Method to do a "join" when using LINQ in db4o? -

whats correct linq navigate end leaf node of following data structure?

root ----symbol (apple)                    |          |------day 1: date: 2010-10-18, string "apples on thursday"    |          |------day 2: date: 2010-10-19, string "apples on friday"    |    |    |-----symbol (pear)                              |------day 1: date: 2010-10-18, string "pears on thursday"               |------day 2: date: 2010-10-19, string "pears on friday" 

in other words, if select "apple", "2010-10-19", should return "apples on friday".

the following not work (it selects cartesian product every single combination) of symbol , date):

var daysforsymbols1 = symbol s in db4o.db                       date t in db4o.db                           (s.symbolglobal == "apple"                              && t.date == new datetime(2010, 10, 18))                           select new                           {                             s.symbolglobal,                             t.date,                             t.meta                           };  foreach (var d in daysforsymbols1) {   console.writeline("{0} - {1} - {2}", d.symbolglobal, d.date.date, d.meta); }  

i'd love use join - db4o object database, , doesn't let reference id each object.

update:

@gamlor gave brilliant answer, works charm.

i should mention current version of db4o not support indexing on collections. since form of 1-to-many hierarchy constructed using ilist collection holds subclasses, breaking database down hierarchy slow things down, both speed , maintenance wise.

hogan answered need use join: example this:

       var daysforsymbols1 = s in database.cast<symbol>()                                   join t in database.cast<tradingday>() on s equals t.symbolglobal                                    (s.symbolglobal == "ibm" && t.date == new datetime(2010, 10, 20))                                   select new                                   {                                       s.symbolglobal,                                       t.date,                                       t.meta                                   }; 

now db4o-parts. can use reference join. don't need id (as shown above)

however there not big issue. linq db4o implementation doesn't support linq-operator @ (you can see using go declaration on join). means falls linq objects. means objects loaded memory , linq object executed on objects. thats extremely slow.

currently query cannot executed efficiently on db4o. way such stuff db4o have day-collection on symbol. collection contains days of symbol. can query symbol , data.

another method split query:

    var ibmsymbols = symbol s in database                      s.symbolglobal == "ibm"                      select s;      // run tradingday selection optimized linq db4o implementation     // rest linq objects     var allsymbols = ibmsymbols.selectmany(s => tradingday t in database                                t.symbolglobal==s && t.date == new datetime(2010, 10, 20)                                select t); 

edit: want add case when symbol has tradingdays in list other collection. can this:

            var ibmsymbols = symbol s in database                              s.symbolglobal == "ibm"                              select s;              var tradingdays = symbol in ibmsymbols                               day in symbol.tradingdays                               day.date == new datetime(2010, 10, 20)                               select day;