dtl

RandomDBView<DataObj, ParamObj>

Category: containers	Component type: type

Description

RandomDBView is a Random Access Container with the property that elements are bound to an underlying database. Rows in the table can be accessed and modified using random access iterators provided by the container. To provide random access to the elements in the table, the container uses an ODBC static cursor to guarantee a particular ordering for the rows. Rows can be inserted or deleted from the table by using the insert() and erase() methods. The specifics of how rows are updated or deleted can be controlled by the user via the SetKey() method which specifies what kind of key to use when updating or deleting rows. Please note that when making changes to the table via insert(), erase() or the non-const iterator, these changes will not be visible until the ReQuery() method is called (see [1] for details).

Definition

Defined in the RandomDBView.h header file.

Refinement of

Random Access Container

Associated types

The types defined by Random Access Container .

Example 1 :

//Create a quick DBView called db_example.

//Rows in this view will be of type db_example_row.

//See DTL_TABLE for details.

DTL_TABLE3(db_example,

   int, int_value,

   string, string_value,

   double, double_value

);





// Random access container example

void RandomDBViewExample()

{

   RandomDBView<db_example_row> view(db_example);



   cout << "Objects read from DB:" << endl;

   copy(view.begin(), view.end(), ostream_iterator<db_example_row>(cout, "\n"));



   cout << "\nElements in reverse order:" << endl;

   copy(view.rbegin(), view.rend(), ostream_iterator<db_example_row>(cout, "\n"));



   // modify the third row in the table

   RandomDBView<db_example_row>::iterator it = view.begin();

   db_example_row row(it[2]);

   row.int_value++;

   *it = row;

   

   // NOTE THAT FOR THE MODIFIED ROW TO SHOW UP WE MUST CALL ReQuery().

   // Note that there is no guarantee that the third row will show the updated record,

   // since other users may have modified the database or the DB may return records in a different

   // order.  See the ReQuery() function for details.

   cout << "Show updated result row:" << endl;

   view.ReQuery();

   cout << it[2] << endl;

 

   // show distance functions 

   cout << "Distance from first to last: " << view.end() - view.begin() << endl;

   cout << "Container size: " << view.size() << endl;



   // insert and delete rows

   db_example_row row_insert(it[2]);

   row_insert.int_value = 666;

   view.insert(row_insert);

   view.erase(it+(ptrdiff_t)2);

   cout << "Show result set with inserted/deleted row:" << endl;

   view.ReQuery();

   copy(view.begin(), view.end(), ostream_iterator<db_example_row>(cout, "\n"));



   // show container comparison operators

   RandomDBView<db_example_row> view2(view);

   cout << boolalpha;

   cout << "view == view2 : " << (view == view2) << endl;

   cout << "view < view2 : " << (view < view2) << endl;

}

Example 2 :

// Random access DBView built from a DynamicDBView

void RandomDynamicDBView()

{

	DynamicDBView<> dynamic_view("DB_EXAMPLE", "*");

	RandomDBView<variant_row, variant_row> random_view(dynamic_view);



	cout << "Items from DB:" << endl;

	copy(random_view.begin(), random_view.end(), ostream_iterator<variant_row>(cout, "\n"));

	cout << "\n\n";



	cout << "Items from DB in reverse order:" << endl;

	copy(random_view.rbegin(), random_view.rend(), ostream_iterator<variant_row>(cout, "\n"));



	// insert and delete rows

	variant_row row_insert(random_view[2]);

	row_insert["INT_VALUE"] = 666;

	random_view.insert(row_insert);

	random_view.erase(random_view.begin()+(ptrdiff_t)2);

	cout << "Show result set with inserted/deleted row:" << endl;

	random_view.ReQuery();

    	copy(random_view.begin(), random_view.end(), ostream_iterator<variant_row>(cout, "\n"));

}

Public Base Classes

RandomDBView

Template parameters

Associated types

As defined in a Random Access Container .

Expression semantics

RandomDBView(const DBView<DataObj, ParamObj>)

New Members

RandomDBView(const DBView<DataObj, ParamObj>)

Members

bool operator==(const RandomDBView&, 

                const RandomDBView&)

bool operator<(const RandomDBView&, 

               const RandomDBView&)

bool operator>(const RandomDBView&, 

               const RandomDBView&)

bool operator<=(const RandomDBView&, 

               const RandomDBView&)

bool operator>=(const RandomDBView&, 

               const RandomDBView&)

[1] RandomDBView design

When we designed RandomDBView we built the container to work against a "static ODBC scrollable cursor" as the underlying cursor for retrieving rows. Using such a cursor gives us a defined and repeatable ordering for the records which was what we needed to obtain random access iterators. Unfortunately, static ODBC cursors have the limitation that if you try to update records directly via the cursor, some ODBC drivers will show the updated records but other drivers will not. To avoid amiguity, we had the container apply updates via an independently executed query so that we could specify how changes would show up rather than having the behavior be ODBC driver dependent. Unfortunately, this means that our non-const random iterators are not quite standard compliant since changes don't show up until a special ReQuery() method is called.

One possible solution to this problem would be to add logging to the container. The idea here would be that when users make a change to a row, we log the updated row. Then, when the iterator goes to retrieve a row via operator * we would simply check whether a more recent version of the row exists in the change log and show it rather than the original. The problems with this approach are twofold:

1. It slows down record access for users that don't want logging.
2. If a lot of records are changed, potentially the log could become as large as the entire table. This sort of defeats the idea of RandomDBView because if the table can be easily held in memory why not just fetch the whole thing into a container or an IndexedDBView?

See also

Random Access Container, DBView

Copyright © 2002, Michael Gradman and Corwin Joy.

Permission to use, copy, modify, distribute and sell this software and its documentation for any purpose is hereby granted without fee, provided that the above copyright notice appears in all copies and that both that copyright notice and this permission notice appear in supporting documentation. Corwin Joy and Michael Gradman make no representations about the suitability of this software for any purpose. It is provided "as is" without express or implied warranty.

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