I'm trying to figure out the proper way to paginate results using the Parse SDK for iOS. The problem I'm having is that the content I'm trying to paginate has the potential to change frequently.
For example:
There is an app with a bunch of objects stored on a server, and the request to get these objects is sorted with newest first and paginated in case there are a lot of objects. When the user scrolls, as they are approaching the last object on the current page, the next page gets requested.
Say the user scrolls to page two, then a new object is added to the list of objects, and the user then scrolls up another page. When the next page is requested, they will get a duplicate of the last message on the second page, and since page 0 has already been loaded, the new object won't be displayed.
How can something like this be handled?
How would the implementation change if the objects were sorted oldest first?
Thanks in advance for the help.
Edit: request pseudo code
- (void)fetchObjectsForPage:(NSUInteger)page completion:(void (^)(NSArray *_Nullable objects, PageInfo *_Nullable pageInfo, NSError *_Nullable error))completion{
PFQuery *query = [SomeObject query];
[query orderByAscending:#"updatedAt"];
[query setLimit:50];
[query setSkip:50 * page];
[query findObjectsInBackgroundWithBlock:^{NSArray *_Nullable objects, NSError *_Nullable error){
...
>>> Do error checking and return array along with paging info (total number of objects, page size) or error in completion block.
}];
}
It's not the request I'm having trouble with, it's figuring out how to properly handle paging in the table when a new object gets added.
What you can do is the following:
Create a query that will return 50 rows (like you did) order by updatedAt
When you get result take the last item from the list and save the item updatedAt in some global property
In the next call do not use setOffset but get the next 50 rows and add another condition of where updatedAt is greater than the updatedAt of your global object. This way you make sure that you will not have duplicate records in your list
At the end your code should look like the following:
PFQuery *query = [PFQuery queryWithClassName:#"SomeClassName"];
[query orderByAscending:#"updatedAt"];
[query setLimit:50];
if (self.lastItemUpdatedAt){
[query whereKey:#"updatedAt" greaterThan:self.lastItemUpdatedAt];
}
[query findObjectsInBackgroundWithBlock:^(NSArray * _Nullable objects, NSError * _Nullable error) {
if (objects.count > 0){
self.lastItemUpdatedAt = [[objects lastObject] updatedAt];
}
}];
Now if you use updatedAt there is a chance that you will still get duplicates because let's assume that someone change the object so the updatedAt will also be changed (so it will be greater than the updatedAt that you saved) in this case you can use createdAt instead of updatedAt since createdAt will never be changed
Related
When I run the following
PFQuery *query = [PFQuery queryWithClassName:#"EventsTable"];
[query setCachePolicy:kPFCachePolicyNetworkOnly];
// Only active items
[query whereKey:#"active" equalTo:[NSNumber numberWithBool:YES]];
// Query by distance from current location
[query whereKey:#"geoLocation" nearGeoPoint:currentLocation];
// Only download objects that match the appropriate key
[query whereKey:#"keyedItems" containedIn:keysArray];
[query findObjectsInBackgroundWithBlock:^(NSArray *eventArray, NSError *error) {
It goes into an infinite loop with no error messages. But If I drop the nearGeoPoint it works fine, or if I keep the nearGeoPoint but drop the active it also works. But dropping the containedIn and keeping the nearGeoPoint and the active also fail the same way. I have also tried rearranging the order but no luck.
Any suggestions?
At first I wasn't able to find an error message so I assumed that it was stuck in the block, but alas here it is
Error: Error Domain=Parse Code=1 "{"code":1,"message":"Internal server error."}" UserInfo={error={"code":1,"message":"Internal server error."}, NSLocalizedDescription={"code":1,"message":"Internal server error."}, code=1} {
NSLocalizedDescription = "{\"code\":1,\"message\":\"Internal server error.\"}";
code = 1;
error = "{\"code\":1,\"message\":\"Internal server error.\"}";
}
This sounds like a parse server error to me - Is that correct?
The issue is that the table was so large it was unable to handle the query. The fix was just as mentioned above - to create an index. In this case I was able to turn on automatic indexing in the MongoDB. That solved the problem.
