ios - How to populate a UIPickerView with results from a NSFetchRequest using Core Data -

i have uipickerview trying populate results nsfetchrequest pulling data managedobjectcontext. when initialize uipickerview following, kcmodalpickerview *pickerview = [[kcmodalpickerview alloc] initwithvalues:_usernames]; xcode doesn't throw , warnings or errors, when build , run app getting following error.

* terminating app due uncaught exception 'nsinvalidargumentexception', reason: '-[account copywithzone:]: unrecognized selector sent instance 0x7a60ec70' * first throw call stack:

now before error due me not implementing copywithzone method in vc, want point out in class files using keyword copy

the method told causinging crash belongs kcmodalpicker class implementation file. , method looks following,

// edit method - (nsstring *)pickerview:(uipickerview *)pickerview titleforrow:(nsinteger)row forcomponent:(nsinteger)component {  return [self.values objectatindex:row]; }  

what need change / edit / add prevent app crashing?

update

_usernames nsarray ...the results following,

nsfetchrequest *fetchrequest = [[nsfetchrequest alloc] init]; nsentitydescription *entity = [nsentitydescription entityforname:@"account" inmanagedobjectcontext:_managedobjectcontext]; [fetchrequest setentity:entity]; fetchrequest.propertiestofetch = [nsarray arraywithobject:[[entity propertiesbyname] objectforkey:@"username"]]; nserror *error = nil; nsarray _usernames = [self.managedobjectcontext executefetchrequest:fetchrequest error:&error];  

kcmodalpickerview expects array of nsstring, give array of account. framework tries copy instance because thinks it's nsstring, conforms nscopying protocol , implements copywithzone:. object not, , there [account copywithzone:]: unrecognized selector sent instance exception.

simply create array of nsstrings using appropriate attribute core data object.

there smarter ways this, obvious solution:

nsmutablearray *names = [nsmutablearray arraywithcapacity:[_usernames count]]; (account *account in _usernames) {     nsstring *accountname = account.name;     if (!accountname) {         accountname = @"<unknown account>";     }     [names addobject:accountname]; } kcmodalpickerview *pickerview = [[kcmodalpickerview alloc] initwithvalues:names]; 

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i saw have set propertiestofetch. additionally have set resultstype of fetchrequest nsdictionaryresulttype. in case executefetchrequest:error: returns array of dictionaries. , should able use nsarray *names = [_usernames valueforkey:@"username"]; instead of loop.