Validating fields that depend on each other in Keystone.js
You shouldn't be ! negating the second part of your validation condition, as you're currently flagging a validation error when it's not empty.
So change it to:
Sample.schema.pre('validate', function(next) { if (this.isPublished && _.isEmpty(this.thumbnailImage.image)) { next(Error('Thumbnail Image is required when publishing a sample')); } else { next(); }});Note that you also need to wrap your error string in an Error object when calling next to report a validation failure.