5. Pre encode and post decode

Converting data during encoding and decoding

We have the most powerful tool which we need to use wisely. For almost each type of data we can pre-process values before encoding and also we can post-process values after decoding data from JSON. Let us a simple example for that. Suppose that we need to convert calendar:datetime() to ISO date format.

%% We serialize log messages where both the message
%% and the time will be binary (string) in JSON.
-json({log, {binary, message},
            {binary, time, [{pre_encode, {?MODULE, time_to_binary}},
                            {post_decode, {?MODULE, binary_to_time}}]
            }
      }).

-export([time_to_binary/2, binary_to_time/2]).

time_to_binary(_LogRecord, Time) ->
    %% We got the whole record and the time before encoding
    %% Time is supposed to be a calendar:datetime() format.
    %% We convert something like 2011-08-28T15:41:09Z
    {{Y, Mo, D}, {H, Mi, S}} = Time,
    io_lib:format("~4..0B-~2..0B-~2..0BT~2..0B:~2..0B:~2..0BZ",
                    [Y, Mo, D, H, Mi, S])).

binary_to_time(_Record, IsoDate) ->
    %% Here we have a structure which is half-ready in the first
    %% parameter. It is in jsx raw format like:
    %% [{<<"attr1">>, <<"value1">>}, ...] where value part can be
    %% a binary, a boolean, a number or undefined
    parse_iso_to_date(IsoDate).

parse_iso_date(<<Y1, Y2, Y3, Y4, $-, Mo1, Mo2, $-, D1, D2, $T,
                 H1, H2, $:, Mi1, Mi2, $:, S1, S2, $Z>>) ->
    {{list_to_integer([Y1, Y2, Y3, Y4]),
      list_to_integer([Mo1, Mo2]),
      list_to_integer([D1, D2])},
     {list_to_integer([H1, H2]),
      list_to_integer([Mi1, Mi2]),
      list_to_integer([S1, S2])}}.

In this example the converter functions are a bit long but covers most of the input possibilities. Here is the workflow during encoding:

• we create a log record as {log, <<"Hello">>, {{2011, 8, 28}, {15, 41, 9}}}
• call ejson:to_json/1 to that value
• ejson calls the pre_encode function for that value
• the intermediate result is [{<<"__rec">>, <<"log">>}, {<<"message">>, <<"Hello">>}, {<<"time">>, <<"2011-08-28T15:41:09Z">>}]
• so the JSON can be resulted from that jsx list

During decoding:

• from JSON string jsx creates the intermediate jsx list: [{<<"__rec">>, <<"log">>}, {<<"message">>, <<"Hello">>}, {<<"time">>, <<"2011-08-28T15:41:09Z">>}]
• ejson knows from the value of <<"__rec">> that the target type is log
• applies normal decoding process, so we will have a {log, <<"Hello">>, <<"2011-08-28T15:41:09Z">>} record
• applies post_decode function for the ISO date value, so the time will be {{2011, 8, 28}, {15, 41, 9}}
• and we get the record we have in the beginning.