Mongoid functionality working for Mongoid 3.0, haven't got to grips with the tests yet, so will push those in next commit

lib/generators/mongoid/impressionist_generator.rb

@@ -0,0 +1,8 @@
+module Mongoid
+  module Generators
+    class ImpressionistGenerator < Rails::Generators::Base
+      # Empty for now, need it for generating the config file without
+      # triggering other ORM's generators.
+    end
+  end
+end

lib/generators/templates/impression.rb

@@ -1,5 +1,5 @@
 # Use this hook to configure impressionist parameters
 Impressionist.setup do |config|
-  # Define ORM. Could be :active_record (default) and :mongo_mapper
+  # Define ORM. Could be :active_record (default), :mongo_mapper or :mongoid
   # config.orm = :active_record
 end

lib/impressionist/controllers/mongoid/impressionist_controller.rb

@@ -0,0 +1,9 @@
+ImpressionistController::InstanceMethods.send(:define_method, :direct_create_statement) do |query_params={}|
+	# creates a statment hash that contains default values for creating an impression.
+	# if :impressionable_id is a valid ObjectId then convert it into one
+	query_params.reverse_merge!(
+		:impressionable_type => controller_name.singularize.camelize,
+		:impressionable_id=> !Moped::BSON::ObjectId.legal?(params[:id]) ? params[:id] : Moped::BSON::ObjectId.from_string(params[:id])
+	)
+	associative_create_statement(query_params)
+end

lib/impressionist/engine.rb

@@ -13,10 +13,17 @@ module Impressionist
         require "impressionist/models/mongo_mapper/impression.rb"
         require "impressionist/models/mongo_mapper/impressionist/impressionable.rb"
         MongoMapper::Document.plugin Impressionist::Impressionable
+      elsif Impressionist.orm == :mongoid
+        require 'impressionist/models/mongoid/impression.rb'
+        require 'impressionist/models/mongoid/impressionist/impressionable.rb'
+        Mongoid::Document.send(:include, Impressionist::Impressionable)
       end
     end
 
     initializer 'impressionist.controller' do
+      if Impressionist.orm == :mongoid
+          require 'impressionist/controllers/mongoid/impressionist_controller.rb'
+      end
       ActiveSupport.on_load(:action_controller) do
         include ImpressionistController::InstanceMethods
         extend ImpressionistController::ClassMethods

lib/impressionist/models/mongoid/impression.rb

@@ -0,0 +1,32 @@
+class Impression
+  include Mongoid::Document
+  include Mongoid::Timestamps
+
+  attr_accessible :impressionable_type, :impressionable_field, :impressionable_id, :user_id,
+  :controller_name, :action_name, :view_name, :request_hash, :ip_address,
+  :session_hash, :message, :referrer
+
+  belongs_to :impressionable, polymorphic: true
+
+  field :user_id
+  field :controller_name
+  field :action_name
+  field :view_name
+  field :request_hash
+  field :ip_address
+  field :session_hash
+  field :message
+  field :referrer
+
+  set_callback(:create, :after) do |doc|
+    unless impressionable_id.nil?
+      impressionable_class = doc.impressionable_type.constantize
+
+      if impressionable_class.impressionist_counter_cache_options
+        resource = impressionable_class.find(doc.impressionable_id)
+        resource.try(:update_impressionist_counter_cache)
+      end
+    end
+  end
+  
+end

lib/impressionist/models/mongoid/impressionist/impressionable.rb

@@ -0,0 +1,52 @@
+module Impressionist
+  module Impressionable
+    extend ActiveSupport::Concern
+
+    module ClassMethods
+      def is_impressionable(options={})
+      	has_many :impressions, as: :impressionable, dependent: :destroy
+        	@impressionist_cache_options = options[:counter_cache]
+        	if !@impressionist_cache_options.nil?
+        		opts = impressionist_counter_cache_options
+        		field opts[:column_name], type: Integer
+        	end
+      end
+
+      def impressionist_counter_cache_options
+        if @impressionist_cache_options
+          options = { :column_name => :impressions_count, :unique => false }
+          options.merge!(@impressionist_cache_options) if @impressionist_cache_options.is_a?(Hash)
+          options
+        end
+      end
+
+      def impressionist_counter_caching?
+        impressionist_counter_cache_options.present?
+      end
+
+      def counter_caching?
+        ::ActiveSupport::Deprecation.warn("#counter_caching? is deprecated; please use #impressionist_counter_caching? instead")
+        impressionist_counter_caching?
+      end
+    end
+
+    def impressionable?
+      true
+    end
+
+    def impressionist_count(options={})
+      options.reverse_merge!(:filter=>:request_hash, :start_date=>nil, :end_date=>Time.now)
+      imps = options[:start_date].blank? ? impressions : impressions.between(created_at: options[:start_date]..options[:end_date])
+      options[:filter] == :all ? imps.count : imps.where(options[:filter].ne => nil).count
+    end
+
+    def update_impressionist_counter_cache
+      cache_options = self.class.impressionist_counter_cache_options
+      column_name = cache_options[:column_name].to_sym
+      count = cache_options[:unique] ? impressionist_count(:filter => :ip_address) : impressionist_count
+      old_count = send(column_name) || 0
+      self.inc(column_name, (count - old_count))
+    end
+
+  end
+end