Compound AND predicate for many to many relationship using SUBQUERY

Question

I have an entity A with a many to many relationship "events" with entity Event. Entity Event has an attribute eventName.

I want to be able to fetch all entity As that have a given set of Event entitys that match the entityName selected by the user. For an ANY fetch, a compound OR predicate works fine. But for an ALL fetch (where only entity As that have ALL of the user chosen events will be fetched) the compound AND predicate returns a nil!

I was told that I would have to use subqueries. My deliema is that I am having a hard time figuring out how to form a subquery string.

The format is as follows...

SUBQUERY(collection_expression, variable_expression, predicate);

I'll have the eventName string selected by the user so that is dynamic. So what should my subquery look like if I want to use the LIKE[d] operator?

the collection_expression would be the relationship "events" if i am not wrong?

kindly help me out with this.

thankyou.

Core Data

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Posted by

ace.neerav

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Answer 1

In reverse order:

Go to the "Content" link, https://forums.developer.apple.com/content

The threads you've created should show up under 'Authored'. The threads you've posted in should show up under 'Participated'.

For

(SUBQUERY(events,$x, $x.event LIKE[d] %@))

it's important to understand that it's an expression which evaluates to a collection, not a single boolean value. That means that

(SUBQUERY(events,$x, $x.event LIKE[d] %@))

by itself isn't a valid predicate because doesn't evaluate to a boolean value. You need to embed the subquery expression in a complete boolean expression like

(SUBQUERY(events,$x, $x.event LIKE[d] %@)).@count > 0

Posted by

NotMyName

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Answer 2

here is what my predicate looks like...

NSPredicate *eventEqualToPredicate = [NSPredicate predicateWithFormat:@"(SUBQUERY(events,$x, $x.event LIKE[d] %@))",event ];

and the error message is as follows...

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Unable to parse the format string "(SUBQUERY(event,$x, $x.event LIKE[d] %@))

by the way, does anyone know how to see all the posts i have made or threads i have started in these new forums?!!

Posted by

ace.neerav

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Answer 3

In reverse order:

Go to the "Content" link, https://forums.developer.apple.com/content

The threads you've created should show up under 'Authored'. The threads you've posted in should show up under 'Participated'.

For

(SUBQUERY(events,$x, $x.event LIKE[d] %@))

it's important to understand that it's an expression which evaluates to a collection, not a single boolean value. That means that

(SUBQUERY(events,$x, $x.event LIKE[d] %@))

by itself isn't a valid predicate because doesn't evaluate to a boolean value. You need to embed the subquery expression in a complete boolean expression like

(SUBQUERY(events,$x, $x.event LIKE[d] %@)).@count > 0

Posted by

NotMyName

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Answer 4

Hi NotMyName,

Nice seeing you back on these new forums.

I'm glad I got a part of the subquery right. now what does .@count>0 do?

Posted by

ace.neerav

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Answer 5

this is what my final predicate looks like when two events are selected...

SUBQUERY(event, $x, $x.event LIKE[d] "Birthday").@count > 0 AND SUBQUERY(event, $x, $x.event LIKE[d] "party").@count > 0

now i have one entry with both these events but i get nil returned on executing the fetch request.

i guess my original AND compound predicate was right. there seems something wrong with my persistent store.

any idea what it could be?

Posted by

ace.neerav

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Answer 6

@count is one of the collection operators, it evaluations the size of the collection.

So the SUBQUERY(............).@count > 0 is a predicate for whether there are any matches to the subquery.

Posted by

NotMyName

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Answer 7

I'm pretty sure you can't actually have a string value that matches two LIKE clauses like that without specify a wildcard in both of them.

Because you're expected to supply the wildcards:

LIKE
The left hand expression equals the right-hand expression: ? and * are allowed as wildcard characters, where ? matches 1 character and * matches 0 or more characters.

In other words, if you're trying to match "Birthday party" you need soething like "Birthday*" and "*party" as your two target values, or "*birthday*" and "*party*".

More importantly, if you're trying to make sure the same event matches both "Birthday*" and "*party" then you need

SUBQUERY(event, $x, $x.event LIKE[d] "Birthday*" && $x.event LIKE[d] "*party").@count > 0

That's the more common use for subquery, by the way. When you have one of the objects in the event to-many relationship and you want to make sure that the same object satisfies multiple criteria.

The way you have it specified it,

SUBQUERY(event, $x, $x.event LIKE[d] "Birthday*").@count > 0 AND SUBQUERY(event, $x, $x.event LIKE[d] "*party").@count > 0

with the added wildcards, if you "Birthday planning" and "Office party" as two different events, your predicate would match.

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NotMyName

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Answer 8

birthday and party are two separate events and they both could be selected in a single entry and it doesnt reduce to a single event called 'birthday party'. Using this app called 'Core Data Editor' I found out that my code was only taking one event where multiple are selected. So my store didnt have any entry with two events Birthday and Party! I guess I found where the bug is.

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ace.neerav

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Answer 9

my orginial code...

                NSPredicate *eventEqualToPredicate= [NSComparisonPredicate predicateWithLeftExpression:eventLHS rightExpression:eventRHS modifier:NSAnyPredicateModifier type:NSLikePredicateOperatorType options:NSDiacriticInsensitivePredicateOption];
                [eventsComparisionPredicates addObject:eventEqualToPredicate];

is working now!

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ace.neerav

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Compound AND predicate for many to many relationship using SUBQUERY

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