[Python] Setting a predecessor constraint in timefold python

I am trying to implement a predecessor constraint similar to the Job Scheduling example given in java.

But I struggle with the ordering constraint definition to consider predecessors.

I have defined my time slots simply as ordered integers :

@dataclass
class Timeslot:
slot : int


And my operations as planning entities with few boolean operations and predecessors as a list of other operations id :

@planning_entity
@dataclass
class Operation:
id: Annotated[int, PlanningId]
name: str
predecessors: list[int]
timeslot: Annotated[Timeslot, PlanningVariable] = field(default=None)

def isPred(self,other):
return other.id in self.predecessors

def isAfter(self,other):
return self.timeslot.slot < other.timeslot.slot


And then my constraint as :

def precondition_conflict(constraint_factory: ConstraintFactory) -> Constraint:
# Respect order constraints
return (
constraint_factory.for_each_unique_pair(Operation)
.filter(lambda op1, op2 : op1.isPred(op2))
.filter(lambda op1, op2 : op1.isAfter(op2))
.penalize(HardSoftScore.ONE_HARD)
.as_constraint("Order conflict")
)


Then I instanciate my problem :

operations.append(Operation(1,"OP N°1 : Bake the cake",[3]))
operations.append(Operation(2,"OP N°2 : Enjoy your meal",[4]))
operations.append(Operation(3,"OP N°3 : Mix flour, eggs and whatever they say in the recipe",[]))
operations.append(Operation(4,"OP N°4 : take it out of the oven",[1]))


But the solver keeps giving me solutions where the operations order is incorrect, with 0 hard constraint violated. For instance :

INFO:timefold.solver:Solving ended: time spent (30056), best score (0hard/0soft), move evaluation speed (219895/sec), phase total (2), environment mode (PHASE_ASSERT), move thread count (NONE).
INFO:app:+------------------+------------------+
INFO:app:|0 |OP N°4 : take it out of the oven|
INFO:app:+------------------+------------------+
INFO:app:|1 |OP N°2 : Enjoy your meal|
INFO:app:+------------------+------------------+
INFO:app:|2 |OP N°3 : Mix flour, eggs and whatever they say in the recipe|
INFO:app:+------------------+------------------+
INFO:app:|3 |OP N°1 : Bake the cake|
INFO:app:+------------------+------------------+


Where obviously I expected 3 -> 1 -> 4 -> 2

Where did I do it wrong?

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