Addition of a JSON Cast Type

[DeanWhitehouse]

Hey Folks,

I came across an interesting challenge today whilst using a JSON field in the database, let me explain further. Details of how I discovered this issue are on Larachat https://larachat.slack.com/archives/C039F76DN/p1652876860353939

I believe that changing the existing object comparison within the object cast will be too much of a breaking change to existing applications, so I believe we should add a specific JSON cast where we can compare JSON changes in a specific way.

Issue 1

Object comparison within Laravel is currently inaccurate for anyone wishing to utilise null values, this is outlined by PHP themselves (kinda) in their docs about property value comparison https://www.php.net/manual/en/language.oop5.object-comparison.php.

Object instances are equal if they have the same attributes and values (values are compared with ==), and are instances of the same class.

We are currently utilising the == comparison within the originalIsEquivalent method in the HasAttributes trait (/framework/src/Illuminate/Database/Eloquent/Concerns/HasAttributes.php).

This means that when you change a property value from null to false, as far as this method is concerned, nothing has changed and therefore it doesn't attempt to commit anything to the database (or mark the model as dirty).

Proposed Solution

Implement a reflection based "deep" comparison of the objects to account for the null/false issue, someone has suggested an approach here https://www.php.net/manual/en/language.oop5.object-comparison.php#121105

Issue 2

JSON changes are currently assessed by their cast, this seems inefficient compared to comparing two serialized JSON strings.

Proposed Solution

Add a JSON cast type https://laravel.com/docs/9.x/eloquent-mutators#array-and-json-casting, this would cast to object by default following the standard of https://www.php.net/manual/en/function.json-decode.php. For comparisons with originalIsEquivalent we would then use a string comparison to determine any changes.