CoreData to-many relationship: "addXxxxlist" - how to replace the list completely

Question

I have a Person entity which has Contacts to-many relationship to an entity name Contact.

CoreData creates a NSSet property for me:

@property (nullable, nonatomic, retain) NSSet<Contact *> *contact_list;

What if I need to completely change the person's contact list? Can I just replace the whole set:

person.contact_list = newContactList

Or should I actually always use CoreData-generated accessors and mutators? e.g.:

person.removeContact_list(person.contact_list) // removing all the current contacts
person.addContact_list(newContactList) // setting a new contact list

Could the former approach cause database errors in the relationship configuration? What is preferred way to completely replace a to-many relationship set?

Answer 1

I assume the core data model is like this:

Question 1: Can I just replace the whole set: person.contact_list = newContactList or should I actually always use CoreData-generated accessors and mutators?

Answer: If all data objects are formerly saved in the database, Core Data creates the same SQL update code (i'm using Swift here but with ObjC it is only syntax differences):

Prepare some objects and save it:

    // Create Persons
    let personObjA: Person = Person(context: mainContext)
    personObjA.name = "Person A"
    let personObjB: Person = Person(context: mainContext)
    personObjB.name = "Person B"
    saveContext()
    // Create Contacts
    let contactObj1: Contact = Contact(context: mainContext)
    contactObj1.name = "Contact A"
    let contactObj2: Contact = Contact(context: mainContext)
    contactObj2.name = "Contact B"
    let contactObj3: Contact = Contact(context: mainContext)
    contactObj2.name = "Contact C"
    let contactObj4: Contact = Contact(context: mainContext)
    contactObj2.name = "Contact D"
    saveContext()

Method by assign to a new contact list:

    // Set Contacts for PersonA
    personObjA.addToContacts([contactObj1,contactObj2])
    saveContext()
    // Replace Contacts for PersonA
    personObjA.contacts = [contactObj3,contactObj4]
    saveContext()

Core Data executes this SQL:

CoreData: sql: BEGIN EXCLUSIVE
CoreData: sql: UPDATE OR FAIL ZPERSON SET Z_OPT = ?  WHERE Z_PK = ? AND Z_OPT = ?
CoreData: details: SQLite bind[0] = (int64)3
CoreData: details: SQLite bind[1] = (int64)1
CoreData: details: SQLite bind[2] = (int64)2
CoreData: sql: UPDATE OR FAIL ZCONTACT SET Z2CONTACTS = ?, Z_OPT = ?  WHERE Z_PK = ? AND Z_OPT = ?
CoreData: details: SQLite bind[0] = (int64)1
CoreData: details: SQLite bind[1] = (int64)2
CoreData: details: SQLite bind[2] = (int64)2
CoreData: details: SQLite bind[3] = (int64)1
CoreData: sql: UPDATE OR FAIL ZCONTACT SET Z2CONTACTS = ?, Z_OPT = ?  WHERE Z_PK = ? AND Z_OPT = ?
CoreData: details: SQLite bind[0] = (int64)1
CoreData: details: SQLite bind[1] = (int64)2
CoreData: details: SQLite bind[2] = (int64)3
CoreData: details: SQLite bind[3] = (int64)1
CoreData: sql: COMMIT

Method by using Core Data accessors and mutators:

    // Set Contacts for PersonB
    personObjB.addToContacts([contactObj1,contactObj2])
    saveContext()
    // Replace Contacts for PersonB
    personObjB.removeFromContacts([contactObj1,contactObj2])
    personObjB.addToContacts([contactObj3,contactObj4])
    saveContext()

Core Data will execute SQL like this:

CoreData: sql: BEGIN EXCLUSIVE
CoreData: sql: UPDATE OR FAIL ZPERSON SET Z_OPT = ?  WHERE Z_PK = ? AND Z_OPT = ?
CoreData: details: SQLite bind[0] = (int64)3
CoreData: details: SQLite bind[1] = (int64)2
CoreData: details: SQLite bind[2] = (int64)2
CoreData: sql: UPDATE OR FAIL ZCONTACT SET Z2CONTACTS = ?, Z_OPT = ?  WHERE Z_PK = ? AND Z_OPT = ?
CoreData: details: SQLite bind[0] = (int64)2
CoreData: details: SQLite bind[1] = (int64)3
CoreData: details: SQLite bind[2] = (int64)2
CoreData: details: SQLite bind[3] = (int64)2
CoreData: sql: UPDATE OR FAIL ZCONTACT SET Z2CONTACTS = ?, Z_OPT = ?  WHERE Z_PK = ? AND Z_OPT = ?
CoreData: details: SQLite bind[0] = (int64)2
CoreData: details: SQLite bind[1] = (int64)3
CoreData: details: SQLite bind[2] = (int64)3
CoreData: details: SQLite bind[3] = (int64)2
CoreData: sql: COMMIT

Question 2: Could the former approach cause database errors in the relationship configuration?

Answer: If there are simple updates all done in same thread, same context (...) as described in former answer to Question 1 there will no errors in relationship.

Question 3: What is preferred way to completely replace a to-many relationship set?

Answer: If there are simple update steps all done in same thread, same context (...) as described in former answer to Question 1 both is good.