undpaul Explains Changing Drupal Field Length with Existing Data

Drupal blocks standard field-storage changes when existing data is present, Steffen Rühlmann explains in a 3 July 2026 undpaul blog post. In Drupal Tip: How to change field length with existing data, he describes the common case of increasing a populated text field from 255 to 512 characters and why changing field configuration alone can trigger a FieldStorageDefinitionUpdateForbiddenException.

The post says Drupal’s Entity API refuses this kind of schema update to protect existing data. Steffen outlines a database-first post-update approach that widens the relevant field columns with Drupal’s schema API, updates the stored field schema in entity.storage_schema.sql, changes active field storage configuration through the raw configuration factory, and refreshes the last-installed field storage definition. The method is presented for widening a text field, not as a general bypass of Drupal’s schema protections.

The undpaul post advises running database updates, rebuilding caches, exporting configuration, and checking Drupal’s status report after the change. It also notes database-driver differences: MySQL, MariaDB, and PostgreSQL can handle the widening operation through the schema API, while SQLite behaves differently because it does not enforce string lengths in the same way. The guidance is most relevant to Drupal developers handling production fields where configuration, schema metadata, and stored data must stay aligned.