Refactor serialization_modification_detection to increase performance

The previous handling was correct, but not performant, as it did
a serialize/deserialize pair both for new objects and after saving.
Change the code to just clear the original values after saving
(which fixes saved objects), and just serialize early for new
objects (which fixes new objects).

To make this either, add a private serialize_deserialized_values
instance method to the serialization plugin, so that the modification
detection plugin can use it.

While here, update the RDoc for the modification detection plugin.

lib/sequel/plugins/serialization.rb

@@ -165,9 +165,9 @@ def initialize_set(values)
           super
         end
 
-        # Serialize all deserialized values
+        # Serialize deserialized values before saving
         def before_save
-          deserialized_values.each{|k,v| @values[k] = serialize_value(k, v)}
+          serialize_deserialized_values
           super
         end
 
@@ -194,6 +194,11 @@ def deserialize_value(column, v)
           end
         end
 
+        # Serialize all deserialized values
+        def serialize_deserialized_values
+          deserialized_values.each{|k,v| @values[k] = serialize_value(k, v)}
+        end
+
         # Serialize the column value.  Called before saving to ensure the serialized value
         # is saved in the database.
         def serialize_value(column, v)

lib/sequel/plugins/serialization_modification_detection.rb

@@ -1,11 +1,16 @@
 module Sequel
   module Plugins
-    # Sequel's built in Serialization plugin doesn't check for modification
-    # of the serialized objects, because it requires an extra deserialization of a potentially
-    # very large object.  This plugin can detect changes in serialized values by
-    # checking whether the current deserialized value is the same as the original
-    # deserialized value.  This does require deserializing the value twice, but the
-    # original deserialized value is cached.
+    # This plugin extends the serialization plugin and enables it to detect
+    # changes in serialized values by checking whether the current
+    # deserialized value is the same as the original deserialized value.
+    # The serialization plugin does not do such checks by default, as they
+    # often aren't needed and can hurt performance.
+    #
+    # Note that for this plugin to work correctly, the values you are
+    # serializing must roundtrip correctly (i.e. deserialize(serialize(value))
+    # should equal value).  This is true in most cases, but not in all.  For
+    # example, ruby symbols round trip through yaml, but not json (as they get
+    # turned into strings in json).
     #
     # == Example
     #
@@ -29,7 +34,7 @@ module InstanceMethods
         # show the column is modified after saving.
         def after_save
           super
-          copy_deserialized_values
+          @original_deserialized_values = {}
         end
 
         # Detect which serialized columns have changed.
@@ -41,16 +46,11 @@ def changed_columns
 
         private
 
-        # For new objects, populate the original deserialized value so that we know it hasn't
-        # changed since initialization.
+        # For new objects, serialize any existing deserialized values so that changes can
+        # be detected.
         def initialize_set(values)
           super
-          copy_deserialized_values
-        end
-
-        def copy_deserialized_values
-          @original_deserialized_values = h = {}
-          @deserialized_values.each{|k, v| h[k] = deserialize_value(k, serialize_value(k, v))}
+          serialize_deserialized_values
         end
 
         # Return the original deserialized value of the column, caching it to improve performance.