Joining multiple (2+) nodes / Multihop mappings in RML (RDF Mapping Language)

Question

I am trying to map an XML source to RDF but cannot seem to get multi-hop mappings or multi-join conditions to work. After having gone through the documentation, examples, and test cases, I am not quite sure if this is even possible.

I appreciate any assistance in resolving this.

Following are a simplified sample XML, the RML mapping I created so far, the current RDF output generated using RMLMapper, as well as the RDF output I am expecting.

Data

<Root>
    <MainNode>
        <name>main1</name>
    </MainNode>
    <OtherNode>
        <name>other1</name>
        <otherCondition>main1</otherCondition>
    </OtherNode>
    <OtherNode>
        <name>other2</name>
        <otherCondition>otherCond2</otherCondition>
    </OtherNode>
    <AnotherNode>
        <name>another1</name>
        <description>anotherDesc1</description>
        <anotherCondition>other1</anotherCondition>
    </AnotherNode>
    <AnotherNode>
        <name>another2</name>
        <description>anotherDesc2</description>
        <anotherCondition>anotherCond2</anotherCondition>
    </AnotherNode>
</Root>

Mapping

@prefix rml: <http://semweb.mmlab.be/ns/rml#> .
@prefix rr: <http://www.w3.org/ns/r2rml#> .
@prefix ql: <http://semweb.mmlab.be/ns/ql#> .
@prefix owl: <http://www.w3.org/2002/07/owl#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .

@prefix testont: <http://www.example.com/ontology/> .
@prefix : <http://www.example.com/rules/> .
@base <http://www.example.com/instance/> .


:TriplesMapAnotherNode a rr:TriplesMap;
  rml:logicalSource [
    rml:source "multihop_data.xml";
    rml:referenceFormulation ql:XPath;
    rml:iterator "//AnotherNode"
  ].

:TriplesMapAnotherNode rr:subjectMap [
  rr:template "{description}"
].

:TriplesMapOtherNode a rr:TriplesMap;
    rml:logicalSource [
        rml:source "multihop_data.xml";
        rml:referenceFormulation ql:XPath;
        rml:iterator "//OtherNode"
    ].

:TriplesMapOtherNode rr:subjectMap [
    rr:template "{name}"
].

:TriplesMapMainNode a rr:TriplesMap;
    rml:logicalSource [
        rml:source "multihop_data.xml";
        rml:referenceFormulation ql:XPath;
        rml:iterator "//MainNode"
    ].

:TriplesMapMainNode rr:subjectMap [
    rr:template "{name}"
].

:TriplesMapMainNode rr:predicateObjectMap [
    rr:predicate rdf:type;
    rr:object testont:MainClass
].

:TriplesMapOtherNode rr:predicateObjectMap [
  rr:predicate testont:dummypredicate;
  rr:objectMap [
    a rr:RefObjectMap;
    rr:parentTriplesMap :TriplesMapAnotherNode;
    rr:joinCondition [
      rr:child "name";
      rr:parent "anotherCondition";
    ]
  ]
].

:TriplesMapMainNode rr:predicateObjectMap [
    rr:predicate testont:hasDescription;
    rr:objectMap [
        a rr:RefObjectMap;
        rr:parentTriplesMap :TriplesMapOtherNode;
        rr:joinCondition [
          rr:child "name";
          rr:parent "otherCondition";
        ]
    ]
].

Current RDF

<http://www.example.com/instance/other1> <http://www.example.com/ontology/dummypredicate> <anotherDesc1>.
<http://www.example.com/instance/main1> <http://www.example.com/ontology/hasDescription> <other1>.
<http://www.example.com/instance/main1> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.example.com/ontology/MainClass>.

Expected RDF

<http://www.example.com/instance/main1> <http://www.example.com/ontology/hasDescription> <anotherDesc1>.
<http://www.example.com/instance/main1> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.example.com/ontology/MainClass>.

In short,

• how do I join 3 nodes based on 2 join conditions?
• how can I skip or avoid a triple from being created?
• can I extend this to join 3+ nodes based on 2+ join conditions?