Nokogiri XPath not finding certain nodes

Question

I am using Nokogiri to modify an existing XML but I am having trouble selecting certain nodes.

Here is the relevant snippet of XML:

<ProductCatalog>
  <ProductLineItem>
    <updi:ProductIdentification>
      <updi:ProductName>800-22283-03</updi:ProductName>

I can locate the lower two nodes with:

doc.xpath("//updi:ProductIdentification") => #<Nokogiri::XML...
doc.xpath("//updi:ProductName") => #<Nokogiri::XML...

However if I try to select one of the upper nodes:

doc.xpath("//ProductLineItem") => []

I get back an empty array. It seems to have something to do with the prefixes. I can locate any element that has a prefix, but can't find elements with no prefix.

UPDATE: Here is the (quite lengthy) namespace:

xsi:schemaLocation="urn:rosettanet:specification:interchange:ProductCatalogInformationDistribution:xsd:schema:01.00 ..\..\XML\Interchange\ProductCatalogInformationDistribution_01_00.xsd"
xmlns:dplcs="urn:rosettanet:specification:domain:Design:ProductLifeCycleStatusCode:xsd:codelist:01.03"
xmlns:rrt="urn:rosettanet:specification:domain:Shared:RateType:xsd:codelist:01.01" 
xmlns:dl="urn:rosettanet:specification:domain:Logistics:xsd:schema:02.15" 
xmlns:ictc="urn:rosettanet:specification:domain:Design:CatalogType:xsd:codelist:01.00" 
xmlns:updi="urn:rosettanet:specification:universal:ProductIdentification:xsd:schema:01.04" 
xmlns:dddt="urn:rosettanet:specification:domain:Design:DateType:xsd:codelist:01.00" 
xmlns:dsdc="urn:rosettanet:specification:domain:Logistics:ShipDateCode:xsd:codelist:01.03" 
xmlns:ucr="urn:rosettanet:specification:universal:Currency:xsd:codelist:01.02" 
xmlns:dpiac="urn:rosettanet:specification:domain:Logistics:PortIdentifierAuthorityCode:xsd:codelist:01.03" 
xmlns:rptc="urn:rosettanet:specification:domain:Shared:PricingTypeCode:xsd:codelist:01.03" 
xmlns:dit="urn:rosettanet:specification:domain:Procurement:InventoryType:xsd:codelist:01.03" 
xmlns:dtt="urn:rosettanet:specification:domain:Procurement:TransactionType:xsd:codelist:01.04" 
xmlns:upd="urn:rosettanet:specification:universal:PhysicalDimension:xsd:schema:01.05" 
xmlns:dcst="urn:rosettanet:specification:domain:Logistics:CustomsType:xsd:codelist:01.03" 
xmlns:dsd="urn:rosettanet:specification:domain:Logistics:ShippingDocument:xsd:codelist:01.02" 
xmlns:uci="urn:rosettanet:specification:universal:ContactInformation:xsd:schema:01.03" 
xmlns:dpcm="urn:rosettanet:specification:domain:Procurement:PurchaseMethod:xsd:codelist:01.03" 
xmlns:rpsc="urn:rosettanet:specification:domain:Shared:ProductStatusCode:xsd:codelist:01.01" 
xmlns:dgrc="urn:rosettanet:specification:domain:Marketing:GeographicRegionCode:xsd:codelist:01.02" 
xmlns:dtrt="urn:rosettanet:specification:domain:Logistics:TrackingReferenceType:xsd:codelist:01.06" 
xmlns:umtq="urn:rosettanet:specification:universal:MimeTypeQualifier:xsd:codelist:01.02" 
xmlns:dcrt="urn:rosettanet:specification:domain:Procurement:CustomerType:xsd:codelist:01.03" 
xmlns:dscd="urn:rosettanet:specification:domain:Logistics:ShipmentChangeDisposition:xsd:codelist:01.03" 
xmlns:uc="urn:rosettanet:specification:universal:Country:xsd:codelist:01.02" 
xmlns="urn:rosettanet:specification:interchange:ProductCatalogInformationDistribution:xsd:schema:01.00" 
xmlns:dpc="urn:rosettanet:specification:domain:Procurement:PaymentCondition:xsd:codelist:01.03" 
xmlns:rpmt="urn:rosettanet:specification:domain:Shared:PaymentType:xsd:codelist:01.01" 
xmlns:dft="urn:rosettanet:specification:domain:Procurement:FinanceTerms:xsd:codelist:01.03" 
xmlns:dtq="urn:rosettanet:specification:domain:Procurement:TotalQualifier:xsd:codelist:01.03" 
xmlns:ume="urn:rosettanet:specification:universal:MonetaryExpression:xsd:schema:01.04" 
xmlns:dcp="urn:rosettanet:specification:domain:Design:Compliant:xsd:codelist:01.02" 
xmlns:drsc="urn:rosettanet:specification:domain:Marketing:RegistrationStatusCode:xsd:codelist:01.03" 
xmlns:uat="urn:rosettanet:specification:universal:AbstractType:xsd:schema:01.02" 
xmlns:dp="urn:rosettanet:specification:domain:Procurement:xsd:schema:02.17" 
xmlns:rpm="urn:rosettanet:specification:domain:Shared:PaymentMethod:xsd:codelist:01.02" 
xmlns:dfrt="urn:rosettanet:specification:domain:Procurement:ForecastReferenceType:xsd:codelist:01.03" 
xmlns:dtec="urn:rosettanet:specification:domain:Procurement:TaxExemptionCode:xsd:codelist:01.03" 
xmlns:ulc="urn:rosettanet:specification:universal:Locations:xsd:schema:01.04" 
xmlns:dccc="urn:rosettanet:specification:domain:Procurement:CreditCardClassification:xsd:codelist:01.03" 
xmlns:drlc="urn:rosettanet:specification:domain:Logistics:ReturnLabelCode:xsd:codelist:01.03" 
