How to filter XML elements by date range in Ruby - ruby

Question

I typically use Nokogiri as my XML parser.
I have the following XML:
<albums>
<aldo_nova album="aldo nova">
<release_date value="19820401"/>
</aldo_nova>
<classix_nouveaux album="Night People"/>
<release_date value="19820501"/>
</classix_nouveaux>
<engligh_beat album="I Just Can't Stop It"/>
<release_date value="19800501"/>
</engligh_beat>
</albums>
I want to get all albums that were released between 1/1/1980 and 4/15/1982:
<aldo_nova album="aldo nova">
<release_date value="19820401"/>
</aldo_nova>
<engligh_beat album="I Just Can't Stop It"/>
<release_date value="19800501"/>
</engligh_beat>
How do I filter/query the XML by a release_date range?

Answer 1

Your XML is malformed. After parsing, here's what Nokogiri has to say about it:
doc.errors
# => [#<Nokogiri::XML::SyntaxError: Opening and ending tag mismatch: albums line 1 and classix_nouveaux>,
# #<Nokogiri::XML::SyntaxError: Extra content at the end of the document>]
That's because:
<classix_nouveaux album="Night People"/>
and
<engligh_beat album="I Just Can't Stop It"/>
are terminated. Instead they should be:
<classix_nouveaux album="Night People">
and
<engligh_beat album="I Just Can't Stop It">
You can use CSS or XPath selectors to find exact matches, or even sub-string matches, but neither CSS or XPath understand "ranges" of dates, nor do they have an idea of what a Date is, so you'd have to extract all nodes, convert the date value into a Date object or integer in this case, then compare to the range:
date_range = 19800501..19820401
selected_albums = doc.search('//release_date').select { |rd| date_range.include?(rd['value'].to_i) }.map { |rd| rd.parent }
selected_albums.map(&:to_xml)
# => ["<aldo_nova album=\"aldo nova\">\n" +
# " <release_date value=\"19820401\"/>\n" +
# "</aldo_nova>",
# "<engligh_beat album=\"I Just Can't Stop It\">\n" +
# " <release_date value=\"19800501\"/>\n" +
# "</engligh_beat>"]
I think your XML is poorly designed because you have varying tag names for what should be an album. <album> should be a child of <albums>. I'd recommend something like this:
<collection>
<albums>
<album band="aldo nova" title="aldo nova" release_date="19820401"/>
<album band="classix nouveaux" title="Night People" release_date="19820501"/>
<album band="english beat" title="I Just Can't Stop It" release_date="19800501"/>
</albums>
</collection>
Once the XML is in a standard form, then it becomes easier to navigate and search:
require 'nokogiri'
doc = Nokogiri::XML(<<EOT)
<collection>
<albums>
<album band="aldo nova" title="aldo nova" release_date="19820401"/>
<album band="classix nouveaux" title="Night People" release_date="19820501"/>
<album band="english beat" title="I Just Can't Stop It" release_date="19800501"/>
</albums>
</collection>
EOT
doc.search('album').last['title'] # => "I Just Can't Stop It"
band = 'aldo nova'
doc.search("//album[#band='#{band}']").map { |a| a['title'] } # => ["aldo nova"]
and searching for dates becomes more straightforward because it's not necessary to find the parent of the node:
date_range = 19800501..19820401
selected_albums = doc.search('album').select { |a| date_range.include?(a['release_date'].to_i) }
selected_albums.map(&:to_xml)
# => ["<album band=\"aldo nova\" title=\"aldo nova\" release_date=\"19820401\"/>",
# "<album band=\"english beat\" title=\"I Just Can't Stop It\" release_date=\"19800501\"/>"]
I'd recommend reading some tutorials on XML itself as it's easy to paint ourselves into corners if the data isn't represented logically and correctly.