Nested Prefabs fileID reference does not exist

Unity Engine
1

I’m making a migration tool for scene’s and am now including prefabs and nested prefabs. To migrate the data I need to know what type of class/MonoBehaviour the document is (can be found with the original fileID and guid). For a normal prefab (without the nesting) this can be done by going in the scene file to the PrefabInstance -> M_Modification -> M_Modifications -> Target -> fileID This fileID then is the flag of the document in the prefab file i’m working with (a quick ctrl + f shows what script it is).

But this does not seem to work when prefabs are nested. It then has a fileID that does not exist anywhere else in the project (that i can find). It seems just randomly generated like the normal prefab and a reference to something i can’t find as it changes when the script is re-added.

I’ve exported it to a package and unzipped it but there isn’t any special mapping other then the normal files. But the import in a new project works fine, which should mean that the fileID is some kind of a reference as the reference stays intact. (I’ve added the package to the post)

Any information about the nested prefab fileID would be appreciated.

5167172–512687–nested prefab test.unitypackage (7.07 KB)

2

fileID for nested prefabs and also variants are generated based on the prefab instance and the corresponding object

(FileID_of_nested_PrefabInstance ^ FileID_of_object_in_nested_Prefab) & 0x7fffffffffffffff;

1 Like
How to get deeply nested Prefab information in UnityYAML(.prefab)?
3

Thanks, this was what I needed! I’ll try it soon and report back if I have a sample for others.

4

Is there any way to calculate the FileID_of_nested_PrefabInstance with the .prefab and .prefab.meta files? Currently i would have to calculate the fileID for every prefab and every yamldocument in that prefab which seems like a workaround.

5

I am not sure what you are asking for?

Lets say you have Prefab B, that nests prefab A. In that case, the file called B.prefab will contain a PrefabInstance object. This object will have a FileID which is randomly generated when prefab B is created and it can’t be calculated.

Getting that FileID can be done with Unity - Scripting API: AssetDatabase.TryGetGUIDAndLocalFileIdentifier

6

Unfortunately I don’t think that the AssetDatabase will work as I just have the files and not the loaded in Unity project.

For context, I’m making a migration tool to move scenes and now prefabs from one project to another. But that also means that i don’t have access to the AssetDatabase in the original project as this code is executed from the new project.

Now to clarify the problem:

I have a scene file called Example.unity in this scene there is a prefab(Prefab.prefab) and in this prefab there is a prefab nested (PrefabNested.prefab). Now I look through the scene and find the PrefabInstance.m_SourcePrefab.guid element and move to that prefab (Prefab.prefab) to also migrate that prefab.

Now in this prefab (Prefab.prefab) I find another PrefabInstance but this time without the guid so i had no idea which prefab this references. For the tool I’m writing I need to change the PrefabInstance.m_Modification.m_Modifications[ ].propertyPath as this field should be migrated. But to know what i need to change it to I need to know which script it actually is which I’m working on. For this I need to look in the nested prefab(PrefabNested.prefab). But the fileID in the Prefab.prefab doesn’t match the fileID in the nested prefab (NestedPrefab.prefab). So first I need find which prefab it is, the script is on.

With the current solution i would have to loop through all prefabs in the project and find all scripts in those prefabs and run the following on it :

(FileID_of_nested_PrefabInstance ^ FileID_of_object_in_nested_Prefab) & 0x7fffffffffffffff;

This would then result in all the fileIDs being calculated and I would then be able to find the script and prefab that I’m looking for.

This is definitely a solution that could work and I think this is currently the way to go, but it would be amazing if I could in some way calculate a fileID that would point me to the correct prefab from within the Prefab.prefab. This would make the processing a lot easier.

But if these fileIDs are generated and I won’t be able to read them then I’ll just have to parse all prefabs.

7

For future reference (because this took me some tinkering to figure out) if a scene has a modification for double nested prefab the fileId in the scene file ‘modification’ is calculated by adding each layer of prefab instance to the xor ‘^’ operation.

- PrefabA
-- PrefabB
--- PrefabC```

the fileId for the modification listed in the scene file would be
```(<PrefabA fileId in scene file> ^ <PrefabB fileId in PrefabA file> ^ <PrefabC fileId in PrefabB file>) & 0x7fffffffffffffff```
1 Like
8

I was thinking… isn’t it posible to have fileid conflicts if you have several nested prefabs? Imagine this hierarchy

- Prefab A
   - Prefab B
      - child 0
      - child 1
      - ...
   - Prefab C
      - child 0
      - child 1
      - ...

I know there is a small chance but there is still a small posibility that any children of B and C end up having the same fileid when being referenced from prefab A since you can edit any of those prefabs separately (you can open PrefabB and start generating GameObjects)

Actually I was able to create this conflict manually (by changing the ids on the prefabs from a text editor) and I ended up with a corrupted prefab

Duplicate identifier -1764472761. File: "Assets/TestPrefab.prefab".
UnityEditor.EditorApplication:Internal_CallUpdateFunctions ()

Error loading the file 'Assets/TestPrefab.prefab'. File has multiple objects with same identifiers. Probably caused by a merge.
UnityEditor.EditorApplication:Internal_CallUpdateFunctions ()

I think it’s posible that this happens… or am I missing something here?