Add the validation for City, User that returns message for requests.

app/Models/City.php

@@ -14,4 +14,25 @@ class City extends Model
         'name',
         'url',
     ];
+
+    public static function validate_with_name(string $name)
+    {
+        $name = $name ?? '';
+
+        if($name == '')
+            return [
+                'error' => 'The city name is empty, please, write the name!!!'
+            ];
+
+        $city = City::where('name', $name)->first();
+
+        if(!$city)
+            return [
+                'error' => 'A city with the name does not exist!!!'
+            ];
+
+        return [
+            'ok' => 'A city with the name is valid.'
+        ];
+    }
 }

app/Models/User.php

@@ -22,4 +22,25 @@ class User extends Authenticatable
         'matrix_username',
         'phone',
     ];
+
+    public static function validate_with_matrix_username(string $matrix_username)
+    {
+        $matrix_username = $matrix_username ?? '';
+
+        if($matrix_username == '')
+            return [
+                'error' => 'The username is empty, please, write username!!!'
+            ];
+
+        $user = User::where('matrix_username', $matrix_username)->first();
+
+        if(!$user)
+            return [
+                'error' => 'A user with the username does not exist!!!'
+            ];
+
+        return [
+            'ok' => 'A user with the username is valid.'
+        ];
+    }
 }