multi-column unique constraint is ignored

I've defined 2 domain classes that are mapped using the "no collections" approach described [here](https://mrpaulwoods.wordpress.com/2011/02/07/implementing-burt-beckwiths-gorm-performance-no-collections/)

``` groovy
class IncomeScenario {

String title

Collection getItems() {
IncomeDelta.findAllByIncomeScenario(this)
}

def beforeDelete() {
withNewSession {
executeUpdate('delete IncomeDelta where incomeScenario = ?', [this])
}
}
}

class IncomeDelta {

IncomeScenario incomeScenario
String name

static constraints = {
// can't have multiple income scenario items with the same name
name unique: 'incomeScenario'
}
}
```

However, it seems this constraint is ignored because I'm not prevented from saving an `IncomeScenario` that has multiple items with the same name, e.g.

``` groovy
def scenario = new IncomeScenario(title: 'foo').save()
new IncomeDelta(name: 'name', incomeScenario: scenario).save()

// this one should fail, but it doesn't
new IncomeDelta(name: 'name', incomeScenario: scenario).save(failOnError: true)
```

Start by running the issue's two IncomeScenario/IncomeDelta save calls and confirm that the duplicate name is accepted despite the `name unique: 'incomeScenario'` constraint. Trace the constraint and persistence behavior for this no-collections mapping; done means the second save is rejected as the example expects.

Written by the indexing model from the issue text.