Connecting remote search results with local database using CoreData - ios

Question

Assume we have simple data model with single entity User; simple tableView_friends with fetchedResultsController_friends for show users - friends.
Assume we have search bar for searching all (not only friends) users in service, and for every typed in it character we perform search request to server, which return to us somehow filtered by character User objects. Some of this objects can already be inside local database. By app logic we don't really must save all this results in local database forever (but ok, we can, we can clear local database time to time); on other hand, if we will perform any action on some searched user, we must store this user. We want to show list of searched user in other tableView_search with fetchedResultsController_search.
Question: should I use same context for fetchedResultsController_friends and fetchedResultsController_search? If no, how can I handle situation, when I wish to edit searched user, which already exists in database and probably already local edited? If yes, how can I setup predicate for fetchedResultsController_search (server perform its own logic for search by character, which can be changed) for show exactly same result as from server?

Answer 1

We recently implemented a search feature in our application and had a similar issue, We had local data in core data and also remote data from our API.
You have a few options that we explored:
Save your data into core data from the API as it is retreived and
then the fetched results controller will do the rest
Manage the merge of the data yourself, you can still use NSFetchedResults controller to an extent but need to do more work
We didn't want to save all of the information returned from the API unless it was needed (the user selected it), so we come up with a simple solution that worked for our app. This may not work directly for your app, you may need a completely different solution or change some of the things we done to suit.
Firstly, To explain what we are dealing with, we had a Article entity in core data which contains around 25 properties, the API returns article objects as JSON data with the same data.
What we decided to do was to create a class which represents a simple version of an article (just enough data to show in a list view and reference it later in the API or core data) which looked something like this:
class SearchResult: NSObject {
var id:String?
var title:String?
var imageUrl:String?
var url:String?
// core data entity
init(article:Article) {
self.id = content.contentId
self.title = content.title
self.featuredImageURL = content.absoluteImagePath()
self.urlAlias = content.urlAlias
self.publishedAt = content.publishedAt
}
init(articleDictionary:NSDictionary) {
self.id = articleDictionary.objectForKeyNotNull("id") as? String
self.title = articleDictionary.objectForKeyNotNull("title") as? String
self.url = articleDictionary.objectForKeyNotNull("url") as? String
if let imageUrl = articleDictionary.objectForKeyNotNull("imageUrl") as? String {
self.imageUrl = imageUrl
}
}
}
Now using this, we can create once of these from either the core data results or from the API results. Our tableview datasource is just an array
var dataSet = [SearchResult]()
We use the NSFectchResultsController delegate methods to add/remove/re-order core data elements from the dataSet after the initial load and when we get API data we'll do something like:
dataSet = Array(Set(apiResponseArray + dataSet))
This will take an array of SearchResult items from the API, merge them with the current result set and remove duplicates. casting to a set and then back to an array will give you an array of unique results as a Set is made of unique values only.
See this reference which should help with how the delegate methods would work