Skip to content

Validation Overview

Validation is a framework for validating data sent to your application. It can help validate things like names, emails and more. It is also extensible, allowing you to easily create custom validators.

Swift & Codable

Swift's strong type system and Codable take care of most of the basic validation that web apps need to do.

struct User: Codable {
    var id: UUID?
    var name: String
    var age: Int
    var email: String?
    var profilePictureURL: String?

For example, when you decode the above User model, Swift will automatically ensure the following:

  • id is a valid UUID or is nil.
  • name is a valid String and is not nil.
  • age is a valid Int and is not nil.
  • email is a valid String or is nil.
  • profilePictureURL is a valid String or is nil.

This is a great first step, but there is still room for improvement here. Here are some examples of things Swift and Codable would not mind, but are not ideal:

  • name is empty string ""
  • name contains non-alphanumeric characters
  • age is a negative number -42
  • email is not correctly formatted
  • profilePictureURL is not a URL without a scheme

Luckily the Validation package can help.


Let's take a look at how the Validation package can help you validate incoming data. We'll start by conforming our User model from the previous section to the Validatable protocol.


This assumes User already conforms to Reflectable (added by default when using one of Fluent's Model protocols). If not, you will need to add conformance to Reflectable manually.

extension User: Validatable {
     /// See `Validatable`.
     static func validations() -> Validations<User> {
         // define validations

let user = User(...)
// since User conforms to Validatable, we get a new method validate()
// that throws an error if any validations fail
try user.validate() 

This is the basic structure of Validatable conformance. Let's take a look at how we can implement the static validations() method.


First let's start by verifying that the name is at least 3 characters long.

extension User: Validatable {
    /// See `Validatable`.
    static func validations() throws -> Validations<User> {
        var validations = Validations(User.self)
        try validations.add(\.name, .count(3...))
        return validations

The count(...) validation accepts Swift Range notation and will validate that a collection's count is within that range. By only placing a value on the left side of ..., we only set a minimum range.

Take a look at all of the available validators here.


Validating that the name is three or more characters is great, but we also want to make sure that the name is alphanumeric characters only. We can do this by combining multiple validators using &&.

try validations.add(\.name, .count(3...) && .alphanumeric)

Now our name will only be considered valid if it is three or more characters and alphanumeric. Take a look at all of the available operators here.


You may want to run validations on optionals only if a value is present. The && and || operators have special overloads that help you do this.

try validations.add(\.email, .email || .nil)

The nil validator checks if a T? optional value is nil.

The email validator checks if a String is a valid email address. However, the property on our User is a String?. This means the email validator cannot be used directly with the property.

We can combine these two operators using || to express the validation we want: validate the email is correctly formatted if it is not nil.


Let's finish up the rest of our validations using our new knowledge.

extension User: Validatable {
    /// See `Validatable`.
    static func validations() throws -> Validations<User> {
        var validations = Validations(User.self)
        try validations.add(\.name, .alphanumeric && .count(3...))
        try validations.add(\.age, .range(18...))
        try validations.add(\.email, .email || .nil)
        try validations.add(\.profilePictureURL, .url || .nil)
        return validations

Now let's try out validating our model., at: "users") { req, user -> User in
    try user.validate()
    return user

When you query that route, you should see that errors are thrown if the data does not meet your validations. If the data is correct, your user model is returned successfully.

Congratulations on setting up your first Validatable model! Check out the API docs for more information and code samples.