How to put an XML document into another with Nokogiri - ruby

Question

I want to include an xml document with arbitrary user provided data in another one using nokogiri.
I have tried Node#add_child and encountered a problem: nokogiri looses namespace definition if there is a uri in child_doc that is also present in main doc but with a different prefix.
Example:
doc = Nokogiri::XML %(
<s:rack xmlns:s="urn://shelf">
<s:shelf></s:shelf>
</s:rack>
)
child_doc = Nokogiri::XML %(
<m:shelf xmlns:m="urn://shelf">
<m:book></m:book>
</m:shelf>
)
doc.root.add_child(child_doc.root)
doc.to_xml
# =>
# <s:rack xmlns:s="urn://shelf">
# <s:shelf/>
# <!-- namespace for prefix m is not declared! -->
# <m:shelf>
# <m:book/>
# </m:shelf>
# </s:rack>
Namespace for prefix m is lost.
Here is the resulting doc object, pretty printed:
#(Document:0x1b2ce28 {
name = "document",
children = [
#(Element:0x1b327c4 {
name = "rack",
namespace = #(Namespace:0x1b320a8 { prefix = "s", href = "urn://shelf" }),
children = [
#(Text "\n "),
#(Element:0x1b45f90 { name = "shelf", namespace = #(Namespace:0x1b320a8 { prefix = "s", href = "urn://shelf" }) }),
#(Text "\n "),
#(Element:0x1a9e5d8 {
name = "shelf",
namespace = #(Namespace:0x1a87450 { prefix = "m", href = "urn://shelf" }),
children = [
#(Text "\n "),
#(Element:0x1a41dec { name = "book", namespace = #(Namespace:0x1a87450 { prefix = "m", href = "urn://shelf" }) }),
#(Text "\n ")]
})]
})]
})
As you can see internally namespaces in added nodes are correct but they are serialized wrong. How do I get nokogiri to save them in the resulting xml?