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RML.io

Generate RDF from an XML file

heypieter profile image Pieter Heyvaert Originally published at rml.io ・8 min read

Table of Contents

  1. Before we start the tutorial
  2. Example
  3. What rules are needed
  4. How to start a document with RML rules
  5. What data to use
  6. How to generate subjects
  7. How to generate predicates and objects
  8. Complete Turtle document with RML rules
  9. Wrapping up
  10. More information

1 Before we start the tutorial

1.1 What you learn

At the end of the tutorial you will be able to generate RDF from an XML file using RML rules.

What you need

We assume that you understand

  • RDF
  • XML
  • vocabularies and ontologies, such as classes, properties, and datatypes

How you use the tutorial

There are two ways to complete this tutorial: you read the explanations and either

  • Read the examples.
  • Try out the examples yourself by writing and executing RML rules on your computer.

For the second option you need a tool that executes RML rules. Suggestions are the RMLMapper and the RMLStreamer.

2 Example

Consider the following XML file called "characters.xml":

<?xml version="1.0" encoding="UTF-8"?>

<characters>
  <character id="0">
    <firstname>Ash</firstname>
    <lastname>Ketchum</lastname>
    <hair>black</hair>
  </character>
  <character id="1">
    <firstname>Misty</firstname>
    <hair>orange</hair>
  </character>
</characters>

It contains the information about two different characters. The id, first name, last name, and hair color are included. The latter two are optional. We want to annotate every character and generate the corresponding RDF triples.

For example, consider the character described by the first XML element:

<character id="0">
  <firstname>Ash</firstname>
  <lastname>Ketchum</lastname>
  <hair>black</hair>
</character>

We want to generate the corresponding RDF triples for this element:

@prefix schema: <http://schema.org/> .
@prefix dbo: <http://dbpedia.org/ontology/> .
@prefix character: <http://example.org/character/> .

character:0 a schema:Person;
  schema:givenName "Ash";
  schema:lastName "Ketchum";
  dbo:hairColor "black".

In the following sections we explain

  1. what rules you need to generate these triples, and
  2. how you write them using RML.

3 What rules are needed

Two sets of rules are needed:

  • rules that describe the XML file
  • rules that define how the RDF terms are generated from the XML file, and how these terms are used to generate triples.

In our example we need rules that define that:

  • The IRI representing a character is generated by concatenating http://example.org/character/ with the character's id.
  • This IRI is used as subject of the triples.
  • A character is annotated with the class schema:Person.
  • The first name is annotated with the property schema:givenName.
  • The last name is annotated with the property schema:lastName.
  • The hair color is annotated with the property dbo:hairColor.

4 How to start a document with RML rules

We write the RML rules in a Turtle document. RML rules are RDF themselves.

We add the following prefixes:

Prefix Description
rml RML ontology
rr The R2RML ontology, which is extended by RML
ql The Query Language vocabulary, which is used together with RML
rdf The RDF Concepts Vocabulary
empty The prefix used for our RML rules
schema The schema.org vocabulary
dbo The DBpedia ontology

The last two are added because they are used for the classes and properties.

The prefixes are added in Turtle like this:

@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 rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix : <http://example.org/rules/> .
@prefix schema: <http://schema.org/> .
@prefix dbo: <http://dbpedia.org/ontology/> .

5 What data to use

In our example the data of the characters is stored in an XML file. We add the following RML rules that define what XML file is used and how we iterate over the elements in it:

:TriplesMap a rr:TriplesMap;
  rml:logicalSource [
    rml:source "characters.xml";
    rml:referenceFormulation ql:XPath;
    rml:iterator "/characters/character"
  ].

The different rules work as follows:

  • :TriplesMap a rr:TriplesMap; defines the Triples Map that groups all rules for the characters.
  • The blank node of :TriplesMap rml:logicalSource [ ... ] contains all rules about the XML file. The class of the blank node is implicitly of the class rml:LogicalSource.
  • [rml:source "characters.xml"] says that we access the XML file characters.xml.
  • [rml:referenceFormulation ql:XPath] says that we use XPath the access the data in the XML file.
  • [rml:iterator "/characters/character"] says that we iterate over all elements that match the XPath expression /characters/character.

6 How to generate subjects

We add the following rules that define how the subject IRI of a character is generated:

:TriplesMap rr:subjectMap [
  rr:template "http://example.org/character/{@id}"
].

The different rules work as follows:

  • :TriplesMap rr:subjectMap [ ... ] contains all the rules about the subject of a triple. The class of the blank node is implicitly of the class rr:SubjectMap.
  • [rr:template "http://example.org/character/{@id}"] says that the IRI of the subject is generated by concatenating http://example.org/character/ with the attribute id of the character element.

7 How to generate predicates and objects

7.1 How to annotate with a class

In our example we need to annotate every character with the class schema:Person. We add the following RML rules:

:TriplesMap rr:predicateObjectMap [
  rr:predicate rdf:type;
  rr:constant schema:Person
].

The different rules work as follows:

  • :TriplesMap rr:predicateObjectMap [ ... ] contains all the rules about a specific predicate of a triple. The class of the blank node is implicitly of the class rr:PredicateObjectMap.
  • [rr:predicate rdf:type] says that we use the predicate rdf:type.
  • [rr:objectMap [ ... ]] contains all the rules about the object of a triple. The class of the blank node is implicitly of the class rr:ObjectMap.
  • [rr:constant schema:Person] says that the object of the triple is schema:Person for every character.

