docs #35: use business objects as mappings

Closes #35

docs/mkdocs.yaml

@@ -85,6 +85,7 @@ nav:
   - Advanced User Guide: 
     - Parameters: parameters.md
     - Rule sets: rule_sets.md
+    - Use your business objects: business_objects.md
     - Custom conditions: custom_conditions.md
     - API Reference: api_reference.md
 extra_css:

docs/pages/business_objects.md

@@ -0,0 +1,99 @@
+You can use business objects (the instances of the classes from your business model) directly with the rule engine, without converting them to dictionaries.
+
+**Arta** processes mappings in the rules and actions to inject data into functions, as you saw in the [simple example](a_simple_example.md) of this documentation.
+
+## The problem with `dict` serialisation
+
+Let’s consider the following example, representing a `Car` with its `Engine` which itself has attributes.
+
+```python
+class Engine:
+    power: int
+    consumption: int  
+
+class Car:
+    engine: Engine
+```
+
+When following the regular usage of Arta, one would convert an instance of a `Car` to a dictionary with a sort of serializer. A candidate code for this might be:
+
+```python
+from typing import Any
+
+def serialize_car_to_dict(car: Car) -> dict[str, Any]:
+    return {
+        "engine": {
+            "power": car.engine.power,
+            "consumption": car.engine.consumption
+        }
+    }
+```
+
+!!! tip "When using Pydantic"
+
+    If you use Pydantic, you might directly use the `model_dump` function in order to represent your object as a dictionnary object.
+
+This way, you can write you conditions as follows:
+
+```yaml
+conditions:
+  HAS_LOWER_CONSUMPTION:
+    description: "Whether an engine as a low consumption".
+    validation_function: is_value_below_threshold
+    condition_parameters:
+      value: input.engine.power
+      threshold: 10
+```
+
+This is where the mapping is important: to go through the `engine` data and access the `power` attribute. We serialised the object in a custom code but there might be a better solution with less code…
+
+## Transform any business object to a mapping
+
+The solution to this issue is to use any business object as a mapping. In Python,  any object may behave as such by subclassing [`Mapping`](https://docs.python.org/3/library/collections.abc.html#collections.abc.Mapping) and implementing the abstract methods.
+
+As an example, we provide the following mixin:
+
+```python
+from collections.abc import Mapping, Iterator
+from typing import Any
+
+class ObjectToMappingMixin(Mapping):
+    def __iter__(self) -> Iterator[Any]:
+        return iter(vars(self))
+
+    def __len__(self) -> int:
+        return len(vars(self))
+
+    def __getitem__(self, key: str, /) -> Any:
+        return getattr(self, key)
+```
+
+Now, we can make our business objects subclass this mixin:
+
+```python
+class Engine(ObjectToMappingMixin):
+    power: int
+    consumption: int  
+    
+class Car(ObjectToMappingMixin):
+    engine: Engine
+```
+
+`Engine` and `Car` now behave as mapping and it’s possible to access the attributes of `Engine` from car using the dict’s `getitem` strategy, such as:
+
+```python
+engine = Engine(power=1, consumption= 12)
+car = Car(engine=engine)
+
+assert car["engine"]["power"] == 1
+assert car["engine"]["consumption"] == 12
+```
+
+Finaly, when using the `RulesEngine.apply_rules` method, there is not longer need to convert your business objects to dictionaries, you can directly use them like:
+
+```python
+from arta import RulesEngine
+
+eng = RulesEngine(config_path="/to/my/config/dir") 
+result = eng.apply_rules(input_data={"car": car})
+```