"there is no need to store this option just for initialization" José Valim dixit.

activerecord/lib/active_record/base.rb

@@ -411,10 +411,6 @@ class Base
     class_attribute :store_full_sti_class
     self.store_full_sti_class = true
 
-    # Determine whether or not to use IdentityMap.
-    class_attribute :identity_map
-    self.identity_map = false
-
     # Stores the default scope for the class
     class_inheritable_accessor :default_scoping, :instance_writer => false
     self.default_scoping = []

activerecord/lib/active_record/railtie.rb

@@ -43,6 +43,11 @@ class Railtie < Rails::Railtie
       ActiveSupport.on_load(:active_record) { self.logger ||= ::Rails.logger }
     end
 
+    initializer "active_record.identity_map" do |app|
+      config.app_middleware.insert_after "::ActionDispatch::Callbacks",
+        "ActiveRecord::IdentityMap::Middleware" if config.active_record.delete(:identity_map)
+    end
+
     initializer "active_record.set_configs" do |app|
       ActiveSupport.on_load(:active_record) do
         app.config.active_record.each do |k,v|
@@ -78,11 +83,6 @@ class Railtie < Rails::Railtie
       end
     end
 
-    initializer "active_record.identity_map" do |app|
-      config.app_middleware.insert_after "::ActionDispatch::Callbacks",
-        "ActiveRecord::IdentityMap::Middleware" if config.active_record.identity_map
-    end
-
     config.after_initialize do
       ActiveSupport.on_load(:active_record) do
         instantiate_observers