```ruby
module Mongoid

  module Autoinc

    class Incrementor
      attr_accessor :model_name, :field_name, :scope_key, :collection, :seed, :step

      def initialize(model_name, field_name, options={})
        self.model_name = model_name
        self.field_name = field_name.to_s
        self.scope_key = options[:scope]
        self.step = options[:step] || 1
        self.seed = options[:seed]
        self.collection = ::Mongoid.default_session['auto_increment_counters']
        create if seed && !exists?
      end

      def key
        "".tap do |str|
          str << "#{model_name.underscore}_#{field_name}"
          str << "_#{scope_key}" unless scope_key.blank?
        end
      end

      def inc
        collection.find(
          '_id' => key
        ).modify(
          {'$inc' => { 'c' => step }},
          :new => true, :upsert => true
        )['c']
      end

      private

      def exists?
        collection.find('_id' => key).count > 0
      end

      def create
        collection.insert({'_id' => key, 'c' => seed})
      end

    end

  end

end
```

Make sure we have a <String>, not a <ModelName>
user
```ruby
module Mongoid

  module Autoinc

    class Incrementor
      attr_accessor :model_name, :field_name, :scope_key, :collection, :seed, :step

      def initialize(model_name, field_name, options={})
        self.model_name = model_name
        self.field_name = field_name.to_s
        self.scope_key = options[:scope]
        self.step = options[:step] || 1
        self.seed = options[:seed]
        self.collection = ::Mongoid.default_session['auto_increment_counters']
        create if seed && !exists?
      end

      def key
        "".tap do |str|
          str << "#{model_name.to_s.underscore}_#{field_name}"
          str << "_#{scope_key}" unless scope_key.blank?
        end
      end

      def inc
        collection.find(
          '_id' => key
        ).modify(
          {'$inc' => { 'c' => step }},
          :new => true, :upsert => true
        )['c']
      end

      private

      def exists?
        collection.find('_id' => key).count > 0
      end

      def create
        collection.insert({'_id' => key, 'c' => seed})
      end

    end

  end

end
```
assistant