when you save the house the array with members has to be empty. After the house is saved you can append the members like this ... .task { if let houses = await getHouses() { do { for house in houses { modelContext.insert(house) try modelContext.save() // you need a var/let variable, that contains the members of this house somewhere separate from the house object // append members to house after house is saved house.members.append(contentsOf: members) } } catch { print(error) } } }