Article: http://myjeeva.com/using-modelmapper-in-grails.html

UsingModelMapperInGrails

// Defining ModelMapper PropertyCondition
// Better place to put this Util class or Helper Class in your application

/**
 * Creating a Mapper field skip condition 
 * 
 * @param skipFields list of string. For eg. ['courses', 'students']
 * @return {@link org.modelmapper.Condition}
 */
static Condition createSkipCondition(skipFields) {
	new Condition() {
		boolean applies(MappingContext context) {
			!(context.mapping.destinationProperties[0].name in skipFields)
			}
		}
}

//=============================================================================================================================

// Defining helper method of createTypeMap, place method where you're globally configuring ModelMapper. 
// Typical spots are Bootstrap.groovy or dedicated ApplicationInitializeService.groovy
private createTypeMap(Class source, Class target, String typeMapName = null, def skipFields = null) {
	if (skipFields && typeMapName) {
		modelMapper.createTypeMap(source, target, typeMapName).propertyCondition = Helper.createSkipCondition(skipFields)
	} else if (typeMapName) {
		modelMapper.createTypeMap(source, target, typeMapName)
	} else if (skipFields) {
		modelMapper.createTypeMap(source, target).propertyCondition = Helper.createSkipCondition(skipFields)
	} else {
		modelMapper.createTypeMap(source, target)
	}
}

//=============================================================================================================================

// Defining Mapping

// Global settings
modelMapper.configuration.setPropertyCondition(Conditions.isNotNull())

/**
 * Domain To DTO/POGO/POJO Mapping
 * 
 * Note: Mapping is required only if you want multiple combination of fields while mapping.
 * For Example-
 * ------------
 * 	Scenario:: Student ==> StudentDto
 * 	Combinations:
 * 		1. Default is skip reports and syllabus always
 * 		2. Skip only reports
 * 		3. Skip only syllabus
 * 		4. All data - Don't skip at all
 */
createTypeMap(Student, StudentDto, null, ['reports', 'syllabus'])
createTypeMap(Student, StudentDto, STUDENT_TO_DTO_REPORTS, ['syllabus'])
createTypeMap(Student, StudentDto, STUDENT_TO_DTO_SYLLABUS, ['reports'])
createTypeMap(Student, StudentDto, STUDENT_TO_DTO_REPORTS_SYLLABUS)

//=============================================================================================================================

// Defining Collection to DTO/POGO/POJO method
private mapDomainToDto(obj, clazz, typeMapName = null) {
	if (typeMapName) {
		modelMapper.map(obj, clazz, typeMapName)
	} else {
		modelMapper.map(obj, clazz)
	}
}

private collectionToDto(def delegate, Class clazz, String name = null) {
	def result = []
	if (name) {
		delegate.each {
			result << mapDomainToDto(it, clazz, name)
		}
	} else {
		delegate.each {
			result << mapDomainToDto(it, clazz)
		}
	}
	result
}

// Defining Closures
Closure toDtoClass = { Class clazz ->
	collectionToDto(delegate, clazz)
}

Closure toDtoClassWithCondition = { Class clazz, String name ->
	collectionToDto(delegate, clazz, name)
}

//=============================================================================================================================

// Initializing Meta methods

// Adding the toDTO(Class) & toDTO(Class, name) methods to collection, list, set, etc
[Collection, ArrayList, HashSet, TreeSet].each {
	it.metaClass.toDto = toDtoClass
	it.metaClass.toDto = toDtoClassWithCondition
}

grailsApplication.domainClasses.each { domainArtefact ->
	// Adding 'as DTO'
	final MetaClass mc = domainArtefact.metaClass
	final asTypeRef = mc.getMetaMethod('asType', [Class ] as Object[])
	mc.asType = { Class clazz ->
		def result

		if (delegate) {
			if (clazz.name.endsWith('Dto')) {  // I know its hacky way :(
				result = mapDomainToDto(delegate, clazz)
			} else {
				result = asTypeRef.invoke(delegate, [clazz ] as Object[])
			}
		}
		result
	}

	// Adding toDto(Class)
	mc.toDto = { Class clazz ->
		mapDomainToDto(delegate, clazz)
	}

	// Adding toDto(Class, name)
	mc.toDto = { Class clazz, String name ->
		mapDomainToDto(delegate, clazz, name)
	}
}