Example ModelAdmin issue

Breed.php

<?php

namespace {

    use SilverStripe\ORM\DataObject;

    class Breed extends DataObject
    {
        private static $db = [
            'BreedName' => 'Varchar(255)',
        ];

        private static $has_many = [
            'Dogs' => Dog::class
        ];

        private static $summary_fields = [
            'BreedName' => 'Breed',
            'Dogs.Count' => 'Dogs'
        ];
    }
}

Dog.php

<?php

namespace {

    use SilverStripe\ORM\DataObject;

    class Dog extends DataObject
    {
        private static $db = [
            'Name' => 'Varchar(255)',
        ];

        private static $has_one = [
            'Breed' => Breed::class
        ];

        private static $summary_fields = [
            'Name'
        ];
    }

}

DogAdmin.php

<?php

namespace {

    use SilverStripe\Admin\ModelAdmin;

    class DogAdmin extends ModelAdmin
    {
        private static $managed_models = [Breed::class];
        private static $url_segment = 'dogs';

    }
}