How can I create a device group in Cumulocity with REST

I am writing a service for forwarding our sensor data to Cumulocity platform. I designed the structure so that all the data is first sent to our main tenant and then device data for each customer is forwarded to corresponding tenants with Data Broker.

I can group devices manually and forward by group but I don't want to deal with it every time a new device is added.Sensor data contains customer name. Probably I can add customer name to device properties (like device_type) and use that as a filter but I want to avoid that if possible. So I thought, when a sensor data hits my endpoint, I do something like this:

Look if the device exists in the database.If it exists just publish measurement data.
If not look at the Group database(Just a key-value store containing customer names and managed Object Ids of the corresponding groups from Cumulocity) to see if there is entry for the customer.
If not add an entry and create a group with customer name.Then add the device to the group.
If it exists, just add the device to the group.

I tried adding devices to groups with REST and it works. The problem is I cannot create a device group with REST.

I looked at the Cumulocity API example requests and tried to tweak them a little.

I tried sending POST request to {{url}}/inventory/managedObjects as:

{
    "name": "TestDeviceGroup",
    "c8y_IsDeviceGroup": {}
}

It returns 201 created but I cannot see the group. When I try to get collection of groups I see it there as a managed object with a new Id.

I tried to add a new device to this object as a child asset.

{{url}}/inventory/managedObjects/{{GroupId}}/childAssets

{
     "managedObject": "id:{{deviceId}}"
}

It returns 201 created but device GROUP is not updated.

If I recreate this scenario with a group created with UI and its Id everything works fine and device is added to the group.

As I understand what I create is not a legit device group and that is the main problem. So my question is How can I create device group with REST?

Solution

To create the group you were already on the right track you are just missing the correct type. Create your group like this:

POST /inventory/managedObjects
{
    "name": "TestDeviceGroup",
    "type": "c8y_DeviceGroup",
    "c8y_IsDeviceGroup": {}
}

To assign your device to a particular group you can EITHER assign an existing device to an existing group like this (replace the placeholders in <> with your IDs):

POST /inventory/managedObjects/<groupId>/childAssets
{
  "managedObject": {"id":"<deviceId>"}
}

Or you can directly create a new device into an existing group like this:

POST /inventory/managedObjects/<groupId>/childAssets

Content-Type: application/vnd.com.nsn.cumulocity.managedobject+json

{
    "name": "my device",
    "c8y_IsDevice": {}
}