1 files changed, 102 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/identity_map.rb b/activerecord/lib/active_record/identity_map.rb
new file mode 100644
index 0000000000..d18b2b0a54
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+++ b/activerecord/lib/active_record/identity_map.rb
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+module ActiveRecord
+  # = Active Record Identity Map
+  #
+  # Ensures that each object gets loaded only once by keeping every loaded
+  # object in a map. Looks up objects using the map when referring to them.
+  #
+  # More information on Identity Map pattern:
+  #   http://www.martinfowler.com/eaaCatalog/identityMap.html
+  #
+  # == Configuration
+  #
+  # In order to enable IdentityMap, set <tt>config.active_record.identity_map = true</tt>
+  # in your <tt>config/application.rb</tt> file.
+  #
+  # IdentityMap is disabled by default.
+  #
+  module IdentityMap
+    extend ActiveSupport::Concern
+
+    class << self
+      def enabled=(flag)
+        Thread.current[:identity_map_enabled] = flag
+      end
+
+      def enabled
+        Thread.current[:identity_map_enabled]
+      end
+      alias enabled? enabled
+
+      def repository
+        Thread.current[:identity_map] ||= Hash.new { |h,k| h[k] = {} }
+      end
+
+      def use
+        old, self.enabled = enabled, true
+
+        yield if block_given?
+      ensure
+        self.enabled = old
+        clear
+      end
+
+      def without
+        old, self.enabled = enabled, false
+
+        yield if block_given?
+      ensure
+        self.enabled = old
+      end
+
+      def get(klass, primary_key)
+        obj = repository[klass.symbolized_base_class][primary_key]
+        obj.is_a?(klass) ? obj : nil
+      end
+
+      def add(record)
+        repository[record.class.symbolized_base_class][record.id] = record
+      end
+
+      def remove(record)
+        repository[record.class.symbolized_base_class].delete(record.id)
+      end
+
+      def remove_by_id(symbolized_base_class, id)
+        repository[symbolized_base_class].delete(id)
+      end
+
+      def clear
+        repository.clear
+      end
+    end
+
+    # Reinitialize an Identity Map model object from +coder+.
+    # +coder+ must contain the attributes necessary for initializing an empty
+    # model object.
+    def reinit_with(coder)
+      @attributes_cache = {}
+      dirty = @changed_attributes.keys
+      @attributes.update(coder['attributes'].except(*dirty))
+      @changed_attributes.update(coder['attributes'].slice(*dirty))
+      @changed_attributes.delete_if{|k,v| v.eql? @attributes[k]}
+
+      set_serialized_attributes
+
+      run_callbacks :find
+
+      self
+    end
+
+    class Middleware
+      def initialize(app)
+        @app = app
+      end
+
+      def call(env)
+        ActiveRecord::IdentityMap.use do
+          @app.call(env)
+        end
+      end
+    end
+  end
+end