customer.rb

require 'rubygems'
require 'bundler/setup'

require 'mongo_mapper'

logger = Logger.new(STDOUT)
MongoMapper.connection = Mongo::Connection.new('localhost', logger: logger)
MongoMapper.database = 'customers'

class Customer
  include MongoMapper::Document

  key :email, String
  key :course, String

  scope :no_course, where(course: '')

  def self.with_email(email)
    first(email: email.downcase)
  end
end

customer_test.rb

require 'minitest/autorun'
require './customer'

class TestCustomer < Minitest::Test
  def setup
    Customer.destroy_all

    Customer.create(email: 'customer1@test.com', course: 'algebra')
    Customer.create(email: 'customer2@test.com', course: '')
  end

  def test_course_scope
    assert_equal 1, Customer.no_course.count
  end

  def test_finder
    refute_nil Customer.with_email('CUSTOMER1@TEST.COM')
  end

  def test_course_scope_and_finder
    assert_nil Customer.no_course.with_email('CUSTOMER1@TEST.COM')
  end

  def test_course_scope_and_finder_2
    assert_nil Customer.no_course.first(email: 'CUSTOMER1@TEST.COM')
  end
end

Gemfile

source 'https://rubygems.org'

gem 'bson_ext'
gem 'mongo_mapper', '0.13.1'