Commit 2af9a369 authored by Tim Peters's avatar Tim Peters

Explain why it's a Bad Idea to use a persistent object as a BTree key.

parent 4dcfe979
......@@ -360,8 +360,29 @@ only with great care:
Any part of a comparison implementation that relies (explicitly
or implicitly) on an address-based comparison result will
eventually cause serious failure.
\item Do not use \class{Persistent} objects as keys, or objects of a
subclass of \class{Persistent}.
\end{enumerate}
That last item may be surprising. It stems from details of how
conflict resolution is implemented: the states passed to conflict
resolution do not materialize persistent subobjects (if a persistent
object P is a key in a BTree, then P is a subobject of the bucket
containing P). Instead, if an object O references a persistent subobject
P directly, and O is involved in a conflict, the states passed to
conflict resolution contain an instance of an internal
\class{PersistentReference} stub class everywhere O references P.
Two \class{PersistentReference} instances compare equal if and only if
they "represent" the same persistent object; when they're not equal,
they compare by memory address, and, as expalined before, memory-based
comparison must never happen in a sane persistent BTree. Note that it
doesn't help in this case if your \class{Persistent} subclass defines
a sane \method{__cmp__()} method: conflict resolution doesn't know
about your class, and so also doesn't know about its \method{__cmp__()}
method. It only sees instances of the internal \class{PersistentReference}
stub class.
\subsubsection{Iteration and Mutation}
......
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