(Quick Reference) # 12.2 Validating Constraints Version: 6.0.0 ## 12.2 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
        }
    }
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