Grails Unit Testing with Composite Keys

I'm trying to unit test a method where an object with a composite key is being inserted into the database. Whenever I run my unit test for this, I get the following.

Failure:  |
testTransaction(com.myapp.foo.TestServiceSpec)
 |
Condition not satisfied:

Transaction.count() == 1
            |       |
            0       false

    at com.myapp.foo.TestServiceSpec.testTransaction(TestServiceSpec.groovy:166)

If I remove the composite key code and nothing else from my domain class, the test passes.

This is my Domain Class, Transaction.groovy:

class Transaction implements Serializable {
    String timestamp
    String source
    String tableName
    String fieldName
    Integer changeNumber
    String fieldValue
    String objectId

    static mapping = {
        id composite: ["timestamp", "source", "tableName", "fieldName", "changeNumber"], generator: 'assigned'
    }

    boolean equals(other) {
        if (!(other instanceof Transaction)) {
            return false
        }

        other.timestamp == timestamp && other.source == source && other.id == id && other.tableName == tableName && other.fieldName == fieldName && other.changeNumber == changeNumber
    }

    int hashCode() {
        def builder = new HashCodeBuilder()
        builder.append timestamp
        builder.append source
        builder.append tableName
        builder.append fieldName
        builder.append changeNumber
        builder.toHashCode()
    }
}

This is the code that's being tested:

def response = [code: 'OK']
def transaction = new Transaction()

transaction.timestamp  = (new Date()).format("yyyy-MM-dd HH:mm:ss.SSS")
transaction.source       = "APP"
transaction.tableName    = "TABLE_NAME"
transaction.fieldName    = "FIELD_NAME"
transaction.fieldValue   = "FIELD_VALUE"
transaction.objectId     = "OBJECT_ID"

def changeNumber = Transaction.createCriteria().get {
    eq("objid", currentTransaction.objid)
    eq("tableName", currentTransaction.tableName)
    projections {
        max("changeNumber")
    }
}

currentTransaction.changeNumber = (changeNumber ?: 0) + 1

if(!transaction.save()) {
    transaction.errors.each {
        println it
    }

    response = [code: 'error transaction', status: 500]
}

return response

And finally, here's my unit test code:

void testTransaction() {
    when:
    def response = testService.loadTransaction() // Creates transaction row

    then:
    assert response == [code: 'OK']
    assert Transaction.count() == 1
}

The domain structure was defined by another party, and I can't change it in any way, so the composite key is a must. Unfortunately many classes in this app use composite keys, so if I can't unit test them, a lot of my code can't be covered by unit testing. Any info to get me going in the right direction would be great.