Add support for variables in export SPARQL queries

The variables will be replaced at runtime with the values of the query
params provided in the request.

Author: erikap

README.md

@@ -40,4 +40,26 @@ services:
       - database:database
 ```
 
-Note: if you extend the `semtech/mu-export-js-template` with an `export.js` file instead of mounting a volume in `/config`, the `ONBUILD` commands of the `semtech/mu-javascript-template` will not be executed in your image since `ONBUILD` is only executed one level deep.
+Note: if you extend the `semtech/mu-export-js-template` with an `export.js` file instead of mounting a volume in `/config`, the `ONBUILD` commands of the `semtech/mu-javascript-template` will not be executed in your image since `ONBUILD` is only executed one level deep.
+
+## Using variables in the export query
+You can also define variables in your SPARQL export query which will be replaced with query param values at runtime. Therefore, we use [ES6's tagged template literals](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals#Tagged_template_literals).
+
+Import the `template` tag function from `/app/template.js` in your `export.js` and apply the tag function on your query. You can then define variables in your SPARQL query using `${'myVariable'}`. At runtime the variable will be replaced with the value provided in the `myVariable` query param of the request.
+
+An example `export.js` file may look as follows:
+
+```javascript
+export default [
+    {
+        path: '/example',
+        format: 'text/csv',
+        query: `SELECT * FROM <http://mu.semte.ch/application> WHERE { ?s a ${'class'} } LIMIT 100`,
+        file: 'export-example.csv'
+    },
+    ...
+];
+
+```
+
+A GET request on `/example?class=<http://xmlns.com/foaf/0.1/Person>` will export the first 100 URIs of type `foaf:Person`.

app.js

@@ -14,7 +14,8 @@ queries.map(function(config) {
             },
             qs: {
                 format: config.format,
-                query: config.query
+                // replace placeholders in query with query params if it's a template
+                query: typeof(config.query) === 'string' ? config.query : config.query(req.query)
             }
         };
 

template.js

@@ -0,0 +1,11 @@
+export default function template(strings, ...keys) {
+    return (function(...values) {
+        var dict = values[values.length - 1] || {};
+        var result = [strings[0]];
+        keys.forEach(function(key, i) {
+            var value = Number.isInteger(key) ? values[key] : dict[key];
+            result.push(value, strings[i + 1]);
+        });
+        return result.join('');
+    });
+}