Spool weight now counts upwards instead

The used filament weight is now kept track of instead of remaining weight. This adds robustness to measurement errors which could accumulate and then the filament usage wouldn't get tracked anymore once the weight hit 0. Now it will just keep going.

spoolman/api/v1/models.py

@@ -91,7 +91,15 @@ class Spool(BaseModel):
     first_used: Optional[datetime] = Field(description="First logged occurence of spool usage.")
     last_used: Optional[datetime] = Field(description="Last logged occurence of spool usage.")
     filament: Filament = Field(description="The filament type of this spool.")
-    weight: float = Field(ge=0, description="Remaining weight of filament on the spool.", example=500)
+    remaining_weight: Optional[float] = Field(
+        default=None,
+        ge=0,
+        description=(
+            "Estimated remaining weight of filament on the spool. Only set if the filament type has a weight set."
+        ),
+        example=500,
+    )
+    used_weight: float = Field(ge=0, description="Consumed weight of filament from the spool.", example=500)
     location: Optional[str] = Field(max_length=64, description="Where this spool can be found.", example="Shelf A")
     lot_nr: Optional[str] = Field(
         max_length=64,
@@ -107,13 +115,19 @@ class Spool(BaseModel):
     @staticmethod
     def from_db(item: models.Spool) -> "Spool":
         """Create a new Pydantic spool object from a database spool object."""
+        filament = Filament.from_db(item.filament)
+        remaining_weight: Optional[float] = None
+        if filament.weight is not None:
+            remaining_weight = max(filament.weight - item.used_weight, 0)
+
         return Spool(
             id=item.id,
             registered=item.registered,
             first_used=item.first_used,
             last_used=item.last_used,
-            filament=Filament.from_db(item.filament),
-            weight=item.weight,
+            filament=filament,
+            used_weight=item.used_weight,
+            remaining_weight=remaining_weight,
             location=item.location,
             lot_nr=item.lot_nr,
             comment=item.comment,