serializing a Proc doesn't seem to work

In Agile Web Development with Rails, there is a section on serializing
Ruby objects.

"We can assign any Ruby object to the last_five column" ... p312

When I attempt to do this with a proc object, something like this is
saved in the db

--- !ruby/object:Proc {}

Obviously, this doesn't do much, and I attempt to use it ...

NoMethodError: undefined method `call' for "--- !ruby/object:Proc {}\n
\n":String
        from (irb):5

Is it possible to save Proc objects in the db?

I am implementing a custom field scheme, where users can create custom
fields. I have Field Types, and I was thinking about using a ruby proc
to convert field entries to the correct object type.

John