Mapstruct map list to an object containing list

Question

I want to have a 2 way mapping from a List of an object to an Object which contains a List.

class Person {
  String firstName;
  String lastName;
}

class Group { // Source
  List<Person> people;
  String groupID; 
  ...
}

class Employee { // target
  String firstName;
  String lastName;
  String employeeNumber;
  ...
}

I used ReportingPolicy.IGNORED to ignore all the irrelevant fields. I just want a mapping between Group to List with the fields firstName and lastName.

Is it possible at all? I have tried but it's giving me error during build "impossible to map iterable to non-iterable."

@Mapping(target="people", source".")
Group map(List<Employee>)

Eugene · Accepted Answer · 2022-02-19 00:38:44Z

6

Mapstruct does not support such kind mapping. See Mapstruct contributor answer regarding this problem.
Like workaround you can implement your own mapping method for converting to wrapped class.

@org.mapstruct.Mapper
public interface Mapper {
    List<Person> map(List<Employee> employees);

    Person map(Employee employee);

    default Group mapGroup(List<Employee> employees) {
        Group group = new Group();
        group.people = map(employees);
        return group;
    }
}

answered Feb 19, 2022 at 0:38

Eugene

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Filip · Accepted Answer · 2022-02-22 16:52:11Z

4

MapStruct supports mapping from a source parameter to a target parameter. This means that you can configure MapStruct to map the source list parameter to a target list property. However, as pointed out in https://stackoverflow.com/a/71181377/1115491 it is not possible to map form a single collection parameter to a target parameter. The only way it works is if there are multiple source parameters.

e.g.

@Mapper
public interface Mapper {

    @Mapping(target = "people", source = "employees")
    @Mapping(target = "groupID", source = "groupId")
    Group map(String groupId, List<Employee> employees);

}

edited Feb 22, 2022 at 16:52

answered Feb 22, 2022 at 15:32

Filip

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4 Comments

Eugene Over a year ago

It is not working in 1.5.0.Beta2. I checked this case.

Filip Over a year ago

You are right, @Eugene. I was thinking about another use case we used to have. It is working correctly if there are 2 source parameters. I'll update the answer

stackyi Over a year ago

This is exactly what I was trying to do. Mapping from an iterable to another iterable inside of an object (and vice versa). But I always get a compilation error "Can't generate mapping method from non-iterable type to iterable type". No matter how I change the source/target.

Eugene Over a year ago

The updated answer is working well. I have just tested it. I will undo the decrease point.

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