How to handle '[<__NSCFString 0x2f1730> valueForUndefinedKey:]: this class is not key value coding-compliant for the key $oid' error

问题

I am hitting the error (stated in the subject) because there are times the property 'id' does not store the hash containing '$oid' in the returned json. For example

Sometimes I get:

"id":{"$oid":"4eea972209f47a0028000140"}

Some other times I get

"id":"4eea972209f47a0028000140"

I am trying to do a check in the following code to cater for such irregularity

if ([[question valueForKey:@"id"] valueForKey:@"$oid"])
{
    question_id = [[question valueForKey:@"id"] valueForKey:@"$oid"];
}
else
{
    question_id = [question valueForKey:@"id"];
}

However, it still doesn't work as the code fails during the checking phase.

How can I implement a check so that I will take question_id from '$oid' only if it exists?

回答1:

Try the following code.

id quesDict = [question valueForKey:@"id"];
if( [quesDict isKindOfClass:[NSDictionary class]] )
{
    question_id = [quesDict valueForKey:@"$oid"];
}
else
{
    question_id = quesDict;
}

回答2:

You need to check what type is being returned by [question valueForKey:@"id"], it looks like sometimes it is a string, and sometimes it is another object which is KVC compliant for your other key. Your error will be in the very first if statement.

来源：https://stackoverflow.com/questions/8754860/how-to-handle-nscfstring-0x2f1730-valueforundefinedkey-this-class-is-no