Relation::morphMap incorrectly merges maps with integer keys

[v0ctor]

• Laravel Version: 5.5.13
• PHP Version: 7.0.24
• Database Driver & Version: MySQL 5.6.37

Description:

When using integer custom polymorphic types, Relation::morphMap does not merge them properly because it uses array_merge(), which treats keys differently depending on their type:

If the input arrays have the same string keys, then the later value for that key will overwrite the previous one. If, however, the arrays contain numeric keys, the later value will not overwrite the original value, but will be appended. PHP Documentation.

This difference in behavior may be a problem in some cases. I'll give an example to reproduce the problem.

Before continuing I would like to clarify that I am aware that it would be more appropriate to use string keys and the enum type, but I can not make changes to the project model, 🤷‍♂️.

Steps To Reproduce:

Imagine you have the following morph map:

// Note that there's no key 0 or 3
$morphMap = [
    1 => Post::class,
    2 => Image::class,
    4 => Video::class,
];

Now, if you call to Relation::morphMap($morphMap), the value of Relation::$morphMap would be the following:

array(3){
    1 => Post::class,
    2 => Image::class,
    4 => Video::class,
}

But if you try to set the morph map again with new values, as it makes no sense removing the old ones before merging them, array_merge will merge them like this:

Relation::morphMap([
    1 => Post::class,
    2 => Image::class,
    4 => Video::class,
    5 => Poll::class,
]);

array(7){
    0 => Post::class,
    1 => Image::class,
    2 => Video::class,
    3 => Post::class,
    4 => Image::class,
    5 => Video::class,
    6 => Poll::class,
}

I think the expected behavior should be the following:

array(4){
    1 => Post::class,
    2 => Image::class,
    4 => Video::class,
    5 => Poll::class,
}

Proposed solution

Since a morph map is an associative array rather than a sequential one, I propose to modify the implementation of Relation::morphMap as follows:

public static function morphMap(array $map = null, $merge = true)
{
    $map = static::buildMorphMapFromModels($map);

    if (is_array($map)) {
        static::$morphMap = $merge && static::$morphMap
            ? $map + static::$morphMap : $map;
    }

    return static::$morphMap;
}

By using the array "+" operator there can not be duplicate values. Note that the $map parameter is the left-hand value of the operation so that it is possible to replace values by calling again to Relation::morphMap.

[hakuno]

@victordzmr Great!