SerializationTypeMismatch when retrieving (not: storing) object

[Ronald Fischer]

I don't understand the following example from the ActiveRecord::Base
section in http://api.rubyonrails.org/ :

======
class User < ActiveRecord::Base
serialize :preferences, Hash
end

user = User.create(preferences: %w( one two three ))
User.find(user.id).preferences # raises SerializationTypeMismatch
======

What was the design choice, that I get the exception in the *last* line?
I could understand, if there is an exception in User.create, because
:preferences is supposed to be a Hash (if I understood correctly the
'serialize' function), and instead an Array is passed. But the
documentation clearly says that the exception happens at retrieval:

======
You can also specify a class option as the second parameter that'll
raise an exception if a serialized object is retrieved as a descendant
of a class not in the hierarchy.
======

Is there a deeper reason, why this error is caught at retrieval, and not
already at the time of storing the value?

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[mike2r]

I'm not sure exactly why, but it is true, the error doesn't get thrown until you try to retrieve it.  Part of the issue is that the serialize method creates a uniquely ruby construct, to the database it's a text field.  Therefore, when you save the record, unless there's logic on the Rails side, there isn't going to be an error from the database from a mismatch.  

I'm not fond of serialization for that reason, because you can't initialize the hashes or whatever you store there, and, by definition, it's limited to 64K (the limit of a text field).  Rails 4 now has the store which, IMO, is better if it meets your needs.  Also, PostgreSQL now supports hash, array, and json column types (I think you need PostgreSQL 9.2 or later).