ios - How do I identify the cause of a "no attribute was found" exception? -

i followed steps in restkit documentation adding project when try start app stops @ line in application:application didfinishlaunchingwithoptions::

entitymapping.identificationattributes = @[@"_id"]; 

the code before is:

nsurl *modelurl = [nsurl fileurlwithpath:[[nsbundle mainbundle] pathforresource:@"mymodel" oftype:@"momd"]]; nsmanagedobjectmodel *managedobjectmodel = [[[nsmanagedobjectmodel alloc] initwithcontentsofurl:modelurl] mutablecopy]; rkmanagedobjectstore *managedobjectstore = [[rkmanagedobjectstore alloc] initwithmanagedobjectmodel:managedobjectmodel];  // init core data stack [managedobjectstore createpersistentstorecoordinator];  nspersistentstore __unused *persistentstore = [managedobjectstore addinmemorypersistentstore:&error]; nsassert(persistentstore, @"faild add persistent store: %@", error);  [managedobjectstore createmanagedobjectcontexts];  // set default shared store instance [rkmanagedobjectstore setdefaultstore:managedobjectstore];  // configure object manager rkobjectmanager *objectmanager = [rkobjectmanager managerwithbaseurl:[nsurl urlwithstring:@"http://localhost:9090"]]; objectmanager.managedobjectstore = managedobjectstore;  [rkobjectmanager setsharedmanager:objectmanager];  rkentitymapping *entitymapping = [rkentitymapping mappingforentityforname:@"pool" inmanagedobjectstore:managedobjectstore]; [entitymapping addattributemappingsfromdictionary:@{  @"id":         @"_id",  @"about_url":  @"abouturl",  @"closed":     @"closed",  @"created_on": @"createdon" }]; entitymapping.identificationattributes = @[@"_id"]; 

the exception getting thrown in rkentitymapping.m on line 100 (if (!attribute) [nsexception raise:nsinvalidargumentexception format:@"invalid attribute '%@': no attribute found given name in '%@' entity.", attributeorname, [entity name]];) part of rkarrayofattributesforentityfromattributesornames.

when po [entity name] says "pool" have pool entity in .xcdatamodeld.

where else might cause of error?

add id property pool entity.
note: naming property id work, recommend changing not objective-c keyword.