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POHow to serialize a NSPredicate object?
 

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  1. POHow to serialize a NSPredicate object?
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    <p>Is there a way to inspect a NSPredicate object for the purposes of serializing it into a URL? I am trying to retrieve the data remotely and need to translate the predicate object into a URL with querystring parameters that the server understands.</p> <p>This was inspired by a talk given in WWDC 2010 called "Building a Server Driven User EXperience" where the speakers talk about using Core-Data and with a server backend. I have followed the session video and slides, but am stuck on the serializing point. For example, there is a <code>Person</code> object, and I'm trying to fetch all people whose first name is "John". I am using a subclass of <code>NSManagedObjectContext</code> called <code>RemoteManagedObjectContext</code>, which overrides the <code>executeFetchRequest</code> method, and is supposed to send the call to the server instead. The fetch request is being created as (ellipsed non-essential parts):</p> <pre><code>@implementation PeopleViewController - (NSArray *)getPeople { RemoteFetchRequest *fetchRequest = [[RemoteFetchRequest alloc] init]; NSEntityDescription *entity = ... NSPredicate *template = [NSPredicate predicateWithFormat: @"name == $NAME AND endpoint = $ENDPOINT"]; NSPredicate *predicate = [template predicateWithSubstitutionVariables:...]; [fetchRequest setEntity:entity]; [fetchRequest setPredicate:predicate]; NSError *error = nil; // the custom subclass of NSManagedObjectContext executes this return [remoteMOC executeFetchRequest:fetchRequest error:&amp;error]; } @end </code></pre> <p>Now inside the custom subclass of <code>NSManagedObjectContext</code>, how can I serialize the fetch request into querystring parameters suitable for the server. So given the above fetch request, the corresponding URL would be:</p> <pre><code>http://example.com/people?name=John </code></pre> <p>It is possible to get a string representation of the predicate which returns,</p> <pre><code>name == "John" AND endpoint == "people" </code></pre> <p>that I can parse to get the parameters <code>name</code>, and <code>endpoint</code>. However, is it possible to do it without parsing the string? Here's a partial implementation of the <code>RemoteManagedObjectContext</code> class.</p> <pre><code>@implementation RemoteManagedObjectContext - (NSArray *)executeFetchRequest:(NSFetchRequest *)request error:(NSError **)error { // this gives name == "John" AND endpoint == "people" // don't know how else to retrieve the predicate data NSLog(@"%@", [[request predicate] predicateFormat]); ... } @end </code></pre>
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