This gist is some of the code for reference for my video https://www.youtube.com/watch?v=EJdzTJX7AJo

UpdateMergePolicy.swift

//
//  UpdateMergePolicy.swift
//  CourseQuery
//
//  Created by Adar Hefer on 13/10/2019.
//  Copyright © 2019 Adar Hefer. All rights reserved.
//

import CoreData

class UpdateMergePolicy: NSMergePolicy {
    override func resolve(constraintConflicts list: [NSConstraintConflict]) throws {
        for conflict in list {
            guard let databaseObject = conflict.databaseObject,
                let conflictingObject = conflict.conflictingObjects.first else {
                try super.resolve(constraintConflicts: list)
                return
            }
            
            switch databaseObject {
            case let course as Course:
                guard let conflictingCourse = conflictingObject as? Course else {
                    try super.resolve(constraintConflicts: list)
                    return
                }
                
                conflictingCourse.isAdded = course.isAdded
                conflictingCourse.color = course.color
                conflictingCourse.justUpdated = true
                
            case let session as CourseSession:
                guard let conflictingSession = conflictingObject as? CourseSession else {
                    try super.resolve(constraintConflicts: list)
                    return
                }
                
                conflictingSession.isAdded = session.isAdded
                conflictingSession.isSelected = session.isSelected
                conflictingSession.color = session.color
                conflictingSession.eventIdentifier = session.eventIdentifier
                conflictingSession.justUpdated = true
                
            case let exam as CourseExam:
                guard let conflictingExam = conflictingObject as? CourseExam else {
                    try super.resolve(constraintConflicts: list)
                    return
                }
                
                conflictingExam.isAdded = exam.isAdded
                conflictingExam.isSelected = exam.isSelected
                conflictingExam.color = exam.color
                conflictingExam.eventIdentifier = exam.eventIdentifier
                conflictingExam.justUpdated = true
            default:
                try super.resolve(constraintConflicts: list)
                return
            }
        }
        
        try super.resolve(constraintConflicts: list)
    }
}