somestuff.php

<?php 
        $invoice = $this->Invoices->newEntity([
            'amount' => 100,
            'description' => 'Simple little description of invoice',
            'invoice_status_id' => 1,
            'due_date' => new Time('2016-05-05')
        ]);

        $invoice->user = $this->Invoices->Users->newEntity([
            'first_name' => 'Bob',
            'last_name' => "McBriddle",
            'username' => 'brob',
            'password' => 'becauseIhaveto'
        ]);

        $invoice->user->detail = $this->Invoices->Users->Detail->newEntity([
            'dba' => 'Coffee House Construction LLC'
        ]);

        $invoice->listing = $this->Invoices->Listings->newEntity([
            'title' => '2005 Volvo Materryder'
        ]);

        $this->Invoices->save($invoice);