Value back references

Sometimes instead of filtering using literal values you want to filter using the value of other columns in a model. Stem supports this by way of a placeholder syntax:

@{ColumnName}

Simply use this in place of a literal value to have this used instead.

For example to select customer records where their outstanding balance is greater than their credit limit you could use a filter like this:

$overLimit = Customer::find(new GreaterThan("OutstandingBalance", "@{CreditLimit}"));

This occurs most frequently when performing intersections, pulling up an aggregate value and then filter. In the above example we supposed that the OutstandingBalance was calculated into a simple column on the customer model. What if it wasn't and had to be calculated on the fly:

$overLimit = Customer::all()
    ->intersectWith(
        Invoice::all()
            // Aggregate the balance of invoices for the customer
            ->addAggregateColumn(new Sum("OutstandingBalance")),
        "CustomerID",
        "CustomerID",
        // Pull up the aggregate value into the customer collection
        [ "SumOfOutstandingBalance" ])
    // Filter on the customer collection back referencing the aggregate value.
    ->filter(new LessThan("CreditLimit", "@{SumOfOutstandingBalance}"));