Error is thrown on undo: Not a child: 0

[stoefln]

Bug report

• [✅] I've checked documentation and searched for existing issues
• [✅] I've made sure my project is based on the latest MST version: v5.1.8
• [😖 ] Fork this code sandbox or another minimal reproduction.
  Sorry, I can't build a repro example, because I don't know what kind of condition exactly leads to that error. We have a huge state tree in our application, no idea which part to copy into the repro example.

Describe the expected behavior
No error

Describe the observed behavior

On undoing a certain operation, the data structure is rolled back, but the following error is thrown:

Error: [mobx-state-tree] Not a child: 0
    at fail$1 (mobx-state-tree.module.js?f7d3:3839)
    at ArrayType.value (mobx-state-tree.module.js?f7d3:5426)
    at ObjectNode.value (mobx-state-tree.module.js?f7d3:1858)
    at resolveNodeByPathParts (mobx-state-tree.module.js?f7d3:3759)
    at eval (mobx-state-tree.module.js?f7d3:1989)
    at Array.forEach (<anonymous>)
    at eval (mobx-state-tree.module.js?f7d3:1983)
    at executeAction (mobx.esm.js?685c:1136)
    at <unnamed action> (mobx.esm.js?685c:1120)
    at runMiddleWares (mobx-state-tree.module.js?f7d3:3246)
    at runWithActionContext (mobx-state-tree.module.js?f7d3:3089)
    at ObjectNode.res [as _applyPatches] (mobx-state-tree.module.js?f7d3:3116)
    at ObjectNode.value (mobx-state-tree.module.js?f7d3:1565)
    at applyPatch (mobx-state-tree.module.js?f7d3:240)
    at eval (mst-middlewares.module.js?e48f:612)
    at skipRecording (mst-middlewares.module.js?e48f:576)
eval	@	TestView.js?e4f0:331

I could catch the error, but the problem is that apparently the UndoManager kind of ignores that that undo has been applied, and so if the user hits UNDO AGAIN, the UndoManager tries to repeatedly apply the undo, which leads to really bad side effects (like duplicate ids in the state tree).

I guess you won't be able to help without repro steps, but is there maybe a way to debug/log what is going on inside of the UndoManager, so I get some idea of what is happening? For me it's quite a black box and I don't even know where to start digging.

[coolsoftwaretyler]

Hey @stoefln - sorry about the bug! Thank you for considering the repro steps. I have a similar situation sometimes with a large enough tree and errors that aren't super clear-cut.

I don't have great advice without the reproduction, as you said. But I can at least point you in the right direction for the UndoManager.

This is the source file for that: https://github.com/mobxjs/mobx-state-tree/blob/master/packages/mst-middlewares/src/undo-manager.ts

Locally, I think you'll probably find your copy of this file in node_modules/mobx-state-tree/dist/middlewares, or somewhere near that if you want to drop some debug statements around your local repo.

If you get more details, post them back here! I have notifications on so hopefully we can work together on this.

[coolsoftwaretyler]

Hey @stoefln - without a reproduction of the problem, I'd like to close out this issue.

I'm hoping the links to the source code were helpful for you.

Feel free to file another issue if you can get a repro together, or if you find the specific bug in the middleware.

Thanks!