Let's say I have accounts, contacts, and contact types. Any given contact can have be any kind of contact type to any account. This is all managed through an account_contacts table with a 3-way composite PK of the contact_id, account_id, and type_id.
All of this translates to 4 entities, the account, contact, type, and accountContact entity. The id of the accountContact entity is the three other entities which is holds.
When I use query builder to retrieve the accountContacts based on some condition, and also fetch join contacts, accounts, and types so that they are eagerly loaded, the ObjectHydrator fails to load the first accountContact returned.
This is because the first time through hydrateRowData, the fetch joined entities are hydrated first. The problem is $this->_rsm->parentAliasMap[$dqlAlias] does not contain an entry for the accountContact (root entity). Not until an accountContact is hydrated does that get set. In the mean time, the other three entities were set to null because there is no parent alias yet (in version 2.4, this is line 405.
Subsequent loops through hydrateRowData work because by this time the parent alias is set. But that first row returned is always an accountContact containing three null objects.