feat: add swap functionality for unit and modem assignments in vibration monitoring locations

backend/routers/project_locations.py

@@ -353,18 +353,18 @@ async def assign_unit_to_location(
             detail=f"Unit type '{unit.device_type}' does not match location type '{location.location_type}'",
         )
 
-    # Check if location already has an active assignment
+    # Check if location already has an active assignment (active = assigned_until IS NULL)
     existing_assignment = db.query(UnitAssignment).filter(
         and_(
             UnitAssignment.location_id == location_id,
-            UnitAssignment.status == "active",
+            UnitAssignment.assigned_until == None,
         )
     ).first()
 
     if existing_assignment:
         raise HTTPException(
             status_code=400,
-            detail=f"Location already has an active unit assignment ({existing_assignment.unit_id}). Unassign first.",
+            detail=f"Location already has an active unit assignment ({existing_assignment.unit_id}). Use swap to replace it.",
         )
 
     # Create new assignment
@@ -433,10 +433,120 @@ async def unassign_unit(
     return {"success": True, "message": "Unit unassigned successfully"}
 
 
+@router.post("/locations/{location_id}/swap")
+async def swap_unit_on_location(
+    project_id: str,
+    location_id: str,
+    request: Request,
+    db: Session = Depends(get_db),
+):
+    """
+    Swap the unit assigned to a vibration monitoring location.
+    Ends the current active assignment (if any), creates a new one,
+    and optionally updates modem pairing on the seismograph.
+    Works for first-time assignments too (no current assignment = just create).
+    """
+    location = db.query(MonitoringLocation).filter_by(
+        id=location_id,
+        project_id=project_id,
+    ).first()
+    if not location:
+        raise HTTPException(status_code=404, detail="Location not found")
+
+    form_data = await request.form()
+    unit_id = form_data.get("unit_id")
+    modem_id = form_data.get("modem_id") or None
+    notes = form_data.get("notes") or None
+
+    if not unit_id:
+        raise HTTPException(status_code=400, detail="unit_id is required")
+
+    # Validate new unit
+    unit = db.query(RosterUnit).filter_by(id=unit_id).first()
+    if not unit:
+        raise HTTPException(status_code=404, detail="Unit not found")
+
+    expected_device_type = "slm" if location.location_type == "sound" else "seismograph"
+    if unit.device_type != expected_device_type:
+        raise HTTPException(
+            status_code=400,
+            detail=f"Unit type '{unit.device_type}' does not match location type '{location.location_type}'",
+        )
+
+    # End current active assignment if one exists (active = assigned_until IS NULL)
+    current = db.query(UnitAssignment).filter(
+        and_(
+            UnitAssignment.location_id == location_id,
+            UnitAssignment.assigned_until == None,
+        )
+    ).first()
+    if current:
+        current.assigned_until = datetime.utcnow()
+        current.status = "completed"
+
+    # Create new assignment
+    new_assignment = UnitAssignment(
+        id=str(uuid.uuid4()),
+        unit_id=unit_id,
+        location_id=location_id,
+        project_id=project_id,
+        device_type=unit.device_type,
+        assigned_until=None,
+        status="active",
+        notes=notes,
+    )
+    db.add(new_assignment)
+
+    # Update modem pairing on the seismograph if modem provided
+    if modem_id:
+        modem = db.query(RosterUnit).filter_by(id=modem_id, device_type="modem").first()
+        if not modem:
+            raise HTTPException(status_code=404, detail=f"Modem '{modem_id}' not found")
+        unit.deployed_with_modem_id = modem_id
+        modem.deployed_with_unit_id = unit_id
+    else:
+        # Clear modem pairing if not provided
+        unit.deployed_with_modem_id = None
+
+    db.commit()
+
+    return JSONResponse({
+        "success": True,
+        "assignment_id": new_assignment.id,
+        "message": f"Unit '{unit_id}' assigned to '{location.name}'" + (f" with modem '{modem_id}'" if modem_id else ""),
+    })
+
+
 # ============================================================================
 # Available Units for Assignment
 # ============================================================================
 
+@router.get("/available-modems", response_class=JSONResponse)
+async def get_available_modems(
+    project_id: str,
+    db: Session = Depends(get_db),
+):
+    """
+    Get all deployed, non-retired modems for the modem assignment dropdown.
+    """
+    modems = db.query(RosterUnit).filter(
+        and_(
+            RosterUnit.device_type == "modem",
+            RosterUnit.deployed == True,
+            RosterUnit.retired == False,
+        )
+    ).order_by(RosterUnit.id).all()
+
+    return [
+        {
+            "id": m.id,
+            "hardware_model": m.hardware_model,
+            "ip_address": m.ip_address,
+        }
+        for m in modems
+    ]
+
+
 @router.get("/available-units", response_class=JSONResponse)
 async def get_available_units(
     project_id: str,