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When 'primary' is set on a relationship, XF\Mvc\Entity\Manager::getRelation parses the condition into a key-value set to lookup by primary key the linked entity.

This has the following issues:

• When this key-value has 1 entry, Finder::find => Finder::whereId do not validate this is a known primary key column (they only do this for an array).
• For multi-column primary keys, the order is dependant on the list of conditions and not the primary key as defined in the entity. This means an unexpected cache miss may occur.
• An empty condition will cause the relationship to always return null, which probably isn't expected.

Putting 'primary' on the wrong relationship is a painful debugging experience, since in all likely hood it just fetches the wrong object