o.mu.RLock()
	defer o.mu.RUnlock()
	return slices.Clone(o.h)
}

// multiIndex stores input and output objects.
// Each input and output can be looked up by its key.
// Additionally, a mapping of input key -> output keys stores the transformation.
type multiIndex[I, O any] struct {
	outputs  map[Key[O]]O
	inputs   map[Key[I]]I
	mappings map[Key[I]]sets.Set[Key[O]]
}

			}
		}
		checkMulIndex = func(t *testing.T) {
			checkColumnType(t, "name", false)
			checkColumnType(t, "nick_name", false)
			if !DB.Migrator().HasIndex(table, mulIndex.Name()) {
				t.Errorf("%v: should has index %s but not", utils.FileWithLineNum(), mulIndex.Name())
			}
		}
	}

	tests := []TestCase{
		{name: "notUnique to notUnique", from: &UniqueStruct1{}, to: &UniqueStruct2{}, checkFunc: checkNotUnique},