The relative particle di may be variably omitted in a few contexts and there is cross-speaker variability of acceptability of such constructions. Omission happens most often with direct objects, but can also be found with subjects and indirect objects (cf. Huttar & Huttar 1994: 98).
Source: Goury and Migge 2003: 157
Source: Huttar and Huttar 1994: 98
Source: Huttar and Huttar 1994: 98