Nokogiri: NodeSet.dup does not deep copy

[dazza-codes]

Take note of the Element object IDs in this console exploration of whether or not NodeSet.dup is actually returning new objects (as a deep copy implies). The Document.dup is doing a deep copy, but NodeSet.dup is not. (This console is using a class wrapping Nokogiri instance variables, but it should be obvious anyway; LMK if you need a "pure" Nokogiri" example.)

> record_setB.print
<?xml version='1.0'?>
<records>
  <REC>
    <UID>WOS:A2</UID>
  </REC>
  <REC>
    <UID>WOS:B2</UID>
  </REC>
</records>

> record_setB.doc
=> #(Document:0x2b17e94437d0 {
  name = "document",
  children = [
    #(Element:0x2b17ea12b1c8 {
      name = "records",
      children = [
        #(Element:0x2b17ea12f804 { name = "REC", children = [
            #(Element:0x2b17ea1333dc { name = "UID", children = [ #(Text "WOS:A2")] })] }),
        #(Element:0x2b17ea13b438 { name = "REC", children = [
            #(Element:0x2b17ea13e188 { name = "UID", children = [ #(Text "WOS:B2")] })] })]
      })]
  })

> record_setB.doc.dup
=> #(Document:0x2b17ea3a2708 {
  name = "document",
  children = [
    #(Element:0x2b17ea3a843c {
      name = "records",
      children = [
        #(Element:0x2b17ea3acba4 { name = "REC", children = [ 
            #(Element:0x2b17ea3b0c18 { name = "UID", children = [ #(Text "WOS:A2")] })] }),
        #(Element:0x2b17ea3b8b0c { name = "REC", children = [
            #(Element:0x2b17ea3c18c4 { name = "UID", children = [ #(Text "WOS:B2")] })] })]
      })]
  })

So far, so good. The Document.dup is returning all new object IDs thanks to a deep copy. Now let's see if the NodeSet.dup is doing a deep copy:

> rec_node_setB = record_setB.doc.search('REC')
=> [#<Nokogiri::XML::Element:0x2b17ea12f804 name="REC" 
  children=[#<Nokogiri::XML::Element:0x2b17ea1333dc name="UID" 
    children=[#<Nokogiri::XML::Text:0x2b17ea137324 "WOS:A2">]>]>,
 #<Nokogiri::XML::Element:0x2b17ea13b438 name="REC" 
  children=[#<Nokogiri::XML::Element:0x2b17ea13e188 name="UID" 
    children=[#<Nokogiri::XML::Text:0x2b17ea14297c "WOS:B2">]>]>]

> rec_node_setB.dup
=> [#<Nokogiri::XML::Element:0x2b17ea12f804 name="REC" 
  children=[#<Nokogiri::XML::Element:0x2b17ea1333dc name="UID" 
    children=[#<Nokogiri::XML::Text:0x2b17ea137324 "WOS:A2">]>]>,
 #<Nokogiri::XML::Element:0x2b17ea13b438 name="REC" 
  children=[#<Nokogiri::XML::Element:0x2b17ea13e188 name="UID" 
    children=[#<Nokogiri::XML::Text:0x2b17ea14297c "WOS:B2">]>]>]

Nope, it's got all the same object IDs.

[dazza-codes]

Also NodeSet.deep_dup does not deep copy (cf. object IDS ^^):

> rec_node_setB.deep_dup
=> [#<Nokogiri::XML::Element:0x2b17ea12f804 name="REC" 
  children=[#<Nokogiri::XML::Element:0x2b17ea1333dc name="UID" 
    children=[#<Nokogiri::XML::Text:0x2b17ea137324 "WOS:A2">]>]>,
 #<Nokogiri::XML::Element:0x2b17ea13b438 name="REC" 
  children=[#<Nokogiri::XML::Element:0x2b17ea13e188 name="UID" 
    children=[#<Nokogiri::XML::Text:0x2b17ea14297c "WOS:B2">]>]>]

[dazza-codes]

WRT documentation, it doesn’t say anything on NodeSet.dup behavior in this doc:

• http://www.rubydoc.info/github/sparklemotion/nokogiri/Nokogiri/XML/NodeSet#dup-instance_method

cf. http://www.rubydoc.info/github/sparklemotion/nokogiri/Nokogiri/XML/Document#dup-instance_method where it states the default behavior is a deep copy.

So, I suppose I’m asking for consistent behavior in the default assumptions for how this method works across Nokogiri objects. I suppose I'm also asking for clarification on how to use the NodeSet.dup method to preserve immutability of the original NodeSet object.

[ar7max]

I agree. Duplication behaviour is confusing me too.

[flavorjones]

Can you help me understand why there is an assumption that NodeSet.dup would do a deep copy?

As a counterexample, Ruby Arrays don't do a deep copy:

$ irb
2.4.1 :001 > obj = Object.new
 => #<Object:0x00000000d52b40> 
2.4.1 :002 > arr = [obj]
 => [#<Object:0x00000000d52b40>] 
2.4.1 :003 > dup = arr.dup
 => [#<Object:0x00000000d52b40>] 

What I mean to say is, this is functioning as intended. I'd love to hear why you think it should do a deep copy, though.

[ar7max]

As for me it is all about black box "Document/Document Fragment" abstraction.

Now I need to dig into doc and copy all by hand, so Im using abstraction over ruby abstractions but I still need to use ruby abstractions for things to work.

It's kinda misleading for many people. Not to say I'm right or you are wrong, just my biased opinion.

[flavorjones]

@ar7max I'm not sure I understand ... I'll ask a different way: why do you expect an Enumerable in this case (NodeSet) to behave differently from the canonical Ruby Enumerable (Array)?

[ar7max]

@flavorjones because word "node" comes first I think.

[flavorjones]

I'm going to go ahead and close this, as I personally feel that NodeSet#dup is doing the right thing; and changing it at this point would be a breaking change to the API that we're not ready to make at this time.

I'd welcome any PRs about the docs that may clarify this issue for future users.

[opoudjis]

Same behaviour appears to obtain for Node.dup.

It's very misleading for many people. Especially if it applies to a single object.

[flavorjones]

@opoudjis I'm not sure I know how to respond, if you'd care to have a conversation please open a new issue?

[opoudjis]

I'll do a pull request on the documentation, to add a note saying that if you're going to strip elements from the node or nodeset clone, don't clone, create a new document based on the node instead.