xmlns:st="http://www.ascc.net/xml/schematron" 
xmlns:dnecc="urn:rosettanet:specification:domain:Logistics:NationalExportControlClassification:xsd:codelist:01.03" 
xmlns:rpktc="urn:rosettanet:specification:domain:Shared:PackageTypeCode:xsd:codelist:01.01" 
xmlns:uwt="urn:rosettanet:specification:universal:WeightType:xsd:codelist:01.01" 
xmlns:dfpt="urn:rosettanet:specification:domain:Logistics:FreightPaymentTerms:xsd:codelist:01.03" 
xmlns:dte="urn:rosettanet:specification:domain:Procurement:TransportEvent:xsd:codelist:01.03" 
xmlns:ul="urn:rosettanet:specification:universal:Language:xsd:codelist:01.02" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xmlns:dbpq="urn:rosettanet:specification:domain:Procurement:BookPriceQualifier:xsd:codelist:01.04" 
xmlns:drl="urn:rosettanet:specification:domain:Logistics:RouteLocation:xsd:codelist:01.03" 
xmlns:ssdh="urn:rosettanet:specification:system:StandardDocumentHeader:xsd:schema:01.16" 
xmlns:dmk="urn:rosettanet:specification:domain:Marketing:xsd:schema:02.12" 
xmlns:rmat="urn:rosettanet:specification:domain:Shared:MonetaryAmountType:xsd:codelist:01.01" 
xmlns:uuom="urn:rosettanet:specification:universal:UnitOfMeasure:xsd:codelist:01.03" 
xmlns:dfe="urn:rosettanet:specification:domain:Procurement:ForecastEvent:xsd:codelist:01.03" 
xmlns:dst="urn:rosettanet:specification:domain:Procurement:ShipmentTerms:xsd:codelist:01.03" 
xmlns:udt="urn:rosettanet:specification:universal:DataType:xsd:schema:01.04" 
xmlns:dacc="urn:rosettanet:specification:domain:Procurement:AccountClassification:xsd:codelist:01.03" 
xmlns:dptt="urn:rosettanet:specification:domain:Logistics:PortType:xsd:codelist:01.03" 
xmlns:sha="urn:rosettanet:specification:domain:Shared:xsd:schema:01.10" 
xmlns:dlv="urn:rosettanet:specification:domain:Design:Level:xsd:codelist:01.02" 
xmlns:rict="urn:rosettanet:specification:domain:Shared:InvoiceChargeType:xsd:codelist:01.02" 
xmlns:utt="urn:rosettanet:specification:universal:TaxType:xsd:codelist:01.02" 
xmlns:ddwsr="urn:rosettanet:specification:domain:Marketing:DesignWinStatusReason:xsd:codelist:01.03" 
xmlns:dsm="urn:rosettanet:specification:domain:Logistics:ShipmentMode:xsd:codelist:01.05" 
xmlns:udct="urn:rosettanet:specification:universal:DocumentType:xsd:codelist:01.09" 
xmlns:dac="urn:rosettanet:specification:domain:Design:ActionCode:xsd:codelist:01.03" 
xmlns:dpsr="urn:rosettanet:specification:domain:Procurement:ProductSubstitutionReason:xsd:codelist:01.03" 
xmlns:sft="urn:rosettanet:specification:system:TPIRFileType:xsd:codelist:01.01" 
xmlns:dltcc="urn:rosettanet:specification:domain:Procurement:LeadTimeClassificationCode:xsd:codelist:01.03" 
xmlns:ri="urn:rosettanet:specification:domain:Shared:Interval:xsd:codelist:01.01" 
xmlns:urss="urn:rosettanet:specification:system:xml:1.0" 
xmlns:dds="urn:rosettanet:specification:domain:Design:xsd:schema:02.15" 
xmlns:dslt="urn:rosettanet:specification:domain:Procurement:SaleType:xsd:codelist:01.04" 
xmlns:udc="urn:rosettanet:specification:universal:Document:xsd:schema:01.08" 
xmlns:dabcc="urn:rosettanet:specification:domain:Design:ABCCode:xsd:codelist:01.02" 
xmlns:dppt="urn:rosettanet:specification:domain:Procurement:ProductProcurementType:xsd:codelist:01.03" 
xmlns:rwtc="urn:rosettanet:specification:domain:Shared:WarrantyType:xsd:codelist:01.01" 
xmlns:dlit="urn:rosettanet:specification:domain:Logistics:InstructionType:xsd:codelist:01.00" 
xmlns:rfob="urn:rosettanet:specification:domain:Shared:FreeOnBoard:xsd:codelist:01.01" 
xmlns:upri="urn:rosettanet:specification:universal:ProcessRoleIdentifier:xsd:codelist:01.08" 
xmlns:ddrn="urn:rosettanet:specification:domain:Marketing:DesignRegistrationNotification:xsd:codelist:01.02" 
xmlns:dsh="urn:rosettanet:specification:domain:Procurement:SpecialHandling:xsd:codelist:01.04" 
xmlns:ud="urn:rosettanet:specification:universal:Dates:xsd:schema:01.03" 
xmlns:dpms="urn:rosettanet:specification:domain:Marketing:ProjectMarketSegment:xsd:codelist:01.02" 
xmlns:rssl="urn:rosettanet:specification:domain:Shared:ShippingServiceLevel:xsd:codelist:01.01" 
xmlns:dldr="urn:rosettanet:specification:domain:Logistics:LotDiscrepancyReason:xsd:codelist:01.03" 
xmlns:rat="urn:rosettanet:specification:domain:Shared:AmountType:xsd:codelist:01.02" 
xmlns:upi="urn:rosettanet:specification:universal:PartnerIdentification:xsd:schema:01.12" 
xmlns:ddp="urn:rosettanet:specification:domain:Marketing:Disposition:xsd:codelist:01.02" 
xmlns:dsfr="urn:rosettanet:specification:domain:Procurement:SpecialFulfillmentRequest:xsd:codelist:01.03" 
xmlns:ucs="urn:rosettanet:specification:universal:CountrySubdivision:xsd:codelist:01.02

