Sokil\Mongo\Collection::readPrimaryOnly PHP Method

    public function readPrimaryOnly()
    {
        $this->getMongoCollection()->setReadPreference(\MongoClient::RP_PRIMARY);
        return $this;
    }

 public function testReadPrimaryOnly()
 {
     $this->collection->readPrimaryOnly();
     $this->assertEquals(array('type' => \MongoClient::RP_PRIMARY), $this->collection->getReadPreference());
 }