When publishing a content object that has an attribute of type object relation and a hidden content object as the value for this attribute, the publishing process will fail with a fatal error if delayed search indexing is DISABLED:

Notice: Undefined index: 0 in kernel/search/plugins/ezsearchengine/ezsearchengine.php on line 319
Undefined index: 0 in kernel/search/plugins/ezsearchengine/ezsearchengine.php on line 314
Fatal error: A database transaction in eZ Publish failed.

The object relation attribute is configured as follows:

required: not checked
searchable: checked
switch of translation: not checked
selection method: browse
no preselected element
allow fuzzy match: not checked

eZ Support could not reproduce it on MySQL.

Steps to reproduce

1. Disable delayed indexing
2. Create a class having an attribute of type object relation
3. Create a content object of that class
4. Select a hidden content object as a value for the object relation attribute
5. Publish the content object