Update of entity fields required

[priyapappachan]

Foreign key fields such as sensor, device and user are not defined as mandatory for the subscription entity. The implementation of subscription is not clear as it is not known whether the user can use the subscriptions for retrieving data now.

Also it is not sure why serial and table id field is used for sensor. Only name field is mandatory for the device and sensor creation. While creating device and sensor, user can select user id. Does it mean user can create device and sensor for other users?