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# Recover from rejected changes

Get an editor working again after [`apply`](<https://docs.dhtmlx.com/planning-engine/rust/api/core/project/project.md#apply>) rejects part of a change set, without restarting the app or losing the user's edits.

When `apply` rejects an operation, the project sets `needs_resync`. From then on, `solve(None, ..)`, `apply` and `level` return [`ProjectError::NeedsResync`](<https://docs.dhtmlx.com/planning-engine/rust/api/core/errors/project-error.md>), and reads that need a baseline (`preview`, `load_profile`, `evaluate`, `suggest_assignments`) return `ProjectError::NoBaseline`. To clear it, reload the full data with `solve(Some(data), ..)`. See [Data Updates](<https://docs.dhtmlx.com/planning-engine/rust/live-projects/editing-a-project.md#rejections-and-resync>) for why.

## Prerequisites

- A long-lived [`Project`](<https://docs.dhtmlx.com/planning-engine/rust/api/core/project/project.md>) solved once with [`solve(Some(data), ..)`](<https://docs.dhtmlx.com/planning-engine/rust/api/core/project/project.md#solve>).
- Your own model that can produce the full [`ProjectData`](<https://docs.dhtmlx.com/planning-engine/rust/api/core/data/project-data.md>) the user expects to see. This is the data you resync from.

## Steps

1. Apply the change set and keep the [`ApplyReport`](<https://docs.dhtmlx.com/planning-engine/rust/api/core/changes/apply-report.md>):

   ```rust
   let report = project.apply(&changes)?;
   ```

   An `Err` here means nothing was applied (`NeedsResync` from an earlier rejection, or `StaleBaseline`). A rejection inside the batch isn't an `Err`.

2. Check `needs_resync`. If it's `false`, continue as usual with [`solve(None, ..)`](<https://docs.dhtmlx.com/planning-engine/rust/api/core/project/project.md#solve>):

   ```rust
   if !report.needs_resync {
       project.solve(None, &SolveOptions::default())?;
   }
   ```

3. Show the user what was refused. Each [`RejectedOperation`](<https://docs.dhtmlx.com/planning-engine/rust/api/core/changes/rejected-operation.md>) has the operation's `path` (such as `tasks.remove[0]`) and a `message`:

   ```rust
   for op in &report.rejected {
       eprintln!("rejected {}: {}", op.path, op.message);
   }
   ```

   The other operations in the batch were applied. A rejected operation leaves no partial edit behind.

4. Make your model hold the state you want. Undo the rejected edits in it, or keep it as is if it's already correct. Don't try to repair the project with more [`apply`](<https://docs.dhtmlx.com/planning-engine/rust/api/core/project/project.md#apply>) calls: they fail with `NeedsResync`.

5. Resync with the full data. Set `include_unchanged` so the result carries the dates of every task, not only the ones that moved:

   ```rust
   let resync = SolveOptions {
       include_unchanged: true,
       ..Default::default()
   };
   let result = project.solve(Some(model.clone()), &resync)?;
   ```

6. Write every entry of `result.updates` back into your UI. The engine's state may differ from what the UI shows, so refresh all task dates, not just the edited ones.

7. Take the new `baseline_id` from `result` for the next [`preview`](<https://docs.dhtmlx.com/planning-engine/rust/api/core/project/project.md#preview>) or change set. Tokens from before the resync are stale.

The full example below stretches one task and, in the same batch, removes a task the engine doesn't have:

```rust
use plen_core::{
    DateTime, Error, Patch, Project, ProjectData, SchedulingChanges, SolveOptions, TaskData,
    TaskUpdate,
};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let jan5 = DateTime::from_ymd_hm(2026, 1, 5, 0, 0).ok_or("invalid date")?;
    let mut design = TaskData::new("design");
    design.start_date = Some(jan5);
    design.duration = Some(2.0);

    // The editor's own model: the data you resync from.
    let mut model = ProjectData {
        tasks: vec![design],
        ..Default::default()
    };
    // 24/7 time keeps the example dates easy to check.
    model.config.use_calendars = false;

    let mut project = Project::default();
    project.solve(Some(model.clone()), &SolveOptions::default())?;

    // The user stretches "design" to 4 days. The UI also sends a removal
    // of "review", a task the engine never had.
    model.tasks[0].duration = Some(4.0);
    let mut longer = TaskUpdate::new("design");
    longer.duration = Patch::Value(4.0);
    let mut changes = SchedulingChanges::default();
    changes.tasks.update.push(longer);
    changes.tasks.remove.push("review".into());

    let report = project.apply(&changes)?;
    if !report.needs_resync {
        project.solve(None, &SolveOptions::default())?;
        return Ok(());
    }

    for op in &report.rejected {
        eprintln!("rejected {}: {}", op.path, op.message);
    }

    let resync = SolveOptions {
        include_unchanged: true,
        ..Default::default()
    };
    match project.solve(Some(model.clone()), &resync) {
        Ok(result) => {
            for m in &result.updates {
                if let (Some(start), Some(end)) = (m.start_date, m.end_date) {
                    println!("{} {}..{}", m.id, start, end);
                }
            }
        }
        Err(Error::Input(e)) => {
            // The project keeps its state and still needs a resync.
            eprintln!("resync data invalid at {:?}: {}", e.path, e.message);
        }
        Err(e) => return Err(e.into()),
    }
    Ok(())
}
```

## When the resync data is invalid

The resync goes through the same validation as the first load. If your model produces invalid data, such as a duplicate task id, [`solve(Some(data), ..)`](<https://docs.dhtmlx.com/planning-engine/rust/api/core/project/project.md#solve>) returns [`Error::Input`](<https://docs.dhtmlx.com/planning-engine/rust/api/core/errors/error.md>) with the field path, for example `tasks[1].id`. The project keeps its previous state and still returns `NeedsResync`. Fix the data and call `solve(Some(data), ..)` again. See [Error handling](<https://docs.dhtmlx.com/planning-engine/rust/error-handling.md>) for the full list of validation errors.

## Drop the edit instead of resyncing

To avoid a full reload, apply the change set to a clone first. A clone is cheap, and a rejection there doesn't touch the original:

```rust
let mut draft = project.clone();
let report = draft.apply(&changes)?;
if report.needs_resync {
    // Drop `draft`. `project` still has its baseline; roll the edit back in your model too.
} else {
    draft.solve(None, &SolveOptions::default())?;
    project = draft;
}
```

Use this when the user should either get the whole change set or none of it.

## Result

The example prints the rejected removal on stderr and the refreshed date of every task on stdout:

```text
rejected tasks.remove[0]: unknown task id
design 2026-01-05 00:00..2026-01-09 00:00
```

After the resync, [`apply`](<https://docs.dhtmlx.com/planning-engine/rust/api/core/project/project.md#apply>), [`solve(None, ..)`](<https://docs.dhtmlx.com/planning-engine/rust/api/core/project/project.md#solve>), `level` and [`preview`](<https://docs.dhtmlx.com/planning-engine/rust/api/core/project/project.md#preview>) work again, and `report.needs_resync` is `false` for the next valid change set.