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erlangmnesia

How does mnesia:write/1 know the table name?

In Joe Armstrong's book on Erlang, there's an example of inserting a row into an mnesia database:

add_shop_item(Name, Quantity, Cost) ->
    Row = #shop{item=Name, quantity=Quantity, cost=Cost},
    F = fun() ->
        mnesia:write(Row)
    end,
    mnesia:transaction(F).

The row gets inserted into a table called shop, but how does that happen if Erlang records like Row are really just tuples, and mnesia:write/1 doesn't take an argument for the table name?

Solution

  • Records are tuples where the first element is an atom that names the record, in this case shop. In the mnesia source you can see how it extracts the first element to use as the table name.

    https://github.com/erlang/otp/blob/maint/lib/mnesia/src/mnesia.erl#L511-L513

    write(Val) when is_tuple(Val), tuple_size(Val) > 2 ->
    Tab = element(1, Val),
    write(Tab, Val, write);