What is the Foundry unset operator?

The '-=' syntax is a special operator in Foundry VTT updates that completely removes a field or flag from a document, rather than simply setting its value to null or undefined.

What is a Foundry VTT data migration?

It is a scripted process that updates existing world data when a module's data structure changes, ensuring compatibility across different versions of your software.

Can I use this for both System and Flag data?

Yes, the skill provides specific patterns for migrating data between system fields and flags, as well as handling nested properties within both structures.

How does schema versioning work in this skill?

The skill implements a 'schemaVersion' world setting that increments with each update, ensuring migrations run exactly once per world when the 'ready' hook fires.

Foundry VTT Data Migrations

Name: Foundry VTT Data Migrations
Author: ImproperSubset

byImproperSubset

Game Development

Implements safe, version-controlled data migrations for Foundry VTT modules and systems.

About

The fvtt-data-migrations skill provides a standardized framework for handling breaking changes in Foundry VTT module development. It guides developers through implementing schema versioning, moving data between storage locations, and cleaning up legacy fields using Foundry's native update syntax. By following proven patterns for idempotency and safe data transformation, this skill ensures that user data remains intact while modules evolve across different versions, preventing data loss and minimizing manual user intervention.

Key Features

Idempotent migration lifecycle management
Standardized schema versioning and tracking patterns
0 GitHub stars
Foundry-specific unset operator (-=) implementation
Safe data transformation logic for Arrays, Objects, and Maps
Batch update optimization for actor and item documents

Use Cases

Moving data from core system fields to module-specific flags
Cleaning up orphaned or deprecated data from game documents
Upgrading module data structures between major releases

About

Key Features

Idempotent migration lifecycle management
Standardized schema versioning and tracking patterns
0 GitHub stars
Foundry-specific unset operator (-=) implementation
Safe data transformation logic for Arrays, Objects, and Maps
Batch update optimization for actor and item documents

Use Cases

Moving data from core system fields to module-specific flags
Cleaning up orphaned or deprecated data from game documents
Upgrading module data structures between major releases