Question

I’m querying rows from my sqlite database using Fetch and loading the results into a NSMutableArray myDataArray. What I want to do is to store the values in myDataArray into my application’s preferences plist file. But when I run the code below I get the following error:

[NSUserDefaults setObject:forKey:]: Attempt to insert non-property value ‘(
…(note: contains 50 rows of values)
)’ of class ‘__NSArrayM’.

I’m storing other NSMutableArrays in my plist file using the same method but cannot figure out a way to store the queried database results.

You might ask: why store the values, just requery the database when I need the values again? The reason is I’m pulling random rows from the database and I need to recall the rows exactly as they were originally randomized.

Any help is appreciated,
lq

 ...
 fetchResults = [managedObjectContext executeFetchRequest:request error:&error];
 self.myDataArray = [NSMutableArray arrayWithArray:fetchResults];

 NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];
 [userDefaults setObject:self.myDataArray forKey:kFetchResultsArray];

Answer 1

The items in myDataArray are not property list objects, so they can’t be automatically serialized to a plist.

From the NSUserDefaults class reference for setObject:forKey::

The value parameter can be only
property list objects: NSData,
NSString, NSNumber, NSDate, NSArray,
or NSDictionary. For NSArray and
NSDictionary objects, their contents
must be property list objects. See
“What is a Property List?” in Property
List Programming Guide.

You may want to consider a mechanism other than user defaults for storing this data. From the Property List Programming Guide:

Many applications require a mechanism
for storing information that will be
needed at a later time. For situations
where you need to store small amounts
of persistent data—say less than a few
hundred kilobytes—property lists offer
a uniform and convenient means of
organizing, storing, and accessing the
data.

In some situations, the property-list
architecture may prove insufficient.
If you need a way to store large,
complex graphs of objects, objects not
supported by the property-list
architecture, or objects whose
mutability settings must be retained,
use archiving. See Archives and
Serializations Programming Guide for
more information.