public String getFirstName() {
            return firstName;
        }
The set of children which we are expecting to be mapped with the BeanMappingBuilder defined by
getChildMapping(), which should not map nulls in the child. Using a sorted set so that we can iterate over
the values and know they are in id order, which means both src and desc sets can easily be compared
core/src/test/java/com/github/dozermapper/core/functional_tests/builder/DeepMappingWithMapIdTest.java