io.objectbox.exception.DbDetachedException: Cannot resolve relation for detached entities

Question

While unit testing the class that is working with ObjectBox database I got this error:

io.objectbox.exception.DbDetachedException: Cannot resolve relation for detached entities

I have implemented all the necessary data into my Entity class for unit testing on macOs. So the entity relation with a to-one and a to-many relation pointing to itself.

@Entity
data class Category(
    @Id var id: Long? = 0,
    var title: String?,
    @JvmField var parent: ToOne<Category>? = null,
    @JvmField @Backlink(to = "parent") var subCategories: ToMany<Category>? = null){

    constructor(): this(id = 0, title =  "", parent = null, subCategories = null)
    constructor(title: String): this(id = 0, title = title, parent = null, subCategories = null)

    // normally transformer would add field, but need to manually for local unit tests
    @JvmField @Transient var __boxStore: BoxStore? = null

    init{
        if(parent == null) parent = ToOne(this, Category_.parent)
        if(subCategories == null) subCategories = ToMany(this, Category_.subCategories)
    }
}

Unit test is very simple:

public class CategoryModelDataMapperTest {

    private CategoryModelDataMapper categoryModelDataMapper = new CategoryModelDataMapper();

    @Before
    public void setUp() throws Exception {
        categoryModelDataMapper = new CategoryModelDataMapper();
    }

    @Test
    public void testShouldTransformDomainModelWithoutParentToEntityCorrectly() throws Exception{
        final Category category = new Category(10L, "Bar", null, null);
        final CategoryModel categoryModel = categoryModelDataMapper.transform(category);

        assertEquals((Long) 10L, categoryModel.getId());
        assertEquals("Bar", categoryModel.getTitle());
        assertNull(categoryModel.getParent());
        assertNotNull(categoryModel.getSubCategories());
        assertTrue(categoryModel.getSubCategories().isEmpty());
    }
}

And actual class that I am testing is normal mapper:

class CategoryModelDataMapper: BaseMapper<Category, CategoryModel>{

    override fun transform(from: Category): CategoryModel {
        val toModel = CategoryModel()
        toModel.id = from.id
        toModel.title = from.title

        toModel.parent = from.parent?.target?.let { transform(it) }

        from.subCategories?.let {
            it.forEach{ toModel.subCategories?.add( transform(it)) }
        }

        return toModel
    }

    override fun transform(fromList: MutableList<Category>): MutableList<CategoryModel> {
        val toList = ArrayList<CategoryModel>(fromList.size)
        return fromList.mapTo(toList) { transform(it) }
    }
}

I have some other tests that are testing the parent and subCategories in more detail, but I got the same error in all of the tests. Before it was working fine, but that something went wrong.

Answer 1

Do you have a full stack trace? Otherwise it is hard to pinpoint the issue.

I am assuming that the exception originates somewhere here (?):

from.subCategories?.let {
    it.forEach{ toModel.subCategories?.add( transform(it)) }
}

And that CategoryModel is also an entity with @Id(assignable=true)?

toModel.id = from.id

In that case make sure to call box.attach(toModel) before modifying the ToMany:

CategoryModel toModel = CategoryModel()
toModel.id = from.id
box.attach(toModel) // need to attach box first
toModel.subCategories.add(...)

Source: See the Updating ToMany section in the ObjectBox Relations documentation.