Use a flat list of measurement metadata

Mimicing the structure of grouped measurements as in `"Foo": {"unit: "u", "Temperature": {"unit": "°C"}}`
leads to confusion as units are assigned to `"Foo":20` values as well as `"Foo" : { "Temperature": 20 }`.

Using a flat structure is less confusing as units for `"Foo"` and `"Foo.Temperature"`
are now defined using two independent property sets.

Signed-off-by: Didier Wenzek <[email protected]>

Author: didier-wenzek

crates/extensions/c8y_mapper_ext/src/json.rs

@@ -93,15 +93,20 @@ impl Units {
     }
 
     pub fn set_unit(&mut self, measurement: String, meta: serde_json::Value) {
-        if let Some(unit) = meta.get("unit") {
-            // "Temperature": {"unit": "°C"},
-            if let serde_json::Value::String(unit_name) = unit {
-                self.units.insert(measurement, unit_name.to_owned());
+        if let Some(serde_json::Value::String(unit)) = meta.get("unit") {
+            match measurement.split_once('.') {
+                None => {
+                    // "Temperature": {"unit": "°C"},
+                    self.units.insert(measurement, unit.to_owned());
+                }
+                Some((group, measurement)) => {
+                    // "Climate.Temperature": {"unit": "°C"},
+                    self.group_units
+                        .entry(group.to_owned())
+                        .or_default()
+                        .set_unit(measurement.to_owned(), meta);
+                }
             }
-        } else {
-            // "Climate": { "Temperature": {"unit": "°C"}, "Humidity": {"unit": "%RH"} }
-            let group = measurement;
-            self.set_group_units(group, meta);
         }
     }
 
@@ -328,15 +333,11 @@ mod tests {
 
         let units = r#"
     {
-      "Climate":{
-        "Temperature": {"unit": "°C"},
-        "Humidity": {"unit": "%RH"}
-      },
-      "Acceleration":{
-        "X-Axis": {"unit": "m/s²"},
-        "Y-Axis": {"unit": "m/s²"},
-        "Z-Axis": {"unit": "m/s²"}
-      }
+      "Climate.Temperature":{"unit": "°C"},
+      "Climate.Humidity": {"unit": "%RH"},
+      "Acceleration.X-Axis": {"unit": "m/s²"},
+      "Acceleration.Y-Axis": {"unit": "m/s²"},
+      "Acceleration.Z-Axis": {"unit": "m/s²"}
     }"#;
 
         let expected_output = r#"

docs/src/references/mappers/c8y-mapper.md

@@ -396,27 +396,24 @@ graph LR
   te --/--- entity_id["<entity id>"] --/--- m --/--- measurement_type["<measurement type>"]  --/--- meta
 ```
 
-The idea is to describe in a single metadata message all the units of the measurements published under the measurement topic.
-This message uses the same shape as the measurements and is published as retained on the meta topic of the measurement topic.
+The idea is to describe in a single retained message all the units of the measurements published under the measurement topic.
 
 ```shell
 tedge mqtt pub -r 'te/device/main///m//meta' '{
-    "Climate":{
-        "Temperature": {"unit": "°C"},
-        "Humidity": {"unit": "%RH"}
-    },
-    "Acceleration":{
-        "X-Axis": {"unit": "m/s²"},
-        "Y-Axis": {"unit": "m/s²"},
-        "Z-Axis": {"unit": "m/s²"}
-    }
+      "Pressure":{"unit": "bar"},
+      "Climate.Temperature":{"unit": "°C"},
+      "Climate.Humidity": {"unit": "%RH"},
+      "Acceleration.X-Axis": {"unit": "m/s²"},
+      "Acceleration.Y-Axis": {"unit": "m/s²"},
+      "Acceleration.Z-Axis": {"unit": "m/s²"}
 }'
 ```
 
 The measurements published to the topic for measurements of the same type:
 
 ```shell
 tedge mqtt pub 'te/device/main///m/' '{
+    "Pressure": 1.013,
     "Climate":{
         "Temperature":23.4,
         "Humidity":95.0
@@ -434,6 +431,9 @@ are then forwarded to C8Y with their units.
 ```json
 {
   "type": "ThinEdgeMeasurement",
+  "Pressure": {
+    "Pressure": {"value":1.013,"unit":"bar"}
+  },
   "Climate": {
     "Temperature": {"value":23.4,"unit":"°C"},
     "Humidity":{"value":95,"unit":"%RH"}
@@ -448,10 +448,12 @@ are then forwarded to C8Y with their units.
 ```
 
 - A message received on `te/device/main///m/<type>` uses the units defined on `te/device/main///m/<type>/meta`, if any.
+- `"Temperature"`, `"Climate": { "Temperature" }` and `"Engine": { "Temperature" }` values can be given different units.
+  - This is done using a flatten structure,
+    assigning metadata respectively to `"Temperature"`, `"Climate.Temperature"` and `"Engine.Temperature"`
 - If the unit for a measurement is unknown, the measurement value is simply sent with no unit.
 - Other metadata such as the precision or the min and max values can be attached to a measurement.
   However, these are ignored by the Cumulocity mapper.
-- `"Temperature"`, `"Climate": { "Temperature" }` and `"Engine": { "Temperature" }` can be given different units.
 - Units and all measurement metadata can be cleared by publishing an empty retained message on `te/device/main///m/<type>/meta`.
 
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