I recently upgraded a Symfony 3.3 application to Symfony 4
Part of the upgrade was loading the DataFixtures.
Symfony 4 recommends you put all your DataFixtures in a single file. I'll get around to that later. However, due to the way I organized the file system for the project, the Doctrine Fixtures Loader could not find the demo data.
To resolve the issue, I created a services_dev.yaml file with the following:
Once I added this file to the development server, the data loaded fine.