Did you try 'user_name' like this:
- (NSDictionary *)asDictionary;
{
return @{
@"type" : @"chat", @"user_id" : @([self userId]), @"user_name" : [self userName], @"user_stores" : [self userStores], @"body" : [self body], @"time" : [self time]
};
}