Mnesia odd behaviour - delete_object

[Dan Gudmundsson]

Hi 

A explanation of the implementation, then you can argument about
semantics :-)

mnesia:delete_object({a, key, [1]}) deletes the object if 
and only if exactly that object exists in the database.

So a mnesia:transaction(fun() -> mnesia:delete_object({a, key, [1]}) end)
will not delete the object {a, key, [1,2]} since they are not the same.

And since transactions doesn't directly operate on the tables, (not 
until the transaction commits), this happens.

Thus when you over write the {a, key, [1]} in your transaction, 
you transaction view shows you the only record in the {a, key, [1]},
and you delete that. This DELETES the WRITE operation you just did.
And when the transaction commits, it tries to delete exactly the object 
{a, key, [1]} from table, which doesn't exist and you are left with 
you old value.

Why this doesn't occur in the async_dirty and sync_dirty cases is 
obvious they don't have the transaction view, writes/deletes are done 
directly on the table.

Hmm.. I need to sleep on this :-) 

/Dan

Martin Bjorklund writes:
 > I've got a funny mnesia situation.  The problem is delete_object
 > vs. delete.
 > 
 > I've got a simple 'set' table a:
 > 
 >   mnesia:create_table(a, []).
 > 
 > Add an object:
 > 
 >   mnesia:dirty_write({a, key, [1,2]}).
 > 
 > Now, I've got this fun:
 > 
 >    F1 = fun() -> 
 >             mnesia:write({a, key, [1]}),  % modify the one&only object
 >             [X] = mnesia:read({a, key}),  % read it
 >             mnesia:delete_object(X)       % delete the one&only object
 >         end.
 > 
 > After executing F1 within a transaction, my table still contains the
 > original object.  Since 'a' is a set, I'd think that the object would
 > be removed.
 > 
 > After executing F1 'async_dirty', the object is removed.
 > 
 > 
 >    F2 = fun() -> 
 >             mnesia:write({a, key, [1]}),  % modify the one&only object
 >             mnesia:delete({a, key})       % delete the one&only object
 >         end.
 > 
 > After executing F2 within a transaction, the object is removed.
 > 
 > 
 > Can someone explain this behaviour??
 > 
 > 
 > I've tested on mnesia 3.9.2 and 4.0.  It's probably not a bug, but I
 > cannot come up with a interpretation that feels consistent...
 > 
 > 
 > 
 > /martin

-- 
Dan Gudmundsson               Project:    Mnesia, Erlang/OTP
Ericsson Utvecklings AB       Phone:      +46  8 727 5762 
UAB/F/P                       Mobile:     +46 70 519 9469
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