## 7.2 Validating Constraints

#### Validation Basics

To validate a domain class you can call the validate method on any 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 essentially 2 phases of validation, the first phase is 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 againts the constraints you defined. For example, by default the persistent save method calls `validate` before executing hence allowing you to write code like:

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