Relation list order

[Verteletsky]

And how can I actually update an existing list in the database, for example, the elements are interchanged? In the put method, I pass a list with the same elements, only the position in the list is changed, but still when the application is restarted, it issues the order that was originally.

###Code

@Override
    public void onItemMove(int oldPosition, int newPosition) {
//        Note targetNote = dataset.get(oldPosition);
//        Note note = new Note(targetNote);
//        dataset.remove(oldPosition);
//        dataset.add(newPosition, note);
//        notifyItemMoved(oldPosition, newPosition);

        if (oldPosition < newPosition) {
            for (int i = oldPosition; i < newPosition; i++) {
                Collections.swap(dataset, i, i + 1);
            }
        } else {
            for (int i = oldPosition; i > newPosition; i--) {
                Collections.swap(dataset, i, i - 1);
            }
        }

        notesBox.put(dataset);
//        dataset.clear();
//        dataset.addAll(noteQuery.find());
        notifyItemMoved(oldPosition, newPosition);
//        notifyDataSetChanged();
    }

[greenrobot]

Currently, relation lists do only have an internal order, which won't help in your case.

Do you have a property that might represent the order already.
Here's what you need to do:

1. define a property in your entity which represents the order, e.g. int order, and keep it up-to-date
2. Implement a Comparator based on that property
3. In your constructor(s) of the entity, call dataset.setComparator(myComparator)

This is clearly not ideal. For the future, I see some options:

1. Track the order in the DB core
2. Define the order based on properties, e.g. @Order("timestamp") List<Note> note;

Both approaches have up- and downsides. E.g. 1.) may introduce O(n) complexity on the data side for a remove(0). It also would be redundant if the logical order is bound to a property anyway. 2.) on the other hand would sometime enforce an artificial property

Will give this some more thought. Meanwhile let's collect some feedback on which option is preferred (1. list based vs. 2. property based).

[Verteletsky]

@greenrobot
Thank you very much for the answer! I added an individual orderPosition field for the entity, the field only works after moving the elements to the moveItem, when I create the object in the constructor, I assign orderPosition = id; (But it does not work) Objects are added to the database in this form:
DB

id = 0 | desc = A | date = ... | orderPosition = 0
id = 1 | desc = B | date = ... | orderPosition = 0
id = 2 | desc = C | date = ... | orderPosition = 0
id = 3 | desc = D | date = ... | orderPosition = 0

Constructor

@Id
long id;

String description;
Date date;
int orderPosition; // int because next time sort by this orderPosition

Task (String description, Date date) {
orderPosition = (int) id;
this.description = description;
this.date = date;
}

I do not understand what the problem is? And how in the designer to set the orderPosition = id of the created element (When creating a default orderPosition should be the same as the element's ID)
Thanks!!

[greenrobot]

Actually, "internal order" is by ID already. I thought you wanted it to be more flexible.

Did you actually persist the entities after assigning the orderPosition (because it is 0 in DB)?

[Verteletsky]

@greenrobot
ID for every item its unique. orderPosition for move UP and DOWN item, on recyclerView, this will help sort the items by dragging. When I run app, works .order(Task_.orderPosition) and show list, undestand me? Sry for my english.

[greenrobot]

Not sure if I understand. Maybe post more code?

[greenrobot-team]

Closing because this issue is missing details. Feel free to re-open with additional details.