_at
      # t.datetime :confirmation_sent_at
      # t.string   :unconfirmed_email # Only if using reconfirmable

      ## Lockable
      # t.integer  :failed_attempts, :default => 0, :null => false # Only if lock strategy is :failed_attempts
      # t.string   :unlock_token # Only if unlock strategy is :email or :both
      # t.datetime :locked_at

      # Uncomment below if timestamps were not included in your original model.
      # t.timestamps
    end

    add_index :users, :email,                :unique => true
    add_index :users, :reset_password_token, :unique => true
    # add_index :users, :confirmation_token,   :unique => true
    # add_index :users, :unlock_token,         :unique => true
  end

  def self.down
    # By default, we don't want to make any assumption about how to roll back a migration when your
    # model already existed. Please edit below which fields you would like to remove in this migration.
    raise ActiveRecord::IrreversibleMigration
  end
end

在迁移类中创建方法
def self.connection ActiveRecord::Base.establish_connection("#{Rails.env}_yourcustomenv").connection end
我花了一些时间才意识到需要一个用self声明的方法，这在Rails4.0上是可行的

你就快找到答案了。只需要做User.connection而不是ActiveRecord::Base.establish_connection。
看看这里的解决方案：https://stackoverflow.com/a/34292909/2499227
ActiveRecord::Base.establish_connection方式总是给我带来schema_migrations和/或连接关闭的问题，但是Model连接运行得很顺利。

在您的配置文件/database.yml中：

development_custom_db:
  database: almoxarifado
  username: user
  password: pass
  host: localhost

在路径config/initializers/中创建一个包含此内容的文件：

module Custom
  class Base < ActiveRecord::Base
    self.abstract_class = true

    connection_name = "#{Rails.env}_custom_db"
    if Rails.application.config.database_configuration.has_key?(connection_name)
      establish_connection connection_name.to_sym
    end
  end

  class Migration < ActiveRecord::Migration
    def connection
      @_connection ||= Custom::Base.connection
    end
  end
end

然后在迁移文件中继承它：

class AddDeviseToUsers < Custom::Migration
 def up
   change_table(:users) do |t|

   end
 end
end

使用了一个简单一点的方法，它对我来说在Rails6上工作。

class AddDimensionsToReportRecords < ActiveRecord::Migration[6.0]
  def change
    add_column :report_records, :sample_length, :decimal
    add_column :report_records, :sample_height, :decimal
    add_column :report_records, :sample_width, :decimal
  end

  def connection
    ReportRecord.connection
  end
end

还有模特...

class ReportRecord < ApplicationRecord
  connects_to database: { writing: :reports_db, reading: :reports_db_replica }
end