# constraints

## Purpose

Allows the definition of declarative validation constraints. See Validation in the user guide.

## Examples

    class Book {
       String title
       Author author
       static constraints = {
            title(blank:false, nullable:false, size:5..150)
            author(nullable:false)
       }
    }

## Description

Constraints are defined using the declarative constraints DSL as described in the Validation section of the user guide. Once evaluated validation can be applied through the use of the validate method:

    def b = new Book()
    assert !b.validate()

A list of api:org.codehaus.groovy.grails.validation.ConstrainedProperty instances applied against the domain class by the `constraints` property is available at runtime:

    b.constraints.each {
          println it.name
          println it.maxSize
    }