Parse - downloading only updated rows of Data - ios

Question

I am running parse server on AWS. Data is being stored on mlab. At app launch I make a query:
let query = PFQuery(className: foodDataClassName_)
query.findObjectsInBackgroundWithBlock({(objects : [PFObject]?,error : NSError?) -> Void in
})
It returns me all the rows of data. I save them locally using CoreData. When any row of data is updated, I delete the previous locally stored data and download all the data again and save it. This is not a good approach. What I want is that I only download the rows which are updated not all the rows. How can I achieve that? Thanks.

Answer 1

what you can do is the following:
The first time the user log in to your app you need to query for all the user rows from the server so in this case you will execute the query without any condition. In this query you can use limit in order to limit the results that will be returned by the server. When you get results from the server you will need to:
Store all rows in your local database
Store the current NSDate inside NSUserDefaults
The next call to your server will be to get only the updated rows. In order to achieve it you will need to:
Get the last sync date from your NSUserDefaults (the one that we save above)
Execute the query but this time with a condition of greaterThan your lastSyncDate
at the end your code to fetch the items should look like the following:
// check if we synced the date before
let lastSyncDate = NSUserDefaults.standardUserDefaults().objectForKey("lastSyncDate")
let query = PFQuery(className: "MyParseObjectClassName")
if (lastSyncDate != nil){
// get only records that were created/updated since the last sync date
query.whereKey("updatedAt", greaterThan: lastSyncDate!)
}
query.findObjectsInBackgroundWithBlock {
(objects: [PFObject]?, error: NSError?) -> Void in
// store objects in parse local data store
if (objects?.count > 0){
PFObject.pinAllInBackground(objects)
}
// save the last sync date in user defaults
NSUserDefaults.standardUserDefaults().setValue(NSDate(), forKey: "lastSyncDate")
NSUserDefaults.standardUserDefaults().synchronize()
}
Please notice here i used parse local data store which allows you to store parse objects to your local data base easily without using core data. Parse local data store is provided by parse iOS SDK and save a lot of time and effort for you so i strongly recommend you to leverage it.
local data store also take care for you to objects that were created/updated and it will automatically create new object and will update existing ones.
you can read more about it in here