Update User, Cart models.

Added isEmpty (no items), setCity, dropItems to Cart.
The cart field, company, was made nullable.

Updated the user field from name to username and made it unique.

app/Models/Cart.php

@@ -12,7 +12,7 @@ class Cart extends Model
     use HasFactory;
 
     protected $fillable = [
-        'status',
+        'status', // CART | DONE
     ];
 
     public function city(): BelongsTo
@@ -29,5 +29,25 @@ class Cart extends Model
     {
         return $this->belongsToMany(Item::class, 'carts_items', 'cart_id', 'item_id');
     }
+
+    public function isEmpty() {
+        return $this->items()->isEmpty();
+    }
+
+    public function setCity($city) {
+        if($this->city == $city)
+            return;
+
+        $this->city = $city;
+
+        if(!$this->isEmpty())
+            $this->dropItems();
+
+        $this->save();
+    }
+
+    public function dropItems() {
+        $this->items()->detach();
+    }
 }