1 files changed, 33 insertions, 8 deletions
diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index 968dad5844..ba053700f2 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -77,8 +77,17 @@ module ActiveRecord
       mattr_accessor :disable_implicit_join_references, instance_writer: false
       self.disable_implicit_join_references = false
 
-      class_attribute :connection_handler, instance_writer: false
-      self.connection_handler = ConnectionAdapters::ConnectionHandler.new
+      class_attribute :default_connection_handler, instance_writer: false
+
+      def self.connection_handler
+        ActiveRecord::RuntimeRegistry.connection_handler || default_connection_handler
+      end
+
+      def self.connection_handler=(handler)
+        ActiveRecord::RuntimeRegistry.connection_handler = handler
+      end
+
+      self.default_connection_handler = ConnectionAdapters::ConnectionHandler.new
     end
 
     module ClassMethods
@@ -176,6 +185,7 @@ module ActiveRecord
       @columns_hash = self.class.column_types.dup
 
       init_internals
+      init_changed_attributes
       ensure_proper_type
       populate_with_current_scope_attributes
 
@@ -246,9 +256,7 @@ module ActiveRecord
       run_callbacks(:initialize) unless _initialize_callbacks.empty?
 
       @changed_attributes = {}
-      self.class.column_defaults.each do |attr, orig_value|
-        @changed_attributes[attr] = orig_value if _field_changed?(attr, orig_value, @attributes[attr])
-      end
+      init_changed_attributes
 
       @aggregation_cache = {}
       @association_cache = {}
@@ -299,9 +307,11 @@ module ActiveRecord
       id.hash
     end
 
-    # Freeze the attributes hash such that associations are still accessible, even on destroyed records.
+    # Clone and freeze the attributes hash such that associations are still
+    # accessible, even on destroyed records, but cloned models will not be
+    # frozen.
     def freeze
-      @attributes.freeze
+      @attributes = @attributes.clone.freeze
       self
     end
 
@@ -336,9 +346,15 @@ module ActiveRecord
       self.class.connection
     end
 
+    def connection_handler
+      self.class.connection_handler
+    end
+
     # Returns the contents of the record as a nicely formatted string.
     def inspect
-      inspection = if @attributes
+      # We check defined?(@attributes) not to issue warnings if the object is
+      # allocated but not initialized.
+      inspection = if defined?(@attributes) && @attributes
                      self.class.column_names.collect { |name|
                        if has_attribute?(name)
                          "#{name}: #{attribute_for_inspect(name)}"
@@ -434,5 +450,14 @@ module ActiveRecord
       @transaction_state        = nil
       @reflects_state           = [false]
     end
+
+    def init_changed_attributes
+      # Intentionally avoid using #column_defaults since overridden defaults (as is done in
+      # optimistic locking) won't get written unless they get marked as changed
+      self.class.columns.each do |c|
+        attr, orig_value = c.name, c.default
+        @changed_attributes[attr] = orig_value if _field_changed?(attr, orig_value, @attributes[attr])
+      end
+    end
   end
 end