Implementing a constraint on when a vehicle has to return to a depot with a delivery

[choward47]

What language and solver does this apply to?
Routing

Describe the problem you are trying to solve.
I working on a vehicle routing problem and am trying to implement a constraint that forces a vehicle to return to the depot by the delivery time requirements for the nodes visited.

Below is what I am currently trying to use to implement this constraint. The code not included is framed from the example problems in the documentation. I'd be happy to elaborate more if needed/show other parts of the code.

for vehicle_id in range(data['num_vehicles']):
    index = routing.Start(vehicle_id)
    for location_idx, delivery_window in enumerate(data['delivery window']):
        if location_idx >= len(df['Record_id']):
            continue
        if delivery_window is not None:
            if routing.VehicleVar(index) == routing.VehicleVar(location_idx):
                idx = routing.End(vehicle_id)
                solver.Add(time_dimension.CumulVar(idx) <= delivery_window)

The solver doesn't seem to be a fan of how I am trying to implement the constraint. Currently, it is not returning a solution despite the model having solutions before this constraint. Based on the data I am testing it on, it should be possible provided the constraint is implemented properly.

To summarize, I am asking for help in understanding how to go about writing a constraint that requires a vehicle returns to the depot before the delivery deadlines of the orders it has picked up. I was wondering if there is a functionality that already handles this that I have missed, or if some functionality could be created to achieve this.

Any help would be greatly appreciated! I've tried a few different things but I am reaching my wit's end.

[Mizux]

routing.VehicleVar(index) == routing.VehicleVar(location_idx):
will generate a IntExpr which is always evaluated to true by the python interpreter, i.e. you can't use it in a if statement BUT you can add it to the solver.Add() call in this case it will be evaluated by the routing solver each time a solution is produced to verify the constraint is not violated.

idx = routing.End(vehicle_id)
same_vehicle = routing.VehicleVar(index) == routing.VehicleVar(location_idx)
routing.solver().Add(same_vehicle * time_dimension.CumulVar(idx) <= delivery_window)

If the constraint same_vehicle is evaluated to false aka 0 by the routing solver then the expression will become
0 * time_dimension.CumulVar(idx) <= delivery_window i.e. 0 <= delivery_window which is OK

If the constraint same_vehicle is evaluated to true aka 1 by the routing solver then the expression will become
1 * time_dimension.CumulVar(idx) <= delivery_window i.e. time_dimension.CumulVar(idx) <= delivery_window which is also OK