Post.groovy

package com.myapp

class Post {
	String content
	static belongsTo = [user: User]
    static constraints = {
    }
}

User.groovy

package com.myapp

class User {
	String username
	static hasMany = [posts: Post]
    static constraints = {
    }
}

UserIntegrationTest.groovy

package com.myapp

import org.junit.Test

class UserIntegrationTests {
	
	@Test
	public void testDeleteFK() {
		def alice = new User(username: 'Alice')
		alice.save(failOnError: true)
		def post
		post = new Post(user: alice, content: 'ketchup & mayo, please')
		post.save(failOnError: true)
		assert Post.count(), 'Post should have been saved'
		
		// Since this is a test we add flush before continuing to the delete part
		alice.save(failOnError: true, flush: true)
		
		def users = User.list()
		users.each {
			// Flush before continuing to the verification part
			it.delete(flush: true) //DataIntegrityViolationException
		}
		
		assert !User.count(), 'All users should have been deleted'
		assert !Post.count(), 'All post should have been cascade deleted'
	}
}

When the it.delete(flush: true) is executed, I get a foreign key restriction error:

| Failure:  testDeleteFK(com.myapp.UserIntegrationTests)
|  org.springframework.dao.DataIntegrityViolationException: could not delete: [com.myapp.User#1]; SQL [delete from user where id=? and version=?]; constraint ["FK3498A01D042DE0: PUBLIC.POST FOREIGN KEY(USER_ID) REFERENCES PUBLIC.USER(ID)";
SQL statement:
delete from user where id=? and version=? [23503-164]]; nested exception is org.hibernate.exception.ConstraintViolationException: could not delete: [com.myapp.User#1]
        at com.myapp.UserIntegrationTests$_testDeleteFK_closure1.doCall(UserIntegrationTests.groovy:97)
        at com.myapp.UserIntegrationTests.testDeleteFK(UserIntegrationTests.groovy:95)
...
Referential integrity constraint violation: "FK3498A01D042DE0: PUBLIC.POST FOREIGN KEY(USER_ID) REFERENCES PUBLIC.USER(ID)"; SQL statement:delete from user where id=? and version=? [23503-164]

If I replace:

post = new Post(user: alice, content: 'ketchup & mayo, please')
post.save(failOnError: true)

with

post = new Post(content: 'ketchup & mayo, please')
alice.addToPosts(post)

the test passes.