[feature] Add GeoJSON API endpoint with device location data

[nemesifier]

Blocked by openwisp/openwisp-users#210.

We need to add a GeoJSON API endpoint (using django-rest-framework-gis) which returns geographic data which can be used to show a general device map.

• the URL should be something like /api/v1/device/geojson/
• it should fetch all the devices accessible to the current logged in user: if it's superuser it can see everything, otherwise the user can see only objects related to organizations they manage (we shall use the code which will be implemented in [feature] Add API classes to filter by organization and to check parent objects openwisp-users#210)
• we need to fetch locations which have devices deployed in them, and for each location we need to know how many devices there are, eg: Location.objects.filter(devicelocation__isnull=False).annotate(device_count=Count('devicelocation'))
• it should return at max 1000 items and paginate if more items are present, without breaking the GeoJSON format (see https://github.com/openwisp/django-rest-framework-gis/#geojsonpagination)
• each element should display all the information available in the fields of the Location model (including the location ID)

We also need another endpoint, which will be called whenever an element is clicked on the map, to fetch more information about a specific point.

• the URL could be something like /api/v1/location/{pk}/device/ (the user must have access to the organization of the location in order to see be able to access the URL)
• it should return a list of devices, each device should include the basic device and config details, and shall include the URL to admin edit page

[purhan]

@nemesisdesign Thanks for the detailed explanation. However, a few things are not clear to me:

• /api/v1/device/geojson/ should display devices or locations?
• Also since both device and location have their own organization field, which one should be the deciding factor?

[nemesifier]

@nemesisdesign Thanks for the detailed explanation. However, a few things are not clear to me:

• /api/v1/device/geojson/ should display devices or locations?

we need to fetch locations which have devices deployed in them, and for each location we need to know how many devices there are, eg:

Location.objects.filter(devicelocation__isnull=False).annotate(device_count=Count('devicelocation'))

• Also since both device and location have their own organization field, which one should be the deciding factor?

The two will always match, use the organizaiton field of the main model of the queryset, so if you follow the suggestion above, use the org field on the location model.