WhateverSpec.groovy

@Grapes([
    @Grab(group='org.mongodb', module='mongo-java-driver', version='3.11.0'),
    @Grab(group='org.spockframework', module='spock-core', version='1.3-groovy-2.5'),
    @Grab(group='cglib', module='cglib-nodep', version='3.2.10'),
])

import org.bson.types.ObjectId
import spock.lang.Specification
import spock.lang.Subject
import spock.lang.Shared

class WhateverSpec extends Specification {

    @Shared
    @Subject
    def mapper = new MsgMapper()

    def 'should map IDs properly'() {
        given: 
        def ids = [
            new MsgId(id: new ObjectId('1' * 24)),
            new MsgId(id: new ObjectId('2' * 24)),
        ]
    
        and:
        def longMsg = Stub(LongMsg)
        longMsg.getPartsIds() >> ids
      
        when:
        def mapped = mapper.map(longMsg.partsIds)
        
        then:
        mapped*.id == ids*.id
    }

}

class MsgId {
    ObjectId id
    
    String toString() {
        id.toString()
    }
}

class LongMsg {
    String[] parts
    String[] partsIds
}

class MsgMapper {

    List<MsgId> map(String[] rawIds) {
        rawIds.collect { new MsgId(id: new ObjectId(it)) }
    }
}