doc: add documentation for database model

Explain the database model, how it is related to generic LwM2M
resources. Furthermore, explain how an application can create custom
aggregation events.

Signed-off-by: Jonas Remmert <jremmert@gmx.net>

doc/source/documentation.rst

@@ -84,20 +84,63 @@ following command:
   OK
   Destroying test database for alias 'default'...
 
-Entity Relationship Diagram (ERD)
-.................................
+Database Model
+~~~~~~~~~~~~~~
+
+The database model is the core of the Django server. It aims to store
+information according to the LwM2M resource model. The advantage is that data
+can be stored in a generic way and the server can be extended with new
+resources without changing the database schema.
+
+The server application logic only has to handle higher level events. Those
+events are situations where multiple resources are associated (e.g.
+Temperature, Pressure, Humidity, Acceleration). Those multiple resources are
+linked in the event, together with a timestamp. The event itself is represented
+by the database model in a generic way, several custom event types can be
+created by the application logic.
+>>>>>>> 3ccaacc (doc: add documentation for database model)
 
 An Entity Relationship Diagram (ERD) is a visual representation of the database
-schema. It is be automatically generated from the Django models.
-
-``sensordata`` is the Django app that contains the application logic.
+schema. It is automatically generated from the Django models. ``sensordata`` is
+the Django app that contains the application logic.
 
 .. figure:: images/erd.svg
 
   Entity Relationship Diagram generated from Django models
 
+**Device:** Represents IoT devices using the LwM2M protocol in the network,
+identifiable by a universally unique ID alongside a human-readable name.
+
+**ResourceType:** Defines resource data points comprehensively, annotating each
+with a unique object-resource ID combination, a descriptive name, and
+specifying the expected data type.
+
+**Resource:** Captures individual data values from IoT devices, annotated with
+timestamps, applicable data types, and linked to both the device and resource
+type for which the data pertains.
+
+**Event:** Serves as a collection point for significant occurrences reported by
+devices, including composite events defined by enclosing object IDs. The server
+application logic has to generate events based on matching timestamps or
+received composite notifications from devices. Although individual resources
+within an event may have different timestamps, the event itself encapsulates a
+single timestamp.
+
+**EventResource:** Acts as a junction table forming a many-to-many relationship
+between events and their constituent resources, enabling flexible association
+without direct modification to the core events or resources tables.
+
+**DeviceOperation:** Represents actionable commands or processes targeted at
+devices, tracking the operation type, status, and scheduling through
+timestamps, also detailing the transmission attempts and last action.
+
+**Firmware:** Stores metadata about firmware binaries that are available for
+devices to download and install. Each record includes a version identifier, the
+name of the file, a URL from where the device can retrieve the firmware, and
+timestamps for tracking when each firmware record was created and last updated.
+
 Leshan LwM2M
-------------
+............
 
 For testing the Lwm2m server can run locally, without the need of a docker
 container: