Rails merge temporary attributes to model object

As soon as you call .attributes you turn the MyModel instance into a hash.

To turn it back into a model you can just pass that hash to the MyModel constructor (assuming you have an attr_accessor for the virtual attribute):

my_obj = MyModel.new(my_obj.attributes.merge(age: compute_age))
my_obj = MyModel.new(my_obj.attributes.merge(name: compute_name))

You can also see here for some other options: Rails update_attributes without save?

edit

to clarify a little more. You can turn a model instance into a hash via .attributes. You can turn it back into a model using Model.new(attributes_hash). While it's a hash, you can set whatever key-vals you want - it's just a hash. But in order to convert it back to a model instance, you need to make sure that all the hash keys correspond to actual attributes on the model (be they persisted attributes defined by the schema, or virtual attributes defined by attr_accessor).

If you just want to turn it into a hash and add multiple arbitrary keyvals there, you can do it with a single merge:

attrs = my_obj.attributes.merge({ age: compute_age, name: compute_name })