Grails - Can't add child record to parent

Trying to follow the example shown here I am trying to create a record which links to a parent record.

In my case I have two classes: Sensor, and Readings. I can create Sensors without any issues, but no matter how I try to create readings I seem to fail :(

I've been spinning my wheels for long enough, I'm throwing in the towel and hoping someone can spot my silly mistake(s).

One more thing - I want to post the data using JSON. But through the debugging process I'm not even looking at the JSON values, I've hard coded them and it still doesn't work.

ReadingsController.groovy

package grailshelloworld

import grails.converters.JSON
import groovy.json.JsonSlurper

class ReadingsController {

    def scaffold=Readings

        def save = {

                def slurper = new JsonSlurper()
                def result = slurper.parseText(request.reader.text)

                def s = new Sensor (sensorid: "SID", sensorname: "name", sensordescription: "description")
                        .addToReadings(reading: "blah")
                        .save()

                render ([ok: false] as JSON)


    }
}

sensor.groovy

package grailshelloworld

class Sensor {

    String sensorid
    String sensorname
    String sensordescription

    static hasMany = [readings: Readings]

    static constraints = {
        sensorid blank:false, nullable: false 
        sensorname blank:false, nullable: false
    }
}

Readings.grooovy

package grailshelloworld
import java.util.Formatter.DateTime;
class Readings {
    String reading
    static belongsTo = [sensor: Sensor]
}

Current error: argument type mismatch...

<dt>Class</dt><dd>java.lang.IllegalArgumentException</dd><dt>Message</dt><dd>argument type mismatch</dd></dl><h2>Around line 15 of <span class="filename">grails-app/controllers/grailshelloworld/ReadingsController.groovy</span></h2>
<pre class="snippet"><code class="line"><span class="lineNumber">12:</span>     def slurper = new JsonSlurper()</code><code class="line"><span class="lineNumber">13:</span>        def result = slurper.parseText(request.reader.text)</code><code class="line"><span class="lineNumber">14:</span></code><code class="line error"><span class="lineNumber">15:</span>     def s = new Sensor (sensorid: &quot;SID&quot;, sensorname: &quot;name&quot;, sensordescription: &quot;description&quot;)</code><code class="line"><span class="lineNumber">16:</span>           .addToReadings(reading: &quot;blah&quot;)</code><code class="line"><span class="lineNumber">17:</span>          .save()</code><code class="line"><span class="lineNumber">18:</span></code></pre><h2>Around line 195 of <span class="filename">PageFragmentCachingFilter.java</span></h2>

Have you tried it by explicitly creating a new Readings?

def s = new Sensor (sensorid: "SID", sensorname: "name", sensordescription: "description")
                    .addToReadings(new Readings(reading: 'blah'))
                    .save()

The error is saying "around line 15" which is the start to the def s = ... statement.

I know the docs say it can be done the way you are attempting - but it's worth a try.