class User
  ...

  field :foo, :type => Array, :default => []
end

>> u = User.last
>> u.foo
[]
>> u.foo << 1
[
    [0] 1
]
>> u.foo
[
    [0] 1
]
>> u.changes
{}
>> u.save(:validate => false)
true
>> u.reload
>> u.foo
[]
>> u.foo += [2]
[
    [0] 2
]
>> u.changes
{
    "foo" => [
        [0] [],
        [1] [
            [0] 2
        ]
    ]
}
>> u.save(:validate => false)
true
>> u.reload
>> u.foo
[
    [0] 2
]