Mocking a method that edits the input

I am trying to mock a dao method that creates a record then sets the id on the input.

here is some psuedo code

sample test

RecordDto input = new RecordDto();
input.setId(null);
input.setName("test");

when(dao.createRecord(any(RecordDto.class)).thenReturn(1);

createSomething(input);

method to mock

public int createRecord(RecordDto input) {
   int updatedRowCount = 0;
   int primaryKey = db call to create record
   rows += primary != null ? 1 : 0;
   input.setId(primaryKey) <-
   return updateRowCount;
}

class under test

public void createSomething(RecordDto input)
   int rowsUpdated = dao.createRecord(input);
   if (input.getId() != null) {
      do something <- this code can not be reached from my test
      because my mock only sets the return value. It needs to also
      modify the input.id.
   }

Once the method is called in my real class input has an id. My mock just returns a 1 for the row count but the id is still null.

If you require some additional mocking behavior other than simply returning a value, you can use thenAnswer instead of thenReturn in your when mocking stub.

I hope this (overly) simplified example helps:

   @Mock
   private Bar executor;

   @Test
   public void testMutation() {
      Foo foo = new Foo();

      when(executor.mutate(foo)).thenAnswer(a -> {
         int mockedId = 10;
         Foo input = a.getArgument(0);
         input.setId(mockedId); // mutate the parameter from the method signature
         return mockedId;
      });

      executor.mutate(foo);
      Assertions.assertEquals(10, foo.getId());
   }

   //---- boiler plate code of the DTO and method that mutates the value
   public class Bar {

      public int mutate(Foo input) {
         input.setId(3);
         return 3;
      }
   }

   public class Foo {
      private int id;

      public int getId() {
         return id;
      }

      public void setId(int id) {
         this.id = id;
      }
   }

My guess is that you are only mocking the value return but not the side effect inside the dao method. You are passing the RecordDto as parameter, doing something and also calling methods that modify the state of the parameter received. The solution for that would be to use doAnswer that lets you not only return a value, you can add some extra logic. So, your test should look like this:

@Test
public void test() {
    dao = mock(Dao.class);
    RecordDto input = new RecordDto();
    Integer someRandomId = 15;
    Integer randomRowsUpdated = 3;
    Answer<Integer> answer = new Answer<Integer>() {
        public Integer answer(InvocationOnMock invocation) {
            RecordDto input = invocation.getArgument(0, RecordDto.class); // get the argument received in `createRecord`
            input.setId(someRandomId); // set the new id
            return randomRowsUpdated; // the value to be returned by `createdRecord`
        }
    };
    doAnswer(answer).when(dao).createRecord(input); // using doAnswer instead of doReturn
    createSomething(input);
}

That would be the quick solution.

I would say that this is not exactly the best approach. Doing side effects inside a method could cause bugs in a future and it will hard to know the root cause of the problem or even debug. Returning a RecordDto or another custom object that lets you now the new id and the affected rows or something similar will make your tests easier to setup and also make the assertion.