Added the functions to get the models connected with the booking.

app/Booking.php

@@ -8,11 +8,48 @@ class Booking extends Model
 {
     public function amount()
     {
-        return $this->hasMany('App\SubBooking')->amount;
+        return $this->subBookings()->sum('amount');
     }
 
     public function signedAmount()
     {
-        return $this->amount;
+        return $this->subBookings()->withSignedAmount()->get()
+                    ->sum('signedAmount');
+    }
+
+    public function subBookings()
+    {
+        return $this->hasMany('App\SubBooking');
+    }
+
+    /**
+     * Return true, if the booking consists only of one sub-booking.
+     * NOTICE: The book instance has to be loaded with the
+     * ->withCount('subBookings') method to be able to access the comments
+     * count.
+     *
+     * @return boolean
+     */
+    public function isSimple()
+    {
+        return $this->sub_bookings_count === 1;
+    }
+
+    public function project()
+    {
+        if ($this->isSimple()) {
+            return $this->subBookings()->first()->project();
+        }
+
+        return null;
+    }
+    
+    public function accountType()
+    {
+        if ($this->isSimple()) {
+            return $this->subBookings()->first()->accountType();
+        }
+
+        return null;
     }
 }