Dependency fields fromTask/toTask are not serialized

[pmiklashevych]

https://www.bryntum.com/forum/viewtopic.php?p=74906#p74906

To reproduce:
Create Gantt/examples/advanced/data/sync.json

{
    "success" : true
}

Edit Gantt/examples/advanced/app.js

const project = new ProjectModel({

    // Let the Project know we want to use our own Task model with custom fields / methods
    taskModelClass : Task,
    transport      : {
        load : {
            url : '../_datasets/launch-saas.json'
        },
        sync : {
            url : 'data/sync.json'
        }
    }
});

Go to http://lh/bryntum-suite/gantt/examples/advanced/
Create a new dependency between Unit tests and UI unit tests / individual screens
Call gantt.project.sync()

See Network: fromTask/toTask should be and ID

=====================
If set a breakpoint into AbstractCrudManagerMixin.getCrudStoreChanges and step into prepareAdded

        if (store.id === 'dependencies') {
            debugger
        }
        if (added.length) added = this.prepareAdded(added, storeDescriptor);

you can see that record.persistableData returns that data. persistableData loops through the defined fields.

In Gantt/examples/_datasets/launch-saas.json the dependencies data is specified as:

    "dependencies" : {
        "rows" : [
            {
                "id"       : 1,
                "fromTask" : 11,
                "toTask"   : 15,
                "lag"      : 2
            },

So fromTask/toTask are specified and the fields are automatically exposed. If you change to fromEvent/toEvent (which are also valid fields), it will work. The request will contain fromEvent/toEvent with serialized ID value. Because they are defined in SchedulingEngine/lib/Engine/data/model/DependencyMixin.ts

=====================
Possible solution:

Add to SchedulingEngine/lib/Engine/data/model/DependencyMixin.ts fromTask/toTask definitions

@generic_field(
    {
        bucket : 'outgoingDeps',
        resolver : function (id : ModelId) { return this.getEventById(id) },
        modelFieldConfig : {
            persist   : true,
            serialize : event => event.id
        },
    },
    ModelReferenceField
)
fromTask           : HasDependencies

@generic_field(
    {
        bucket : 'incomingDeps',
        resolver : function (id : ModelId) { return this.getEventById(id) },
        modelFieldConfig : {
            persist   : true,
            serialize : event => event.id
        },
    },
    ModelReferenceField
)
toTask             : HasDependencies