[5.8] Prevent unnecessary JSON UPDATE queries on MySQL

[staudenmeir]

MySQL normalizes JSON strings by adding a space between key and value ("foo":"bar" → "foo": "bar") and sorting the keys. This can prevent Eloquent from detecting equivalent values and causes unnecessary UPDATE queries:

class User extends Model
{
    protected $casts = ['options' => 'array'];

    protected $guarded = [];
}

Schema::create('users', function ($table) {
    $table->increments('id');
    $table->json('options');
    $table->timestamps();
});

$options = ['b' => 'bar', 'a' => 'foo'];

$user = User::create(['options' => $options]);
$user->refresh();

$user->options = $options;
$user->save(); // Executes unnecessary UPDATE query.

For collection and object casts, a simple value like ['a' => 'foo'] already causes this issue: Due to the additional whitespace, $current === $original fails and the created stdClass/Collection objects aren't identical.

We can fix this by recursively sorting the keys on the current and original value before comparing them. To minimize the overhead, this only happens when the previous comparisons fail.

This PR doesn't apply to nested objects, but they are probably rather rare.

In Laravel 5.9, we could make sortKeys() (optionally) recursive and remove sortKeysRecursively().

Fixes #27946.

[taylorotwell]

I don't particularly think the code overhead of fixing this is worth it for a fairly minor "bug".

[zeyad82]

When logging or auditing the model updates, it's spamming and it creates many unnecessary records so it's better to find a better way to handle it than ignoring it at all.

[sigismund]

You can bypass this bug if you cast JSON attribute as object instead of an array.

[Tofandel]

@sigismund This saved my life, I was about to write a function to deeply sort the object before an update as I was using the isDirty check to recompute some other models and it was always true

[goyote]

This is not fixed for AsCollection