(Quick Reference)
                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, size: 5..150
        author nullable: true
    }
}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 
ConstrainedProperty instances applied against the domain class by the 
constraints property is available at runtime:
for (constraint in b.constraints) {
    println it.name
    println it.maxSize
}