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• Updating in a JavaScript app

Updating in a JavaScript app

Comunica SPARQL (@comunica/actor-init-sparql) allow you to initiate queries to update data in a certain store. In this guide, we will build upon the guide on querying in a JavaScript app, and show how you can not only read, but also update data.

1. Creating a new query engine and store

The easiest way to create an engine and store is as follows:

const newEngine = require('@comunica/actor-init-sparql').newEngine;
const N3 = require('n3');

const myEngine = newEngine();

const store = new N3.Store();

We make use of the Store from N3.js for these examples.

2. Executing INSERT DATA queries

Once you engine has been created, you can use it to execute any SPARQL Update query, such as a INSERT DATA query:

// Initiate the update
const result = await myEngine.query(`
  PREFIX dc: <http://purl.org/dc/elements/1.1/>
  INSERT DATA
  { 
    <http://example/book1> dc:title "A new book" ;
                           dc:creator "A.N.Other" .
  }`, {
  sources: [ store ],
});

// Wait for the update to complete
await result.updateResult;

// Prints '2' => the store is updated
console.log(store.size);

When you obtain the result of the query method, this means that your query has been initiated. In order to wait until the update operation has been completed, you need to await the updateResult.

3. Executing DELETE/INSERT WHERE queries

DELETE/INSERT WHERE queries allow you to delete and insert new quads, based on quads that are already available:

// Insert initial data
await (await myEngine.query(`
  PREFIX foaf: <http://xmlns.com/foaf/0.1/>
  INSERT DATA
  { 
    <http://example/president25> foaf:givenName "Bill" .
    <http://example/president25> foaf:familyName "McKinley" .
    <http://example/president27> foaf:givenName "Bill" .
    <http://example/president27> foaf:familyName "Taft" .
    <http://example/president42> foaf:givenName "Bill" .
    <http://example/president42> foaf:familyName "Clinton" .
  }`, {
  sources: [ store ],
})).updateResult;

// Rename all occurrences of "Bill" to "William"
await (await myEngine.query(`
  PREFIX foaf:  <http://xmlns.com/foaf/0.1/>
  DELETE { ?person foaf:givenName 'Bill' }
  INSERT { ?person foaf:givenName 'William' }
  WHERE
  {
    ?person foaf:givenName 'Bill' 
  }`, {
  sources: [ store ],
})).updateResult;

4. Configure a custom destination

By default, update queries will modify data within the given source. In some cases, you may want to direct changes to another place. For example, if you have multiple sources, but you want to direct all changes to a single source.

This can be done by passing a destination into the query context:

// Insert friends based on common friends from Ruben's
await (await myEngine.query(`
  PREFIX foaf:  <http://xmlns.com/foaf/0.1/>
  INSERT
  {
    <http://example/person> foaf:knows ?friend
  }
  WHERE
  {
    <https://www.rubensworks.net/#me> foaf:knows ?friend .
    <https://ruben.verborgh.org/profile/#me> foaf:knows ?friend . 
  }`, {
  sources: [
    'https://www.rubensworks.net/',
    'https://ruben.verborgh.org/profile/',
  ],
  destination: store,
})).updateResult;