Answer 1

The simplest quick-hack solution is to remove namespaces from your document altogether:

require 'nokogiri'
xml = Nokogiri.XML "<root xmlns='foo' xmlns:bar='whee'><a/><bar:b /></root>"

p xml.xpath('//b').length     #=> 0
p xml.xpath('//bar:b').length #=> 1
p xml.xpath('//a').length     #=> 0
xml.remove_namespaces!
p xml.xpath('//a').length     #=> 1
p xml.xpath('//b').length     #=> 1

However, the above is not a valid solution if you need to preserve namespaces (e.g. modifying your document and resaving it, or you have conflicting element or attribute names amongst the various namespaces). If you can't nuke the namespaces, you can either create a prefix and tell Nokogiri what it corresponds to…

xml = Nokogiri.XML "<root xmlns='foo' xmlns:bar='whee'><a/><bar:b /></root>"
p xml.xpath('//x:a','x'=>'foo').length  #=> 1

…where the string foo is the URI for the namespace of the owning element in your document that has a default namespace (usually on the root), and the string x is whatever you want it to be (that doesn't conflict with another namespace already declared in your document). Or, more simply, you can just use xmlns as a prefix for the default namespace:

p xml.xpath('//xmlns:a').length  #=> 1

Alternatively, if you need to leave the namespaces and can construct a reasonable CSS-style selector to get the nodes you need then you can use the css method:

require 'nokogiri'
xml = Nokogiri.XML "<root xmlns='foo' xmlns:bar='whee'>
  <a/>
  <bar:b />
  <c xmlns='jim'><d/></c>
</root>"

p xml.css('a').length, #=> 1
  xml.css('b').length, #=> 0
  xml.css('c').length, #=> 0
  xml.css('d').length  #=> 0

As shown above, note that this only works for nodes that are in the same namespace as the root element.