Restricting results of $expand parameter in SensorThings API

Question

I am attempting to truncate the results of an $expand parameter from SensorThingsAPI eg

http://example.org/v1.0/Things?$expand=Datastreams    

However, $top only restricts the trunk of the query (eg Things). Is there a way to truncate the results of the 'leaves' of an $expand ?

Answer 1

In this case, the server-side pagination should be controlling the 'leaves' of an $expand .

For example, if the service limit 100 entities for each response and the expanded entities (or the collection) have more than 100, the service will return the top 100 entities following a service-defined order. A @iot.nextLink will also be returned, so that the client know how to fetch the next 100 entities (ie, next page). Using the above query as an example, an example nextLink to retrieve the Datastreams will be

Datastreams@iot.nextLink:"http://URL_to_retrieve_the_next_page/"

You can use this OGC SensorThings sandbox to see an example return of $expand: http://scratchpad.sensorup.com/OGCSensorThings/v1.0/Datastreams?$expand=Observations

The following JSON shows an example response of the following query with $expand : http://scratchpad.sensorup.com/OGCSensorThings/v1.0/Datastreams?$expand=Observations :

 { "@iot.count": 1, "value": [{ "@iot.id": 8, "@iot.selfLink": "http://scratchpad.sensorup.com/OGCSensorThings/v1.0/Datastreams(8)", "description": "Daily Water level", "observationType": "http://www.opengis.net/def/observationType/OGC-OM/2.0/OM_Observation", "unitOfMeasurement": { "symbol": "m", "name": "meter", "definition": "https://en.wikipedia.org/wiki/Metre" }, "Observations@iot.nextLink": "http://scratchpad.sensorup.com/OGCSensorThings/v1.0/Datastreams(8)/Observations?$top=3&$skip=3", "Observations@iot.count": 1826, "Observations": [{ "@iot.id": 1835, "@iot.selfLink": "http://scratchpad.sensorup.com/OGCSensorThings/v1.0/Observations(1835)", "phenomenonTime": "2015-12-30T16:00:00.000Z", "result": "1375.44", "resultTime": null, "Datastream@iot.navigationLink": "http://scratchpad.sensorup.com/OGCSensorThings/v1.0/Observations(1835)/Datastream", "FeatureOfInterest@iot.navigationLink": "http://scratchpad.sensorup.com/OGCSensorThings/v1.0/Observations(1835)/FeatureOfInterest" }], "ObservedProperty@iot.navigationLink": "http://scratchpad.sensorup.com/OGCSensorThings/v1.0/Datastreams(8)/ObservedProperty", "Sensor@iot.navigationLink": "http://scratchpad.sensorup.com/OGCSensorThings/v1.0/Datastreams(8)/Sensor", "Thing@iot.navigationLink": "http://scratchpad.sensorup.com/OGCSensorThings/v1.0/Datastreams(8)/Thing" },{},{}] }