array('relation1', 'relation2')
Eager load relation1 and relation2 without any callbacks
array('relation1' => $callable1, 'relation2' => $callable2)
Eager load relation1 with callback $callable1 etc
array('relation1', 'relation2' => $callable2)
Eager load relation1 without a callback, relation2 with callback $callable2
The callback must have the signature function(\JDatabaseQuery $query) and doesn't return a value. It is
supposed to modify the query directly.
Please note that eager loaded relations produce their queries without going through the respective model. Instead
they generate a SQL query directly, then map the loaded results into a DataCollection.