Repository in Ruby according to Eric Evans (2006): Domain-Driven Design. Tackling complexity in the heart of software, Upper Saddle River, NJ: Addison-Wesley.

in_memory_strategy.rb

class InMemoryStrategy
  def matching(criteria)
    results = []
    domain_objects.each do |object|
      results << object if criteria.satisfied_by?(object)
    end
  end
end

relational_database_strategy.rb

class RelationalDatabaseStrategy
  def matching(criteria)
    query = Query.new # --> QueryObject pattern
    query.add_criteria(criteria)
    return query.execute
  end
end

repository.rb

# Repository with pluggable strategy.
# Strategy can be replaced during tests, too.
class Repository
  def matching(criteria)
    repository_strategy.matching(criteria)
  end

  private
  def repository_strategy
    @repository_strategy ||= InMemoryStrategy.new
  end
end