11 Validation

Constraints in Grails are a way to declaratively specify validation rules. Most commonly they are applied to domain classes, however URL Mappings and Command Objects also support constraints.

11.1 Declaring Constraints

class User {
    String login
    String password
    String email
    Integer age
    static constraints = {
      …
    }
}

You then use method calls that match the property name for which the constraint applies in combination with named parameters to specify constraints:

class User {
    ...
    static constraints = {
        login size: 5..15, blank: false, unique: true
        password size: 5..15, blank: false
        email email: true, blank: false
        age min: 18
    }
}

In this example we've declared that the login property must be between 5 and 15 characters long, it cannot be blank and must be unique. We've also applied other constraints to the password, email and age properties.

By default, all domain class properties are not nullable (i.e. they have an implicit nullable: false constraint).

A complete reference for the available constraints can be found in the Quick Reference section under the Constraints heading.

Note that constraints are only evaluated once which may be relevant for a constraint that relies on a value like an instance of java.util.Date.

class User {
    ...
    static constraints = {
        // this Date object is created when the constraints are evaluated, not
        // each time an instance of the User class is validated.
        birthDate max: new Date()
    }
}

A word of warning - referencing domain class properties from constraints

It's very easy to attempt to reference instance variables from the static constraints block, but this isn't legal in Groovy (or Java). If you do so, you will get a MissingPropertyException for your trouble. For example, you may try

class Response {
    Survey survey
    Answer answer
    static constraints = {
        survey blank: false
        answer blank: false, inList: survey.answers
    }
}

See how the inList constraint references the instance property survey? That won't work. Instead, use a custom validator:

class Response {
    …
    static constraints = {
        survey blank: false
        answer blank: false, validator: { val, obj -> val in obj.survey.answers }
    }
}

In this example, the obj argument to the custom validator is the domain instance that is being validated, so we can access its survey property and return a boolean to indicate whether the new value for the answer property, val, is valid.

11.2 Validating Constraints

Validation Basics

Call the validate method to validate a domain class instance:

def user = new User(params)
if (user.validate()) {
    // do something with user
}
else {
    user.errors.allErrors.each {
        println it
    }
}

The errors property on domain classes is an instance of the Spring Errors interface. The Errors interface provides methods to navigate the validation errors and also retrieve the original values.

Validation Phases

Within Grails there are two phases of validation, the first one being data binding which occurs when you bind request parameters onto an instance such as:

def user = new User(params)

At this point you may already have errors in the errors property due to type conversion (such as converting Strings to Dates). You can check these and obtain the original input value using the Errors API:

if (user.hasErrors()) {
    if (user.errors.hasFieldErrors("login")) {
        println user.errors.getFieldError("login").rejectedValue
    }
}

The second phase of validation happens when you call validate or save. This is when Grails will validate the bound values against the constraints you defined. For example, by default the save method calls validate before executing, allowing you to write code like:

if (user.save()) {
    return user
}
else {
    user.errors.allErrors.each {
        println it
    }
}

11.3 Sharing Constraints Between Classes

Global Constraints

In addition to defining constraints in domain classes, command objects and other validateable classes, you can also define them in grails-app/conf/application.groovy:

grails.gorm.default.constraints = {
    '*'(nullable: true, size: 1..20)
    myShared(nullable: false, blank: false)
}

These constraints are not attached to any particular classes, but they can be easily referenced from any validateable class:

class User {
    ...
    static constraints = {
        login shared: "myShared"
    }
}

Note the use of the shared argument, whose value is the name of one of the constraints defined in grails.gorm.default.constraints. Despite the name of the configuration setting, you can reference these shared constraints from any validateable class, such as command objects.

The '*' constraint is a special case: it means that the associated constraints ('nullable' and 'size' in the above example) will be applied to all properties in all validateable classes. These defaults can be overridden by the constraints declared in a validateable class.

Importing Constraints

Grails 2 introduced an alternative approach to sharing constraints that allows you to import a set of constraints from one class into another.