So in my app I am using Parse for user accounts. I have a class "Follow" with "to" and "from" fields containing user object ids, allowing users to follow each other. Now, if I delete a user somehow, the follow relation remains in tact and querying empty user data raises an object not found error. What I want to know is how I can delete a follow relation if any of the "to" or "from" fields contains an objectID of an object that does not exist.
I have tried querying objects by their objectID, but any attempt at checking empty data (like checking if a user.username is nil) has resulted in a missing object error, and I can't check if an object is nil because Xcode says it never will be.
Thanks!
I think you need to acquire it manually, if you have deleted user, then you can delete all of its relations to maintain Database integrity, following is the code you would calling after deleting any User.
PFQuery * _to = [PFQuery queryWithClassName:#"Follow"];
[_to whereKey:#"to" equalTo:#"Replace UserID of Deleted User"];
PFQuery * _from = [PFQuery queryWithClassName:#"Follow"];
[_from whereKey:#"from" equalTo:#"Replace UserID of Deleted User"];
PFQuery *query = [PFQuery orQueryWithSubqueries:#[_to, _from]];
[query findObjectsInBackgroundWithBlock:^(NSArray *results, NSError *error) {
if (!error) {
// The find succeeded.
NSLog(#"Successfully retrieved %d relations.", results.count);
// Do something with the found objects, lets delete them all to intact relationship
[PFObject deleteAllInBackground:results];
} else {
// Log details of the failure
NSLog(#"Error: %# %#", error, [error userInfo]);
}
}];
Please follow this link for detailed description and information:
Parse Compound Queries
I am trying to implement functionality to store data about user's friends (request friend, received friend request, accepted request). I have two tables.
First table is _User with username column.
Second table is Friends. This table is keeping track of who are friends of the user. This table has two fields:
A pointer column to user table
An array column called receivedRequest. In this array I keep the _User's objectId who are send request to that user.
Table _User has one to one relation with Friends table but meanwhile there is an array field keeping information of user's friend. In this array I am saving objectId of other users. I am using an array to avoid repeating rows for each friend's request.
Fist I want to know if this is a good idea or there is any alternative better that this. Actually I have extra array columns which is recived requests. Send requests. And accepted requests. All of them are array.
Second I want to know how can I write a query to go to Friends table. Find current user row. Go to friendList column. Return name of each friends whose name is in that array?
Currently I am doing this:
- (PFQuery *)queryForTable {
//Query user's friend request
PFQuery *query = [PFQuery queryWithClassName:#"Friends"];
[query whereKey:#"user" equalTo:[PFUser currentUser]];
[query includeKey:#"receivedRequest"];
return query;
}
This is returning only Id of the use's added my current user. I need their name from _User table.
Here's what I'd do:
(1) the User class ought to be about the user's relationship with the app, a place for data that's just between the user and the app.
(2) For data that users want to share, have a Persona class that has an image, nickname, etc. Persona should contain a pointer to User and vice versa.
(3) Personae (Personas in common usage) make friend invitations and become friends.
(4) arrays of string object ids = bad, arrays of pointers = good. In fact, I can't think of a circumstance where I'd prefer a string object id over a pointer.
(5) A FriendInvitation ought to be its own object, where inviter and invitee are pointers to Persona.
(6) A friendship is a bilateral and symmetrical relationship (at least we always hope they are). A good representation for that might be a Friendship class that has an array of pointers to exactly two Persona objects.
Here are a few functions, given a data model:
Persona has a pointer to User, call it 'user', and User has a persona pointer. FriendInvitation has an inviter and invitee, both pointers to Persona. Friendship has an array of two pointers to Persona, call it friends
// get the current user's FriendInvitations
- (void)friendInvitationsWithCompletion:(void (^)(NSArray *, NSError *))completion {
PFObject *persona = [PFUser currentUser][#"persona"];
PFQuery *query = [PFQuery queryWithClassName:#"FriendInvitation"];
[query whereKey:#"invitee" equalTo:persona];
[query includeKey:#"inviter"];
[query findObjectsInBackgroundWithBlock:completion];
}
// get the current user's friendships
// remember, these are not the friends, but the objects that record pairings of friends.
