[7.x] New method withSum for sum one or more columns for the relations

[Alexmg86]

Added new method withSum for sum one or more columns for the relations without second sql query.

It's working like withCount. Three type:

1. Invoice::withSum('items:price,price2')->first(); where price, price2 are columns
2. $invoices = Invoice::first(); $invoices->loadSum('items:price');
3. $invoices = Invoice::withSum(['items:price,price2' => function (Builder $query) {
   $query->where('id','<', 30);
   }])->get();

I often use this in my work and I hope it will be useful to you!

[taylorotwell]

I am closing this pull request because it lacks sufficient explanation, tests, or both. It is difficult for us to merge pull requests without these things because the change may introduce breaking changes to the framework.

Feel free to re-submit your change with a thorough explanation of the feature and tests - integration tests are preferred over unit tests. Please include it's benefit to end users; the reasons it does not break any existing features; how it makes building web applications easier, etc.

Thanks!

[khalilst]

@Alexmg86
I am really sorry your PR rejected here. However, I think it should be because it not DRY at all.
You have just copy and pasted the withCount method and its relatives to add withSum and nothing else.
You claimed here #34965 that I have stolen your code from this PR and your package with your insulting words.
The PRs are not similar at all, they are not even close together.
Your package also https://github.com/Alexmg86/laravel-sub-query
has main differences with my PR (codes-style, naming, method signatures & ...)
But assume if I copied from your unknown package, why I copied only some parts and not everything from there e.g. withMath from yours?!
And in addition to you should know the exact meaning of open-source, apparently you do not!
I have never seen your poor codes before and I know you are upset that your PR rejected and your package is useless now.
But you have to be polite and try harder to become a good programmer. Your are far away from both and this is the main reason that your are upset now.

[driesvints]

@khalilst I closed that other PR for a reason. Please stop bringing this up.