How do I replace one node in an XML file with a node from another XML file?

I have two XML files. The first is:

<a>
  <b>
    <c1>1</c1>
  </b>
  <b>
    <c1>2</c1>
  </b>
  <b "id" = "true">
    <c1>3</c1>
    <d "do" ="me"></d>
  </b>
  <b id ="true">
    <c1>4</c1>
  </b>
</a>

And the second is:

<a>
  <b>
    <c1>5</c1>
  </b>
</a>

I want to update an element from first.xml:

<b "id" = "true">
  <c1>3</c1>
  <d "do" ="me"></d>
</b>

with an element from second.xml:

<b>
<c1>5</c1>
</b>

I tried to achieve that by deleting all the <b> nodes from first.xml and add the node <b> taken from second.xml file. I am able to delete all the nodes <b> but not able get an element from second.xml and add that to the first.xml.

Solution

After cleaning up the source XML, this seems to be what you're looking for:

xml1 = <<EOT
<a>
  <b>
    <c1>1</c1>
  </b>
  <b>
    <c1>2</c1>
  </b>
  <b id="true">
    <c1>3</c1>
    <d do="me"></d>
  </b>
  <b id="true">
    <c1>4</c1>
  </b>
</a>
EOT

xml2 = <<EOT
<a>
  <b>
    <c1>5</c1>
  </b>
</a>
EOT

require 'nokogiri'

doc1 = Nokogiri::XML(xml1)
doc2 = Nokogiri::XML(xml2)

doc1_b = doc1.at('//b[@id="true"]/c1/..')
doc2_b = doc2.at('b')

doc1_b.replace(doc2_b)

puts doc1.to_html

Which outputs:

<a>
  <b>
    <c1>1</c1>
  </b>
  <b>
    <c1>2</c1>
  </b>
  <b>
    <c1>5</c1>
  </b>
  <b id="true">
    <c1>4</c1>
  </b>
</a>

doc1.at('//b[@id="true"]/c1/..')' means "find the first occurrence of a b tag with id="true" with a child c1 node".