More implementation.

Modified intent of ts_db_records. timestamp value is no longer a
  {Date, Time} value, it is now gregorian seconds since year 0.
  This was done to make queries that are based on date comparisons
  easier to write and more efficient.
Added date time comparison function to ts_common.
Implemented update/1 and list/3 for ts_entry.

src/ts_common.erl

@@ -1,5 +1,5 @@
 -module(ts_common).
--export([list/3]).
+-export([list/3, compare_dates/2]).
 
 -include_lib("stdlib/include/qlc.hrl").
 
@@ -27,3 +27,21 @@ list(Query, Start, Length) ->
         List
     end),
     Result.
+
+% This is somewhat ridiculous.
+order_datetimes({{Y1, Mon1, D1}, {H1, Min1, S1}},
+                {{Y2, Mon2, D2}, {H2, Min2, S2}}) ->
+
+    % this is actually a deep-nested set of case statements, but it seems
+    % cleaner to keep the indentation level and follow a strict pattern
+
+    % compare field.If /=, return that value, else drop through to next field
+    case Y1   = Y2   of false -> Y1   > Y2;   true ->
+    case Mon1 = Mon2 of false -> Mon1 > Mon2; true ->
+    case D1   = D2   of false -> D1   > D2;   true ->
+    case H1   = H2   of flase -> H1   > H2;   true ->
+    case Min1 = Min2 of false -> Min1 > Min2; true ->
+    case S1   = S2   of false -> S1   > S2;   true -> true
+
+  % sec min hr  day mon year
+    end end end end end end.

src/ts_db_records.hrl

@@ -6,7 +6,7 @@
 
 -record(ts_entry, {
     ref,        % {username, timelineid, eventid}
-    timestamp,  % {{year, month, day}, {hour, minute, second}}
+    timestamp,  % gregorian seconds
     mark,       % String description of entry
     notes       % String with further notes about the entry
 }).

src/ts_entry.erl

@@ -17,3 +17,41 @@ new(ER = #ts_entry()) ->
         ok = mnesia:write(NewRow),
         NewRow end),
     {ok, NewRow}.
+
+update(ER = #ts_entry()) ->
+    
+    % look for existing record
+    case mnesia:dirty_read(ts_entry, ER#ts_entry.ref) of
+        % record does not exist
+        [] -> no_record;
+        % record exists, update it
+        [_Record] -> mnesia:dirty_write(ER)
+    end.
+
+list({Username, Timeline}, Start, Length)
+when is_integer(Start) and is_integer(Length) ->
+    ts_common:list(
+        qlc:q([E || E <- mnesia:table(ts_entry),
+                    E#ts_entry.ref =:= {Username, Timeline, _}]_,
+            Start, Length).
+
+list({Username, Timeline}, StartDateTime, EndDateTime) ->
+
+    {atomic, Entries} = mnesia:transaction(fun() ->
+
+        StartSeconds = calendar:datetime_to_gregorian_seconds(StartDateTime),
+        EndSeconds = calendar:datetime_to_gregorian_seconds(EndDateTime),
+
+        % create the query that will select these records
+        Q = qlc:q([E || E <- mnesia:table(ts_entry),
+                        E#ts_entry.timestamp >= StartSeconds,
+                        E#ts_entry.timestamp < EndSeconds]),
+
+        % sort by timestamp
+        SortedQ = qlc:sort(Q, {order, fun(A, B) ->
+            A#ts_entry.timestamp > B#ts_entry.timestamp end}),
+
+        % return
+        qlc:e(SortedQ)
+    end),
+    Entries.

src/ts_timeline.erl

@@ -22,13 +22,12 @@ update(TR = #ts_timeline{}) ->
         % record does not exist
         [] -> no_record;
         % record exists, update
-        [Record] -> mnesia:dirty_write(TR)
+        [_Record] -> mnesia:dirty_write(TR)
     end.
 
 list(Username, Start, Length) -> 
     ts_common:list(
         qlc:q([T || T <- mnesia:table(ts_timeline),
-                    T#ts_timeline.ref = {Username, _}]),
-        Start, Length).
-
+                    T#ts_timeline.ref =:= {Username, _}]),
+            Start, Length).