Boolean field indicating that record has belonging association with given field value

My models:

class CountryVisit < ActiveRecord::Base
  belongs_to :country
  belongs_to :user

  validates :country, uniqueness: { scope: user, message: 'already visited' }
end

class User < ActiveRecord::Base
  has_many :country_visits

  ...
end

class Country < ActiveRecord::Base
  has_many :country_visits

  ...
end

I want each country to have a virtual attribute indicating that it has country_visit for given user

Country.for(user)

Here's my incomplete solution (raises ActiveRecord::StatementInvalid: SQLite3::SQLException if I try to perform .count on result):

def self.for(user_id)
  Country
    .select("countries.*, country_visits.id AS visited")
    .joins(
      "LEFT OUTER JOIN country_visits ON 
      country_visits.country_id = countries.code 
      AND country_visits.user_id = #{user_id}"
    )
end