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POFinding objects in Core Data by array attribute, performantly in >10k elements
 

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  1. POFinding objects in Core Data by array attribute, performantly in >10k elements
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    <p>Short:<br> I need to <strong>find core data objects by</strong> a key, which holds a <strong>unique immutable array</strong> (fixed length, but chosen at runtime) of arbitrary objects (for which not only <strong>element membership</strong>, but also <strong>element order</strong> determines <strong>uniqueness</strong>). <strong>NSManagedObject</strong> however <strong>forbids overriding</strong> <code>[isEqual:]</code>. Now what?</p> <hr> <p>Long:<br> I have an entity (see diagram image for entity "…Link") in my Core Data model for which I have to guarantee uniqueness based on an attribute key ("tuple"). So far so good.</p> <p>The entity's <strong>unique attribute</strong> however has to be an <strong>NSArray</strong>.<br> And to make things a bit more difficult I <strong>neither know the class type</strong> of the <strong>tuple's elements</strong>.<br> Nor do I know the tuple's element count. Well, actually the count is the same for every tuple (per core data context at least), but not known before the app runs.</p> <p>There must <strong>only ever be one instance</strong> of my link entity <strong>with a given tuple</strong>.<br> And for obvious reason only ever one tuple instance with a given array of arbitrary objects.<br> Whereas two tuples are to be considered equal if <code>[tuple_1 isEqual:tuple_n]</code> returns <code>YES</code>. <strong>NSManagedObject forbids the overriding</strong> of <code>[isEqual:]</code> and <code>[hash]</code> though, otherwise things would be pretty much a piece of cake.</p> <p><strong>"…Tuple" objects</strong> are created together with their array of tokens (via a convenience method) and <strong>are immutable</strong> (and so is each "…Token" and its data attribute). (think of "…Tuple" as a "…Link"'s dictionary key.)</p> <p>"…Tuple" implements <code>"- (NSArray *)tokens;"</code>, which <strong>returnes a neatly ordered array of tokens</strong>, based on the "order" keys of "…TokenOrder". (<strong>Tuples</strong> are expected to <strong>contain at most 5 elements</strong>.)</p> <p>I however <strong>expect to have tens of thousands</strong> (potentially even more in some edge cases) of "…Link" objects, which I have to (frequently) <strong>find based on their "tuple" attribute</strong>.</p> <p>Sadly I couldn't find any article (let alone solution) for such a scenario in any literature or the web.</p> <p>Any ideas?</p> <p><img src="https://i.stack.imgur.com/sn2kd.jpg" alt="core data model"></p> <p>A possible solution I've come up with so far would be:</p> <ol> <li><p>Narrow amount of elements to compare by tuple by adding another attribute to "…Tuple" called "<code>tupleHash</code>", which is pre-calculated on object creation via: <strong>Snippet 1</strong></p></li> <li><p>Query with NSPredicate for objects of matching tupleHash (narrowing down the list of candidates quite a bit).</p></li> <li><p>Find "…Link" featuring given tuple in narrowed candidate list by: <strong>Snippet 1</strong></p></li> </ol> <p><strong>Snippet 1:</strong></p> <pre><code>NSUInteger tupleHash = [[self class] hash]; for (id token in self.tokens) { tupleHash ^= [token.data hash]; } </code></pre> <p><strong>Snippet 2:</strong></p> <pre><code>__block NSArray *tupleTokens = someTokens; NSArray *filteredEntries = [narrowedCandidates filteredArrayUsingPredicate: [NSPredicate predicateWithBlock: ^(id evaluatedObject, NSDictionary *bindings) { return [evaluatedObject.tuple.tokens isEqualToArray:tupleTokens]; }]]; </code></pre> <p><em>(Sorry, markdown appears to oppose mixing of lists with code snippets.)</em></p> <p>Good idea of or just insane?</p> <p>Thanks in advance!</p>
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    1. COI might actually do that. SQLite, that is. :(
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