belongsTo performing differently in each scenario
class Post {
String content
Date dateCreated
static constraints = {
content(blank: false)
}
static belongsTo = [ user : User ]
}

class User {
// existing code here
static hasMany = [ posts : Post ]
}

void testFirstPost() {
def user = new User(userId: 'joe',password: 'secret').save()
def post1 = new Post(content: "First post... W00t!")
user.addToPosts(post1)
def post2 = new Post(content: "Second post...")
user.addToPosts(post2) }

class User {
    String login
    String password
    Profile profile
    Status status
    static hasMany = [ holidays : Holiday ]
    static constraints = {
        login(unique:true,size:2..20)
        password(size:2..20) //more to be added later!
        profile(nullable:true)
        company(nullable:true)
    }
    static belongsTo = [ company : Company ]
}

class Company {
    String shortName;
    String name
    Date dateCreated
    String region
    String email
    Address address
    Status status
    Long tel
    Long fax
    static hasMany = [ users : User]
        static constraints = {
        }
    static mapping = {
        address lazy:false
        status lazy:false
    }
}

void testSaveUser() {
        def status1 = new Status(name:"Busy")
        status1.save(flush:true)
        def user = new User(login:"anto",password:"anything",
            status:status1)
        assert user.save(flush:true, failOnError: true)
    }

Validation Error(s) occurred during save(): - Field error in object 'mnm.User' on field 'company': rejected value [null]; codes [mnm.User.company.nullable.error.mnm.User.company,mnm.User.company.nullable.error.company,mnm.User.company.nullable.error.mnm.Company,mnm.User.company.nullable.error,user.company.nullable.error.mnm.User.company,user.company.nullable.error.company,user.company.nullable.error.mnm.Company,user.company.nullable.error,mnm.User.company.nullable.mnm.User.company,mnm.User.company.nullable.company,mnm.User.company.nullable.mnm.Company,mnm.User.company.nullable,user.company.nullable.mnm.User.company,user.company.nullable.company,user.company.nullable.mnm.Company,user.company.nullable,nullable.mnm.User.company,nullable.company,nullable.mnm.Company,nullable]; arguments [company,class mnm.User]; default message [Property [{0}] of class [{1}] cannot be null]
grails.validation.ValidationException: Validation Error(s) occurred during save():
- Field error in object 'mnm.User' on field 'company': rejected value [null]; codes [mnm.User.company.nullable.error.mnm.User.company,mnm.User.company.nullable.error.company,mnm.User.company.nullable.error.mnm.Company,mnm.User.company.nullable.error,user.company.nullable.error.mnm.User.company,user.company.nullable.error.company,user.company.nullable.error.mnm.Company,user.company.nullable.error,mnm.User.company.nullable.mnm.User.company,mnm.User.company.nullable.company,mnm.User.company.nullable.mnm.Company,mnm.User.company.nullable,user.company.nullable.mnm.User.company,user.company.nullable.company,user.company.nullable.mnm.Company,user.company.nullable,nullable.mnm.User.company,nullable.company,nullable.mnm.Company,nullable]; arguments [company,class mnm.User]; default message [Property [{0}] of class [{1}] cannot be null]

    at mnm.UserIntegrationTests.testSaveUser(UserIntegrationTests.groovy:18)