My Rails 4.2.6 app uses two databases. Can I only use one for testing purposes?

I have a web application in Rails 4.2.6 that was originally using MongoDB (through Mongoid 5) as its main database. However, now I need to connect to a MySQL database to only read some additional data for my app.

So far, I have required ActiveRecord in my project and was able to establish the connection with the aforementioned database in development environment. My configuration files for this matter are:

mongoid.yml

development:
  clients:
    default:
      database: mongo_development
      hosts:
        - localhost:27017
      options:
test:
  clients:
    default:
      database: mongo_test
      hosts:
        - localhost:27017
      options:
        read:
          mode: :primary
        max_pool_size: 1
  options:
    raise_not_found_error: false

database.yml

development:
  adapter: mysql2
  encoding: utf8
  reconnect: false
  database: mysql_development
  pool: 5
  username: user
  password: pass
  socket: /var/run/mysqld/mysqld.sock

The problem arrives when I run my test suite. I am using RSpec and executing bundle exec rspec spec yields the following error message:

 /home/user/.rvm/gems/ruby-2.1.6@global/gems/activerecord-4.2.6/lib/active_record/connection_adapters/connection_specification.rb:248:in `resolve_symbol_connection': 'test' database is not configured. Available: ["development"] (ActiveRecord::AdapterNotSpecified)

ActiveRecord complains for not having a test database to connect, but the problem is that I don't have a MySQL database for testing purposes: the specs I wrote only interact with Mongoid models.

I was considering creating an empty MySQL database as a hack, but I want to know first if I could configure ActiveRecord not to connect to a test database and use only mongo_test.

Is it possible to do what I want? Does anybody know how to do this?

I decided to answer my own question as the provided answers didn't fit my needs.

After many trials and errors I was able to avoid connecting to a MySQL database in test environment adapting the information given in this question post.

The only thing I needed to do was to add the following gem to my Gemfile:

group :test do
  gem 'activerecord-nulldb-adapter'
end

This gem installs the NullDB database adapter in the project, which enables ActiveRecord to establish a connection without the need to define a real database.

Last but not least, I updated my config/database.yml adding the parameters of a database connection in test environment that uses the new adapter:

test:
  adapter: nulldb
  encoding: utf8
  reconnect: false
  database: mysql_test
  pool: 5
  username: user
  password: pass
  socket: /var/run/mysqld/mysqld.sock