update all line of table in erlang - erlang

Question

I have this table :
-record(person, {id, firstname, lastname, address}).
for example this table contains this values :
2 alen dumas paris
5 franco mocci parma
3 ali othmani london
Now I have the variable Key which contains this value 10
I want to develop a function in erlang which will modify all id of the table person
the new value of this id will be the previous value + the value of Key
mean the table person will became like this
12 alen dumas paris
15 franco mocci parma
13 ali othmani london
meaning each id will be added by 10 (is the value of Key) :(2+10) (5+10) (3+10)
I try with your code :
testmodify()->
Key=22,
[ P#person{id=P#person.id+Key} || P <- Persons ].
but I have this error in the sysntax : variable Persons is unbound
I try to resolve this problem with this code :
testmodify()->
Key=22,
[ P#person{id=P#person.id+Key} || P <- mnesia:table(person) ].
but I have this error :
1> model:testmodify().
** exception error: no function clause matching
model:'-testmodify/0-lc$^0/1-0-'({qlc_handle,
{qlc_table,
#Fun<mnesia.20.112329951>,
true,
#Fun<mnesia.21.62211129>,
#Fun<mnesia.22.75429001>,
#Fun<mnesia.23.26336897>,
#Fun<mnesia.26.62819299>,
#Fun<mnesia.25.51075493>,
#Fun<mnesia.24.47804912>,
'=:=',undefined,
no_match_spec}})

Answer 1

Assuming your table is stored as a list:
[ P#person{id=P#person.id+Key} || P <- Persons ].
UPDATE: For an Mnesia table, you can retrieve similar results with QLC:
-include_lib("stdlib/include/qlc.hrl").
⋮
[ P#person{id=P#person.id+Key} || P <- mnesia:table(person) ].
Note that this only gives you a list of transformed person records. To update the records, you'll probably have to remove the existing records and write the new records in a transaction, since a record with a modified key (assuming that's what id is) is treated as a different record — something like this:
mnesia:transaction(fun() ->
Old = [ P || P <- mnesia:table(person) ],
New = [ P#person{id=P#person.id+Key} || P <- Old ],
[ mnesia:delete({person, P#Person.id}) || P <- Old ],
[ mnesia:write(P) || P <- New ]
end)
You might be able to do this in a single pass with mnesia:foldr, but I don't know what happens if you issue mnesia:delete and mnesia:write inside mnesia:foldr. You might get stuck in an infinite loop (but don't quote me on that).