Let's say you have a domain class like so:

class User {
    String firstName
    String lastName
    String passwordHash
    static constraints = {
        firstName blank: false, nullable: false
        lastName blank: false, nullable: false
        passwordHash blank: false, nullable: false
    }
}

You then want to create a command object, UserCommand, that shares some of the properties of the domain class and the corresponding constraints. You do this with the importFrom() method:

class UserCommand {
    String firstName
    String lastName
    String password
    String confirmPassword
    static constraints = {
        importFrom User
        password blank: false, nullable: false
        confirmPassword blank: false, nullable: false
    }
}

This will import all the constraints from the User domain class and apply them to UserCommand. The import will ignore any constraints in the source class (User) that don't have corresponding properties in the importing class (UserCommand). In the above example, only the 'firstName' and 'lastName' constraints will be imported into UserCommand because those are the only properties shared by the two classes.

If you want more control over which constraints are imported, use the include and exclude arguments. Both of these accept a list of simple or regular expression strings that are matched against the property names in the source constraints. So for example, if you only wanted to import the 'lastName' constraint you would use:

…
static constraints = {
    importFrom User, include: ["lastName"]
    …
}

or if you wanted all constraints that ended with 'Name':

…
static constraints = {
    importFrom User, include: [/.*Name/]
    …
}

Of course, exclude does the reverse, specifying which constraints should not be imported.

11.4 Validation on the Client

Displaying Errors

Typically if you get a validation error you redirect back to the view for rendering. Once there you need some way of displaying errors. Grails supports a rich set of tags for dealing with errors. To render the errors as a list you can use renderErrors:

<g:renderErrors bean="${user}" />

If you need more control you can use hasErrors and eachError:

<g:hasErrors bean="${user}">
  <ul>
   <g:eachError var="err" bean="${user}">
       <li>${err}</li>
   </g:eachError>
  </ul>
</g:hasErrors>

Highlighting Errors

It is often useful to highlight using a red box or some indicator when a field has been incorrectly input. This can also be done with the hasErrors by invoking it as a method. For example:

<div class='value ${hasErrors(bean:user,field:'login','errors')}'>
   <input type="text" name="login" value="${fieldValue(bean:user,field:'login')}"/>
</div>

This code checks if the login field of the user bean has any errors and if so it adds an errors CSS class to the div, allowing you to use CSS rules to highlight the div.

Retrieving Input Values

Each error is actually an instance of the FieldError class in Spring, which retains the original input value within it. This is useful as you can use the error object to restore the value input by the user using the fieldValue tag:

<input type="text" name="login" value="${fieldValue(bean:user,field:'login')}"/>

This code will check for an existing FieldError in the User bean and if there is obtain the originally input value for the login field.

11.5 Validation and Internationalization

Constraints and Message Codes

The codes themselves are dictated by a convention. For example consider the constraints we looked at earlier:

package com.mycompany.myapp
class User {
    ...
    static constraints = {
        login size: 5..15, blank: false, unique: true
        password size: 5..15, blank: false
        email email: true, blank: false
        age min: 18
    }
}

If a constraint is violated Grails will by convention look for a message code of the form:

[Class Name].[Property Name].[Constraint Code]

In the case of the blank constraint this would be user.login.blank so you would need a message such as the following in your grails-app/i18n/messages.properties file:

user.login.blank=Your login name must be specified!

The class name is looked for both with and without a package, with the packaged version taking precedence. So for example, com.mycompany.myapp.User.login.blank will be used before user.login.blank. This allows for cases where your domain class message codes clash with a plugin's.

For a reference on what codes are for which constraints refer to the reference guide for each constraint.

Displaying Messages

The renderErrors tag will automatically look up messages for you using the message tag. If you need more control of rendering you can handle this yourself:

<g:hasErrors bean="${user}">
  <ul>
   <g:eachError var="err" bean="${user}">
       <li><g:message error="${err}" /></li>
   </g:eachError>
  </ul>
</g:hasErrors>

In this example within the body of the eachError tag we use the message tag in combination with its error argument to read the message for the given error.

11.6 Applying Validation to Other Classes

The Validateable Trait

Classes which define the static constraints property and implement the Validateable trait will be validateable. Consider this example:

// src/groovy/com/mycompany/myapp/User.groovy
package com.mycompany.myapp
import grails.validation.Validateable
class User implements Validateable {
    ...
    static constraints = {
        login size: 5..15, blank: false, unique: true
        password size: 5..15, blank: false
        email email: true, blank: false
        age min: 18
    }
}