nokogiri and xpath -- nested looping with data set - ruby-on-rails

Question

I'm trying to loop through the elements in each but am having issues with the inner loop below. It appears to me the xpath pattern '*/td' is not returning any results. I'm expecting to see the data inside the tags printed to stdout. I'm using nokogiri.
I'm pasting this into my rails console:
require 'nokogiri'
f = File.open("public/index.html")
doc = Nokogiri::HTML(f)
f.close
doc.xpath('//*[#id="WhoIsOnDutyTableLevel4"]/tbody/tr').each do |row|
puts "row= " + row.to_s
row.xpath('*/td').each do |td|
puts "td= " + td
end
end
And here's the output from the console:
row= <tr id="208894">
<td headers="WhoIsOnDutyTableLevel1:header:1">User 1</td>
<td headers="WhoIsOnDutyTableLevel1:header:2">PERSON</td>
<td headers="WhoIsOnDutyTableLevel1:header:3">0</td>
</tr>
row= <tr id="207792">
<td headers="WhoIsOnDutyTableLevel1:header:1">User 2</td>
<td headers="WhoIsOnDutyTableLevel1:header:2">PERSON</td>
<td headers="WhoIsOnDutyTableLevel1:header:3">5</td>
</tr>
=> 0
Here's the html I'm parsing:
<table class="duty-report-level1" id="WhoIsOnDutyTableLevel1">
<caption></caption>
<thead>
<tr>
<th id="WhoIsOnDutyTableLevel1:header:1" class="duty-report-lt-header">c</th>
</tr>
</thead>
<tfoot></tfoot>
<tbody>
<tr>
<td headers="WhoIsOnDutyTableLevel1:header:1">
<table class="duty-report-level2" id="WhoIsOnDutyTableLevel2">
<caption></caption>
<thead>
<tr>
<th id="WhoIsOnDutyTableLevel1:header:1">Group Name</th><th id="WhoIsOnDutyTableLevel1:header:2">Group Time Zone</th><th id="WhoIsOnDutyTableLevel1:header:3">Default Devices</th><th id="WhoIsOnDutyTableLevel1:header:4">Supervisors</th>
</tr>
</thead>
<tfoot></tfoot>
<tbody>
<tr>
<td headers="WhoIsOnDutyTableLevel1:header:1">Team 1</td><td headers="WhoIsOnDutyTableLevel1:header:2" class="centered-text">US/Pacific</td><td headers="WhoIsOnDutyTableLevel1:header:3" class="centered-text"><img src="/static/images/icon_boolean_false.png" alt="No" border="0"></td><td headers="WhoIsOnDutyTableLevel1:header:4">
<values>
</values>Mgr 1
<br>
</td>
</tr>
<tr>
<td headers="WhoIsOnDutyTableLevel1:header:1" class="no-padding" colspan="4">
<table class="duty-report-level3" id="WhoIsOnDutyTableLevel3">
<caption></caption>
<thead>
<tr>
<th id="WhoIsOnDutyTableLevel1:header:1" class="th-left">a</th><th id="WhoIsOnDutyTableLevel1:header:2" class="">b</th>
</tr>
</thead>
<tfoot></tfoot>
<tbody>
<tr>
<td headers="WhoIsOnDutyTableLevel1:header:1" class="no-padding" colspan="2">
<table class="duty-report-level4" id="WhoIsOnDutyTableLevel4">
<caption></caption>
<thead>
<tr>
<th id="WhoIsOnDutyTableLevel1:header:1">Recipient</th><th id="WhoIsOnDutyTableLevel1:header:2">Category</th><th id="WhoIsOnDutyTableLevel1:header:3">Escalation</th>
</tr>
</thead>
<tfoot></tfoot>
<tbody>
<tr id="208894">
<td headers="WhoIsOnDutyTableLevel1:header:1">User 1</td><td headers="WhoIsOnDutyTableLevel1:header:2">PERSON</td><td headers="WhoIsOnDutyTableLevel1:header:3">0</td>
</tr>
<tr id="207792">
<td headers="WhoIsOnDutyTableLevel1:header:1">User 2</td><td headers="WhoIsOnDutyTableLevel1:header:2">PERSON</td><td headers="WhoIsOnDutyTableLevel1:header:3">5</td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
</table>

Answer 1

You need a minor change to your XPath:
doc.xpath('//*[#id="WhoIsOnDutyTableLevel4"]/tbody/tr').each do |row|
# puts "row= " + row.to_s
row.xpath('./td').each do |td|
puts "td= " + td.text
end
end
Which outputs:
td= User 1
td= PERSON
td= 0
td= User 2
td= PERSON
td= 5
Using ./td as the XPath for td basically means "from this point look down one".
Personally, unless you absolutely need XPath, I recommend using CSS accessors. They are more readable, and often much more simple:
doc.search('#WhoIsOnDutyTableLevel4 tbody tr').each do |row|
row.search('td').each do |td|
puts "td= " + td.text
end
end
I recommend using search instead of css or xpath and at instead of at_css or at_xpath. There is no real magic going on when you choose one over the other and you only have to remember two different methods.

Answer 2

The XPath expression in the inner loop is evaluated relative to each tr, so you want to use td (which selects children td elements of the context tr) and not */td (which selects grandchildren td elements).
Full code:
doc.xpath('//*[#id="WhoIsOnDutyTableLevel4"]/tbody/tr').each do |row|
puts "row= " + row.to_s
row.xpath('td').each do |td|
puts "td= " + td
end
end