// see the next function for the friends
- (void)friendshipsWithCompletion:(void (^)(NSArray *, NSError *))completion {
PFObject *persona = [PFUser currentUser][#"persona"];
PFQuery *query = [PFQuery queryWithClassName:#"Friendship"];
[query whereKey:#"friends" equalTo:persona];
[query includeKey:#"friends"];
[query findObjectsInBackgroundWithBlock:completion];
}
// get the current user's friends' personae
- (void)friendsWithCompletion:(void (^)(NSArray *, NSError *))completion {
PFObject *persona = [PFUser currentUser][#"persona"];
[self friendshipsWithCompletion:^(NSArray *friendships, NSError *error) {
if (!error) {
NSMutableArray *result = [#[] mutableCopy];
for (PFObject *friendship in friendships) {
NSArray *friends = friendship[#"friends"];
NSInteger indexOfFriend = ([friends indexOfObject:persona] == 0)? 1 : 0;
[result addObject:friends[indexOfFriend]];
}
completion(result, nil);
} else {
completion(nil, error);
}
}];
}
// agree to be friends with someone
- (void)becomeFriendsWith:(PFObject *)friend completion:(void (^)(BOOL, NSError *))completion {
PFObject *persona = [PFUser currentUser][#"persona"];
PFObject *friendship = [PFObject objectWithClassName:#"Friendship"];
friendship[#"friends"] = #[ persona, friend ];
[friendship saveInBackgroundWithBlock:completion];
}
// we could go on, but this should convey the basic ideas
Friends table should not have arrays, but single IDs (actually pointers). So for every incoming request or friendship, there should be a single, separate entry on the database. So your Friends object (or for a better name, Relationship, or Friendship, but that's my personal preference of course) should roughly have following properties:
first (_User)
second (_User)
type (String. Possible values: 'friends' or 'request', maybe even 'blocked')
And for every accepted request, make sure you are creating two entries, one with first=user1/second=user2 and one with first=user2/second=user1. While you could technically go without making double entries, it will just complicate things in the long run, making everything harder to maintain.
i'm using Parse.com I wanted to ask you something if you can ... How do I delete from the list the total number of users CurrentUser? I would avoid that the report will also be made with oneself: D Thank you all
Perfect Guys .. then I found the method ... In layman's terms to exclude the PFUser Current User from a query to show only the App users but not the CurrentUser is just the use of notEqualTo in the query but it should be associated with the string that we want to remove from the query
I hope this can help other people :)
Do not pay too much attention to the constants: D
PFQuery * query = [self.RelazioniUtenti query];
[query whereKey: FF_USER_NOMECOGNOME notEqualTo: [[PFUser currentUser] objectForKey: FF_USER_NOMECOGNOME]];
[query orderByAscending: FF_USER_NOMECOGNOME];
[query findObjectsInBackgroundWithBlock: ^ (NSArray * objects, NSError * error) {
Is there any way to do a query in Parse to get the most recently added PFObject type?
I know that I can do a query with a greater than criteria, but it would be very useful if there was a function to get the most recent object added when no date is known.
Just add a descending order on createdAt and get the first object:
PFQuery *query = [PFQuery queryWithClassName:#"YourClassName"];
[query orderByDescending:#"createdAt"];
[query getFirstObjectInBackgroundWithBlock:^(PFObject *object, NSError *error) {
// code
}];
In Swift:
var query = PFQuery(className: "YourClassName")
query.orderByDescending("createdAt")
query.getFirstObjectInBackgroundWithBlock {(object: PFObject?, error: NSError?) -> Void in
// code
}
This may not address the use case in question. I often want to get back the objectId of the object i just added in code. While it might be automatically possible, i have missed any info in this regard. So i usually have my own unique identifier that i add, and then retrieve the object by that identifier in the async block.. thus getting the objectId..