Assigning a mixed property with a collection should replace instead of merge

[sync-by-unito]

The current API for setting a mixed field in all SDKs should behave as a replace, even if the server is capable of merging lists. This is because SDKs only expose assignment operations - i.e. after:

foo.mixed = [1, 2, "abc"]

users would expect var bar = foo.mixed matches what they assigned to it, not something merged. We haven't designed an API that would allow merging, but if we did, it would look differently and be more explicit about the expected outcome - like:

var list = foo.mixed.getOrCreateList(); list.add([1, 2, "abc"]);

At the moment this test is failing

@MainActor func testAssignMixedListWithSamePrimaryKey() async throws {
        let realm1 = try await openRealm()
        let results1 =  try await realm1.objects(SwiftTypesSyncObject.self).where { $0.stringCol == name }.subscribe()
 
        let realm2 = try await openRealm()
        let results2 =  try await realm2.objects(SwiftTypesSyncObject.self).where { $0.stringCol == name }.subscribe()
 
        let primaryKey = ObjectId.generate()
 
        let object = SwiftTypesSyncObject(id: primaryKey)
        object.stringCol = name
        object.anyCol = AnyRealmValue.fromArray([.string("John")])
        try realm1.write {
            realm1.add(object)
        }
 
        let object2 = SwiftTypesSyncObject(id: primaryKey)
        object2.stringCol = name
        object2.anyCol = AnyRealmValue.fromArray([.string("Marie")])
        try realm2.write {
            realm2.add(object2)
        }
 
        try await realm1.syncSession?.wait(for: .upload)
        try await realm2.syncSession?.wait(for: .upload)
        try await realm1.syncSession?.wait(for: .download)
        try await realm2.syncSession?.wait(for: .download)
 
        XCTAssertEqual(results1.first?.anyCol.listValue?.count, 1)
        XCTAssertEqual(results2.first?.anyCol.listValue?.count, 1)
        XCTAssertEqual(results1.first?.anyCol.listValue?[0], results2.first?.anyCol.listValue?[0])
    }

Even though we clear the collection before assigning to it, this is because the clear instruction is not been sent to the server.

[sync-by-unito]

➤ PM Bot commented:

Jira ticket: RCORE-2168

[sync-by-unito]

➤ dianaafanador3 commented:

This branch https://github.com/realm/realm-core/tree/dp/clear-before-assign-collections-in-mixed fixes the issue but I don't know the implications of sending a clear instructions even if the collection is empty in other uses cases

[sync-by-unito]

➤ danieltabacaru commented:

I find the title a bit misleading. What I think we want to do is the SDKs to clear the list before inserting the elements and core to replicate the clear instruction.

[nirinchev]

Yeah, to clarify, the request here is that Core unconditionally replicates the clear instruction, even if the collection is empty. Then SDKs can implement the replace semantics for collections in mixed.