Method call on wrong class instance from valueForKey return (Ribbit)

Hi all! I have a question about making method calls on the object returned by an -objectForKey call on an NSDictionary object.

I'm currently working through the Ribbit app, on the "Saving the Media" video. I have created an NSString property to hold a captured video's file path, aptly named videoFilePath.

When I use the code:

        self.videoFilePath = [[info objectForKey:UIImagePickerControllerMediaURL] path];

I receive a compiler error about ARC casting rules. Apparently Xcode believes calling "path" on the return value from the objectForKey call will return a CGPathRef object, because it thinks "path" is a property of a "CAAnimation" object, whatever that is.

Some Googling has led me to typecasting, so I've gone ahead and used:

        self.videoFilePath = [(NSURL *)[info objectForKey:UIImagePickerControllerMediaURL] path];

instead. But I've read in a few places that using typecasting like this is bad practice or "unsafe".

Since the return type of objectForKey is "id", this has led me to a question that I can't seem to find an answer to anywhere else. If different objects have different methods, but those methods may share the same name, either because they overwrite an inherited method or just reuse the name by coincidence, how does Xcode know which class's "path" method is being called for a return value of type "id"? And why does it think (unless I'm doing something very wrong here, incorrectly) that the returned value here is going to be of the "CAAnimate" class?

Thanks for reading this long post, and thanks for answering if you do!