Use a model (resource) attribute (accessor) with metrics in Laravel Nova

I have a MySQL table called purchases and a model called Purchase in Laravel that is related to it. My table has two columns, quantity and unit_cost. The total cost of a purchase is not a column in my table. Instead, it is defined by an accessor in Laravel that multiplies unit_cost by quantity and returns it as total_cost.

Now I am trying to define a trend metric in Laravel nova that is supposed to output the value of my daily sales in the selected range. When I try to use return $this->sumByDays($request, Purchase::class, 'total_cost') it gives me an error that says total_cost is not a column.

Fair enough! So, I tried to use Purchase::selectRaw(DB::RAW('unit_cost * quantity AS total_cost')) in the second argument, but it didn’t work. I saw multiple posts, none of them could solve the problem of accessing a model accessor in Laravel Nova metrics. Any help is appreciated.

Answers:

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Method 1

Unfortunately you can’t use the built-in sumByDays helper function.

But good news! You can still implement it yourself within the calculate function. Just make sure your function returns a LaravelNovaMetricsTrendResult.

Something like this:

$purchases = DB::table('purchases')
    ->select(DB::raw('quantity * unit_cost as total_cost'))
    ->groupBy(DB::raw('DATE(created_at) DESC'))
    ->get();

return new TrendResult()
    ->trend($purchases);

I don’t have time to test this at the moment but it should get you started.

Method 2

While trying to fix this problem, I checked through the sum by day or hours code and I realised that the column is an instance of Query Expression.

So that means you can still use the built in nova sum by days.

Make sure you import use IlluminateDatabaseQueryExpression;

Just do this

$xpression = new Expression('quantity * unit_cost');
return $this->sumByDays($request, Purchase::class, $xpression);

It worked for me easily, I hope it does for others too.

The expression could be used in various ways in my case it was to perform a different calculation.

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