JOIN TABLES parse.com - ios

Question

I have two parse tables POST and COMMENT.
I want to get all post where there are new comments. I am making checks based on viewedDate field in post table and createdAt field in Comment Table.
SELECT *
FROM POST p
INNER JOIN COMMENT c
ON p.objectId==c.pId and c.createdAt > p.viewedDate;
PFQuery *post = [ParsePost query];
[post findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
PFQuery *post = [ParsePost query];
PFQuery *comment = [ParseComment query];
//how add greater than for keys from other table?
//line below crashes
[comment whereKey:#"createdAt" greaterThan:[objects valueForKey:#"viewedDate"]];
[comment whereKey:#"post" matchesKey:#"objectId" inQuery:post];
[post whereKey:#"objectId" matchesKey:#"post" inQuery:comment];
[post findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
}];
}];
UPDATE
*This is how i do it currently*
PFQuery *postView = [ParsePost query];
[postView whereKey:#"author" equalTo:[ParseLigoUser currentUser]];
[postView findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
for (ParsePost *post in objects) {
PFQuery *comment = [ParseComment query];
[comment orderByDescending:#"createdAt"];
if (post.viewedDate) {
[comment whereKey:#"createdAt" greaterThan:post.viewedDate];
}else {
continue;
}
[comment whereKey:#"post" equalTo:post];
[comment countObjectsInBackgroundWithBlock:^(int number, NSError *error) {
if (number>0) {
if (!self.newposts) {
self.newposts = [NSMutableDictionary dictionary];
}
[self.newposts setObject:#"NOTSEEN" forKey:[post.objectId stringByAppendingString:#"Comment"]];
self.postCount += 1;
}
}];
}
}];
UPDATE 2 with relations
PFQuery *postView = [ParsePost query];
[postView whereKey:#"author" equalTo:[ParseLigoUser currentUser]];
[postView setLimit:100];
[postView findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
for (ParsePost *post in objects) {
PFRelation *rel = [post relationForKey:#"hasComment"];
PFQuery *query = [rel query];
[query orderByDescending:#"createdAt"];
if (post.viewedDate) {
[query whereKey:#"createdAt" greaterThan:post.viewedDate];
}else {
continue;
}
[[rel query] countObjectsInBackgroundWithBlock:^(int number, NSError *error) {
if (number>0) {
if (!self.newposts) {
self.newposts = [NSMutableDictionary dictionary];
}
[self.newposts setObject:#"NOTSEEN" forKey:[post.objectId stringByAppendingString:#"Comment"]];
self.postCount += 1;
}
}];
}
}];

Answer 1

Your problem isn't really the lack of join table queries on parse, but rather that you designed your database with a sql mindset instead of a NoSQL mindset (which of course is common for all who comes from the relation db world).
When designing your database "schema" for parse (or any other NoSQL database) you need to free yourself from thinking in relations and normalization etc to thinking in data access. How will you access your data? Start by thinking on how you will query your data, and then design your db to optimize for these queries. The most important for your mobile app is to minimize the number of connections to a remote server, and to minimize client side handling.
There are several ways to fix solve your current problem. You can, of course, create some queries that will work around the lack of join table queries, and handle some of this on the client. That could probably work short-term if this feature needs to be implemented quickly.
A long-term approach would be to redesign your schema to meet your requirement of easily retrieving posts that have new comments.
One solution (of several possible):
Create a new property in the Post class: “newcomments” that is a boolean.
Create a cloud code snippet that updates the newcomments property in Post whenever a new comment is created (sets it to TRUE).
This snippet should be run in the afterSave hook for Comment (https://parse.com/docs/cloud_code_guide#functions-aftersave)
Then, whenever you open a post to see the new comments, you reset this field to FALSE in the background.
Now you can query for posts where newcomments equalTo false
Or, instead of newcomments being a boolean, you could just as well use it for storing an array of pointers to the actual new comments (the afterSave hook updates this array with the pointer to the new comment). This way, you don’t need a second query for getting the new comments once you open the post. Here, you clear the newcomments property as soon as you’ve read the comments (or opened the post and have an array of the comments).
This storing of an array probably strikes a bad note in your SQL mindset, but this is one of many differences between SQL and NoSQL, since the latter is more focused on query efficiency than storage and consistency.
Or, if you don’t want to store this in the Post object, you could create a new PostTracker (or whatever) class to handle this. Maybe there are other things you would want to track (certainly, there might be in the future, even if you don’t have an idea for that at the moment).