* Specifies the relation associated with the pivot table.
* @param string $relationName the relation name. This refers to a relation declared in [[primaryModel]].
* @param string $relationName the relation name. This refers to a relation declared in the [[ActiveRelationTrait::primaryModel|primaryModel]] of the relation.
* @param callable $callable a PHP callback for customizing the relation associated with the pivot table.
* Its signature should be `function($query)`, where `$query` is the query to be customized.