Each element in the $indexes array must have a "key" document, which
contains fields mapped to an order or type. Other options may follow.
For example:
$indexes = [
Create a unique index on the "username" field
[ 'key' => [ 'username' => 1 ], 'unique' => true ],
Create a 2dsphere index on the "loc" field with a custom name
[ 'key' => [ 'loc' => '2dsphere' ], 'name' => 'geo' ],
];
If the "name" option is unspecified, a name will be generated from the
"key" document.