[ruby-core:44848] help understanding weird bug

Here is a sample application:

https://github.com/rosenfeld/weird-sequel-bug

And here is the strange behavior:

https://github.com/rosenfeld/weird-sequel-bug/blob/master/example.rb

children = Sq::Node[1].children
p children.class
parent = children.first.parent
p children.map &:id
2.times { p children.map{|c| c.parent = parent; c.id} }

Output:

Array
[2, 3]
[2, 2]
[3, 3]

For some strange reason, changing the parent of the element inside 
children.map{} block will modify the children array itself.

for example:

p children.map &:id
children[0].parent = parent
p children.map &:id

outputs:
[2, 3]
[3, 2]

Maybe this is a Sequel bug (I didn't look for the code of associations 
yet), but I'd like to understand how this could happen in Ruby.

For example, how can I know if this is an ordinary Array or not? 
children.class is Array, so how can I differentiate it from a normal Array.

If I change the first line to the one below it works:
# children = Sq::Node[1].children
children = Sq::Node[1].children.map{|c| c}

Any explanations on what could be happening?

Thanks in advance,
Rodrigo.