Active record error when migrating from rails 2.3 to rails 3.1

I am migrating from a rails 2.3 application to Rails 3.1, i am getting this error when i try to save a record. This was not happening earlier and i am not sure what could be the reason, any help would be appreciated, Thanks.

Here are previews of my models

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class Order < ActiveRecord::Base 
has_many :items, :dependent => :delete_all
end

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class Item < ActiveRecord::Base 
belongs_to :order
validates_presence_of :order_id
end

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Below is error message log when i try to save

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ActiveRecord::RecordInvalid: Validation failed: Items order can't be blank
    from /Users/branch/.rvm/gems/ruby-1.9.3-p484/gems/activerecord-3.1.12/lib/active_record/validations.rb:56:in `save!'
    from /Users/branch/.rvm/gems/ruby-1.9.3-p484/gems/activerecord-3.1.12/lib/active_record/attribute_methods/dirty.rb:33:in `save!'
    from /Users/branch/.rvm/gems/ruby-1.9.3-p484/gems/activerecord-3.1.12/lib/active_record/transactions.rb:246:in `block in save!'
    from /Users/branch/.rvm/gems/ruby-1.9.3-p484/gems/activerecord-3.1.12/lib/active_record/transactions.rb:295:in `block in with_transaction_returning_status'
    from /Users/branch/.rvm/gems/ruby-1.9.3-p484/gems/activerecord-3.1.12/lib/active_record/connection_adapters/abstract/database_statements.rb:194:in `transaction'
    from /Users/branch/.rvm/gems/ruby-1.9.3-p484/gems/activerecord-3.1.12/lib/active_record/transactions.rb:208:in `transaction'
    from /Users/branch/.rvm/gems/ruby-1.9.3-p484/gems/activerecord-3.1.12/lib/active_record/transactions.rb:293:in `with_transaction_returning_status'
    from /Users/branch/.rvm/gems/ruby-1.9.3-p484/gems/activerecord-3.1.12/lib/active_record/transactions.rb:246:in `save!'
    from (irb):16
    from /Users/branch/.rvm/gems/ruby-1.9.3-p484/gems/railties-3.1.12/lib/rails/commands/console.rb:45:in `start'
    from /Users/branch/.rvm/gems/ruby-1.9.3-p484/gems/railties-3.1.12/lib/rails/commands/console.rb:8:in `start'
    from /Users/branch/.rvm/gems/ruby-1.9.3-p484/gems/railties-3.1.12/lib/rails/commands.rb:40:in `<top (required)>'
    from script/rails:6:in `require'
    from script/rails:6:in `<main>'
1

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