Putting all rules we have so far together results in

@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 rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix : <http://example.org/rules/> .
@prefix schema: <http://schema.org/> .
@prefix dbo: <http://dbpedia.org/ontology/> .

:TriplesMap a rr:TriplesMap;
  rml:logicalSource [
    rml:source "characters.xml";
    rml:referenceFormulation ql:XPath;
    rml:iterator "/characters/character"
  ].

:TriplesMap rr:subjectMap [
  rr:template "http://example.org/character/{@id}"
].

:TriplesMap rr:predicateObjectMap [
  rr:predicate rdf:type;
  rr:objectMap [
    rr:constant schema:Person
  ]
].

You can download the Turtle file here. If we execute these rules, the following triples are generated:

@prefix schema: <http://schema.org/> .

<http://example.org/character/0> a schema:Person .
<http://example.org/character/1> a schema:Person .

Two triples are generated: one for each character. There is a unique subject IRI for each character and each character is annotated with the class schema:Person.

7.2 How to annotate with a property

In our example we need to annotate the values in the tags firstname
with the property schema:givenName. We add the following rules:

:TriplesMap rr:predicateObjectMap [
  rr:predicate schema:givenName;
  rr:objectMap [
    rml:reference "firstname"
  ]
].

The rules are different from when annotating with a class: rml:reference is used instead of rr:constant because the object is not the same for every character. More specific, [rml:reference "firstname"] says that the data in the tag firstname is used for the object.

Putting all rules we have so far together results in

@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 rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix : <http://example.org/rules/> .
@prefix schema: <http://schema.org/> .
@prefix dbo: <http://dbpedia.org/ontology/> .

:TriplesMap a rr:TriplesMap;
  rml:logicalSource [
    rml:source "characters.xml";
    rml:referenceFormulation ql:XPath;
    rml:iterator "/characters/character"
  ].

:TriplesMap rr:subjectMap [
  rr:template "http://example.org/character/{@id}"
].

:TriplesMap rr:predicateObjectMap [
  rr:predicate rdf:type;
  rr:objectMap [
   rr:constant schema:Person
 ]
].

:TriplesMap rr:predicateObjectMap [
  rr:predicate schema:givenName;
  rr:objectMap [
    rml:reference "firstname"
  ]
].

You can download the Turtle file here. If we execute these rules, the following triples are generated:

@prefix schema: <http://schema.org/> .

<http://example.org/character/0> a schema:Person;
  schema:givenName "Ash" .

<http://example.org/character/1> a schema:Person;
  schema:givenName "Misty" .

Two triples are added: one for the first name of each character.

We add the following rules to annotate the last name and the hair color in the same way as the first name:

:TriplesMap rr:predicateObjectMap [
  rr:predicate schema:lastName;
  rr:objectMap [
    rml:reference "lastname"
  ]
].

:TriplesMap rr:predicateObjectMap [
  rr:predicate dbo:hairColor;
  rr:objectMap [
    rml:reference "hair"
  ]
].

8 Complete Turtle document with RML rules

The complete Turtle document with RML rules is

@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 rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix : <http://example.org/rules/> .
@prefix schema: <http://schema.org/> .
@prefix dbo: <http://dbpedia.org/ontology/> .

:TriplesMap a rr:TriplesMap;
  rml:logicalSource [
    rml:source "characters.xml";
    rml:referenceFormulation ql:XPath;
    rml:iterator "/characters/character"
  ].

:TriplesMap rr:subjectMap [
  rr:template "http://example.org/character/{@id}"
].

:TriplesMap rr:predicateObjectMap [
  rr:predicate rdf:type;
  rr:objectMap [
   rr:constant schema:Person
 ]
].

:TriplesMap rr:predicateObjectMap [
  rr:predicate schema:givenName;
  rr:objectMap [
    rml:reference "firstname"
  ]
].

:TriplesMap rr:predicateObjectMap [
  rr:predicate schema:lastName;
  rr:objectMap [
    rml:reference "lastname"
  ]
].

:TriplesMap rr:predicateObjectMap [
  rr:predicate dbo:hairColor;
  rr:objectMap [
    rml:reference "hair"
  ]
].

You can download the Turtle file here. If we execute these rules, the final triples are generated:

@prefix dbo: <http://dbpedia.org/ontology/> .
@prefix schema: <http://schema.org/> .

<http://example.org/character/0> a schema:Person;
  dbo:hairColor "black";
  schema:givenName "Ash";
  schema:lastName "Ketchum" .

<http://example.org/character/1> a schema:Person;
  dbo:hairColor "orange";
  schema:givenName "Misty" .

9 Wrapping up

Congratulations! You have created your own RML rules that generate RDF from data in an XML file. Nice work! We hope you now feel like you have a decent grasp on how RML rules work.

10 More information

You can find more information about RML in its specification. There is also a human readable text-based representation available for RML rules called YARRRML. It is a subset of YAML, a widely used data serialization language designed to be human-friendly.

If you have any questions or remarks, don't hesitate to contact me via email or via Twitter.

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