modify the hash in-place as an optimization

lib/mongoid/contextual/mongo.rb

@@ -989,6 +989,10 @@ def recursive_demongoize(field_name, value, is_translation)
       # of the given hash as if it were the raw representation of a document of
       # the given klass.
       #
+      # @note this method will modify the given hash, in-place, for performance
+      # reasons. If you wish to preserve the original hash, duplicate it before
+      # passing it to this method.
+      #
       # @param [ Document ] klass the Document class that the given hash ought
       #   to represent
       # @param [ Hash | nil ] hash the Hash instance containing the values to
@@ -1001,28 +1005,26 @@ def recursive_demongoize(field_name, value, is_translation)
       def demongoize_hash(klass, hash)
         return nil unless hash
 
-        hash.each_with_object({}) do |(key, value), new_hash|
+        hash.each_key do |key|
+          value = hash[key]
+
           # does the key represent a declared field on the document?
           if (field = klass.fields[key])
-            new_hash[key] = field.demongoize(value)
+            hash[key] = field.demongoize(value)
             next
           end
 
           # does the key represent an emebedded relation on the document?
           aliased_name = klass.aliased_associations[key] || key
           if (assoc = klass.relations[aliased_name])
-            new_hash[key] = case value
-                            when Array then value.map { |hash| demongoize_hash(assoc.klass, hash) }
-                            when Hash then demongoize_hash(assoc.klass, value)
-                            else value
-                            end
-            next
+            case value
+            when Array then value.each { |h| demongoize_hash(assoc.klass, h) }
+            when Hash then demongoize_hash(assoc.klass, value)
+            end
           end
-
-          # if its not a field or a relation, then we just pass it through
-          # literally
-          new_hash[key] = value
         end
+
+        hash
       end
 
       # Demongoize the value for the given field. If the field is nil or the