django: db model: allow empty data type field

An empty data type field represents an executable resource. The lwm2m
specification does not define a datatype for an executable resource e.g.
reboot, keeping the data type field empty comes closest to it.

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

server/django/sensordata/migrations/0002_alter_resourcetype_data_type.py

@@ -0,0 +1,17 @@
+# Generated by Django 5.0.6 on 2024-05-27 21:24
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("sensordata", "0001_initial"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="resourcetype",
+            name="data_type",
+            field=models.CharField(blank=True, max_length=50),
+        ),
+    ]

server/django/sensordata/models.py

@@ -10,7 +10,16 @@ class ResourceType(models.Model):
     object_id = models.IntegerField()
     resource_id = models.IntegerField()
     name = models.CharField(max_length=255)
-    data_type = models.CharField(max_length=50)  # 'int', 'float', 'str', 'bool'
+    # LwM2M data types:
+    # - string: UTF-8 encoded sequence of characters
+    # - integer: 16-bit signed integer
+    # - float: 64-bit IEEE 754 floating point
+    # - boolean: 0 or 1
+    # - opaque: sequence of binary data
+    # - time: POSIX time, number of s since 1970 in UTC (signed integer)
+    # - objlnk: link to another object instance
+    # - none (''): no data, used for executable resources
+    data_type = models.CharField(max_length=50, blank=True)
 
     class Meta:
         unique_together = ('object_id', 'resource_id')