public function getAcquaintances()
    {
        $acquaintances = new Collection();

        // Past message senders
        foreach ($this->messages()->received()->with('sender')->get() as $message) {
            $acquaintances = $acquaintances->push($message->sender);
        }

        // Past message recipients
        foreach ($this->messages()->sent()->with('recipient')->get() as $message) {
            $acquaintances = $acquaintances->push($message->recipient);
        }

        // Past sellers
        foreach ($this->purchases()->with('item.seller')->get() as $purchase) {
            $acquaintances = $acquaintances->push($purchase->item->seller);
        }

        // Past buyers
        foreach ($this->items()->with('purchases.buyer')->get() as $item) {
            foreach ($item->purchases as $purchase) {
                $acquaintances = $acquaintances->push($purchase->buyer);
            }
        }

        return $acquaintances->unique();
    }