Modifying XML

Purpose

Learn how to modify existing XML documents by adding, updating, and removing elements, attributes, and content.

Modifying element content

Update text content of elements:

xml = '<library><book><title>Old Title</title></book></library>'
doc = Moxml.new.parse(xml)

# Find and update
title = doc.at_xpath('//title')
title.text = 'New Title'

puts doc.to_xml
# => <library><book><title>New Title</title></book></library>

Adding elements

Add new elements to existing documents:

xml = '<library><book id="1"><title>Ruby Basics</title></book></library>'
doc = Moxml.new.parse(xml)

book = doc.at_xpath('//book[@id="1"]')

# Add author element
author = doc.create_element('author')
author.text = 'Jane Smith'
book.add_child(author)

# Add price element
price = doc.create_element('price')
price.text = '29.99'
price['currency'] = 'USD'
book.add_child(price)

# Add ISBN element
isbn = doc.create_element('isbn')
isbn.text = '978-0-123456-78-9'
book.add_child(isbn)

puts doc.to_xml(indent: 2)

Removing elements

Remove elements from documents:

xml = <<~XML
  <library>
    <book id="1">
      <title>Ruby Basics</title>
      <author>Jane Smith</author>
      <draft>true</draft>
    </book>
  </library>
XML

doc = Moxml.new.parse(xml)

# Find and remove element
draft = doc.at_xpath('//draft')
draft.remove

# Remove by parent
book = doc.at_xpath('//book')
author = book.at_xpath('.//author')
book.remove_child(author)

puts doc.to_xml

Modifying attributes

Update, add, and remove attributes:

xml = '<book id="1" status="draft">Ruby Basics</book>'
doc = Moxml.new.parse(xml)

book = doc.root

# Update existing attribute
book['id'] = '100'

# Add new attribute
book['edition'] = '2nd'
book['category'] = 'programming'

# Remove attribute
book.remove_attribute('status')

# Get all attributes
book.attributes.each do |attr|
  puts "#{attr.name}=#{attr.value}"
end
# => id=100
# => edition=2nd
# => category=programming

Replacing nodes

Replace elements with new content:

xml = '<book><title>Old Title</title><author>Old Author</author></book>'
doc = Moxml.new.parse(xml)

# Replace title element
old_title = doc.at_xpath('//title')
new_title = doc.create_element('title')
new_title.text = 'New Title'
new_title['lang'] = 'en'

old_title.replace(new_title)

# Replace text node
author = doc.at_xpath('//author')
author.children.first.replace(doc.create_text('New Author'))

puts doc.to_xml

Adding siblings

Insert elements relative to existing nodes:

xml = <<~XML
  <book>
    <title>Ruby Programming</title>
    <price>29.99</price>
  </book>
XML

doc = Moxml.new.parse(xml)

# Add before price
price = doc.at_xpath('//price')
author = doc.create_element('author')
author.text = 'Jane Smith'
price.add_previous_sibling(author)

# Add after title
title = doc.at_xpath('//title')
subtitle = doc.create_element('subtitle')
subtitle.text = 'A Comprehensive Guide'
title.add_next_sibling(subtitle)

puts doc.to_xml(indent: 2)

Output:

<book>
  <title>Ruby Programming</title>
  <subtitle>A Comprehensive Guide</subtitle>
  <author>Jane Smith</author>
  <price>29.99</price>
</book>

Batch modifications

Update multiple elements at once:

xml = <<~XML
  <library>
    <book><price currency="USD">29.99</price></book>
    <book><price currency="USD">39.99</price></book>
    <book><price currency="USD">19.99</price></book>
  </library>
XML

doc = Moxml.new.parse(xml)

# Apply 10% discount to all books
doc.xpath('//price').each do |price|
  current = price.text.to_f
  discounted = (current * 0.9).round(2)
  price.text = discounted.to_s
  price['original'] = current.to_s
end

puts doc.to_xml(indent: 2)

Preserving structure

Maintain document structure during modifications:

xml = <<~XML
  <?xml version="1.0" encoding="UTF-8"?>
  <library>
    <book id="1">Ruby Basics</book>
  </library>
XML

doc = Moxml.new.parse(xml)

# Modifications preserve declaration
book = doc.at_xpath('//book')
book['edition'] = '2nd'

# Original structure maintained
puts doc.to_xml(indent: 2)
# => Still has <?xml ... ?> declaration

Common modification patterns

Update or create pattern 

def ensure_element(parent, name, text)
  elem = parent.at_xpath(".//#{name}")

  if elem
    # Update existing
    elem.text = text
  else
    # Create new
    elem = parent.document.create_element(name)
    elem.text = text
    parent.add_child(elem)
  end

  elem
end

book = doc.at_xpath('//book')
ensure_element(book, 'author', 'Jane Smith')
ensure_element(book, 'price', '29.99')

Conditional modification 

doc.xpath('//book').each do |book|
  price = book.at_xpath('.//price')
  next unless price

  # Add discount for expensive books
  if price.text.to_f > 30
    book['discount'] = '10%'
    price.text = (price.text.to_f * 0.9).to_s
  end
end

Best practices

  1. Find before modifying - always locate elements first

  2. Check element exists before calling methods on it

  3. Use transactions for complex modifications if needed

  4. Validate structure after major changes

  5. Preserve document metadata (declarations, encoding)

See also