What does the "primary" key in entity's relation mean?

[asprin]

Just trying to wrap my head around a basic concept in XF2. What's the difference between (1) and (2)?

PHP:

Code:

//#1
$structure->relations['Bars'] = [
    'entity'    => 'ABC\DEF:Bar',
    'type'      => self::TO_MANY,
    'conditions' => ['foo_id', '=', '$bar_id'],
    'primary'   => true
];


//#2
$structure->relations['Bars'] = [
    'entity'    => 'ABC\DEF:Bar',
    'type'      => self::TO_MANY,
    'conditions' => ['foo_id', '=', '$bar_id'],
    'primary'   => false
];

And does...

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