doesNotMatchKey:inQuery: return records that match

问题:

I'm trying to do a simple query in xcode using the Parse SDK where I am showing products to users but i don't want to show them products they've already "liked."

So I have two tables in Parse that I'm using:
- Products: has an objectId
- Likes: contains columns: Like (says yes/ no), Pointer to a product, Pointer to a user

Any my code is:

//query will get products that the user has liked
PFQuery *likeQuery = [PFQuery queryWithClassName:@"Likes"];
[likeQuery whereKey:@"User_Obj" equalTo:[PFUser currentUser]];

//product query will get products that are not in like query
PFQuery *prodQuery = [PFQuery queryWithClassName:@"Products"];
[prodQuery whereKey:@"objectID" doesNotMatchKey:@"Product" inQuery:likeQuery];

//restrict to 10 items
[prodQuery setLimit:10];
[prodQuery findObjectsInBackgroundWithTarget:self
                                 selector:@selector(getCallback:error:)];

However, when I try and run this previously liked items by that user are being returned.

If i were doing this in SQL I would write something like:

SELECT product FROM product WHERE product NOT IN (SELECT product FROM likes WHERE useriD = currentUser AND like = yes)

Any help would be much appreciated!

Thanks, Tim

回答1:

Working with Parse and other NoSQL services, you need to free yourself from the SQL mindset (with normalization etc). NoSQL "schemas" are focused on query efficiency more than storage and consistency. You could, as Petro suggests, store all your likes in an array in the user table. You could then query Parse with a constraint stating that records should not match any objectid in your query, like this:

[query whereKey:@"objectId" notContainedIn:user.productLikes];

where productLikes is the array of product ids.

Another solution would be to store the product likes in a different table, but still as an array (not separate records for each like, with pointers to products and users like you would have done with a SQL database). In this case you could nest the queries much like you have in your question.