Understanding equality in RealmSwift - swift

Question

I have some items which I fetch from Realm:
let items = realm.objects(ShoppingListItem.self)
print("\(items.count) items") // 10 items
Each item has a subdepartment, and each subdepartment has a department:
let departments = items.flatMap({ $0.product?.subdepartment?.department })
print("\(departments.count) departments") // 10 departments
My goal is to find the unique Department objects from this list. The expected result is 4. My standard approach here is to use Set:
let uniqueDepartments1 = Set(departments)
print("\(uniqueDepartments1.count) unique departments via Set") // 9 unique departments via Set - but changes every time!?
I figure there must be something I'm missing related to the way Realm treats equality. But to check, I also attempt to get the unique departments via enumeration:
var uniqueDepartments2 = [Department]()
for department in departments {
if uniqueDepartments2.contains(department) {
continue
}
uniqueDepartments2.append(department)
}
print("\(uniqueDepartments2.count) unique departments via enumeration") // 4 unique departments via enumeration
This is indeed the expected result.
Why doesn't Set work here as I expected it to? And why is the count changing each time I run it?
Edit 2/27
Here are the models in play:
class ShoppingListItem: Object {
dynamic var product: Product?
convenience init(ingredient: Ingredient) {
self.init()
self.product = ingredient.product
}
}
class Product: Object {
dynamic var productID, subdepartmentID, title: String?
dynamic var subdepartment: Subdepartment?
}
class Department: Object {
dynamic var departmentID, title: String?
}
class Subdepartment: Object {
dynamic var subdepartmentID, departmentID, title: String?
dynamic var department: Department?
}

Answer 1

In short: in order for an Object subclass to be correctly hashable you must currently declare a property as the primary key.
Set is built on top of a hash table. This means it checks for the existence of a specific object by first computing the hash value of the object, mapping that hash value to a bucket within the hash table, and then checking each entry in that bucket for equality with the specified object.
The nature of this implementation means that for Set to work correctly on objects of a given type, both the hashValue property and == operator must obey specific rules. In particular, any two objects for which == returns true must return the same value from their hashValue property (the reverse is not required; it's completely valid for two unequal objects to have the same hashValue). Realm's implementation's of hashValue and == don't currently meet this criteria unless your class declares a property as the primary key. When no primary key is declared, Object's default hashValue calculation defers to -[NSObject hash], which simply returns the object's address in memory. Since Object.== allows two objects with different addresses to compare equal, this violates the relationship between hashValue and == I outlined above, leading to incorrect behavior when used with Set or as the key of a Dictionary. I've filed a bug against Realm requesting that the behavior of == be fixed to be compatible with the value returned by the hashValue property.