In simplified form: I have two content types representing documents and folders. Thus, content type MyFolder has parent type Folder, content type MyDocument has parent type Document.
What I want to do now is to have a column MyColumn in the MyDocument content type that is a reference/link to the column MyColumn in MyFolder. The desired behaviour is that the value of that column in a MyDocument instance is always equal to the value of the same column of the MyFolder that contains the MyDocument. It should not be just a copy of that value, it should be the very same value at all times.
How can I achieve this? Do I need a calculated column for this behaviour?