In previous chapters in this book, we built applications that load and display several DB databases. In this example, we will extend those examples to use secondary databases. Specifically:
In Database Usage Example we built an application that can open and load data into several databases. In Secondary Databases with example_database_load we will extend that application to also open a secondary database for the purpose of indexing inventory item names.
            In Cursor Example we
            built an application to display our inventory database (and related
            vendor information). In 
            Secondary Databases with example_database_read
            we will extend that application to
            show inventory records based on the index we cause to be loaded using
            example_database_load.
        
            In order to update example_database_load 
            to maintain an index of inventory item names, all we really need 
            to do is:
        
Create a new database to be used as a secondary database.
Associate our new database to the inventory primary database.
We also need a function that can create our secondary keys for us.
            Because DB maintains secondary databases for us; once this work
            is done we need not make any other changes to example_database_load. 
            
        
Remember that you can find the complete implementation of these functions in:
DB_INSTALL/examples_cxx/getting_started
        
            where DB_INSTALL is the location where you
            placed your DB distribution.
        
            To begin, we go to gettingStartedCommon.hpp and
            we write our secondary key extractor function. This is a fairly
            trivial function to write because we have already done most of the
            work when we wrote the InventoryData class.
            Recall that when we wrote that class, we provided a constructor that
            accepts a pointer to a buffer and unpacks the contents of the buffer
            for us (see InventoryData Class
            for the implementation). We now make use of that constructor.
        
// File: gettingStartedCommon.hpp
// Forward declarations
class Db;
class Dbt;
// Used to extract an inventory item's name from an
// inventory database record. This function is used to create
// keys for secondary database records.
int
get_item_name(Db *dbp, const Dbt *pkey, const Dbt *pdata, Dbt *skey)
{
    // Obtain the buffer location where the we placed the item's name. In
    // this example, the item's name is located in the primary data. It is
    // the first string in the buffer after the price (a double) and
    // the quantity (a long).
    size_t offset = sizeof(double) + sizeof(long);
    char * itemname = (char *)pdata->get_data() + offset;
    // unused
    (void)pkey;
    // If the offset is beyond the end of the data, then there is a
    // problem with the buffer contained in pdata, or there's a
    // programming error in how the buffer is marshalled/unmarshalled.
    // This should never happen!
    if (offset > pdata->get_size()) {
        dbp->errx("get_item_name: buffer sizes do not match!");
        // When we return non-zero, the index record is not
        // added/updated.
        return (-1);
    }
    // Now set the secondary key's data to be the item name 
    skey->set_data(itemname);
    skey->set_size(strlen(itemname) + 1);
    return (0);
}; 
        
        Having written our key extractor callback, we now need to make
        a trivial update to our MyDb implementation.
        Because an item name is used by multiple inventory records, we need our
        secondary database to support sorted duplicates. We therefore must
        update MyDb to handle this detail.
    
        The MyDb class definition changes to add a
        boolean to the constructor (remember that new code is in
        bold):
    
// File: MyDb.hpp
#include <db_cxx.h>
class MyDb
{
public:
    // Constructor requires a path to the database,
    // and a database name.
    MyDb(std::string &path, std::string &dbName,
         bool isSecondary = false);
    // Our destructor just calls our private close method.
    ~MyDb() { close(); }
    inline Db &getDb() {return db_;}
private:
    Db db_;
    std::string dbFileName_;
    u_int32_t cFlags_;
    // Make sure the default constructor is private
    // We don't want it used.
    MyDb() : db_(0, 0) {}
    // We put our database close activity here.
    // This is called from our destructor. In
    // a more complicated example, we might want
    // to make this method public, but a private
    // method is more appropriate for this example.
    void close();
}; 
        And the implementation changes slightly to take advantage of the new boolean. Note that to save space, we just show the constructor where the code actually changes:
// File: MyDb.cpp
#include "MyDb.hpp"
// Class constructor. Requires a path to the location
// where the database is located, and a database name
MyDb::MyDb(std::string &path, std::string &dbName,
           bool isSecondary)
    : db_(NULL, 0),               // Instantiate Db object
      dbFileName_(path + dbName), // Database file name
      cFlags_(DB_CREATE)          // If the database doesn't yet exist,
                                  // allow it to be created.
{
    try
    {
        // Redirect debugging information to std::cerr
        db_.set_error_stream(&std::cerr);
        // If this is a secondary database, support
        // sorted duplicates
        if (isSecondary)
            db_.set_flags(DB_DUPSORT);
        // Open the database
        db_.open(NULL, dbFileName_.c_str(), NULL, DB_BTREE, cFlags_, 0);
    }
    // DbException is not a subclass of std::exception, so we
    // need to catch them both.
    catch(DbException &e)
    {
        std::cerr << "Error opening database: " << dbFileName_ << "\n";
        std::cerr << e.what() << std::endl;
    }
    catch(std::exception &e)
    {
        std::cerr << "Error opening database: " << dbFileName_ << "\n";
        std::cerr << e.what() << std::endl;
    }
} 
        
        That done, we can now update
        example_database_load to open our new secondary
        database and associate it to the inventory database.
    
        To save space, we do not show the entire implementation for this program
        here. Instead, we show just the main() function,
        which is where all our modifications occur. To
        see the rest of the implementation for this command, see 
          example_database_load.
    
// Loads the contents of vendors.txt and inventory.txt into
// Berkeley DB databases.
int
main(int argc, char *argv[])
{
    // Initialize the path to the database files
    std::string basename("./");
    std::string databaseHome("./");
    // Database names
    std::string vDbName("vendordb.db");
    std::string iDbName("inventorydb.db");
    std::string itemSDbName("itemname.sdb");
    // Parse the command line arguments here and determine
    // the location of the flat text files containing the
    // inventory data here. This step is omitted for clarity.
    //  Identify the full name for our input files, which should
    //  also include some path information.
    std::string inventoryFile = basename + "inventory.txt";
    std::string vendorFile = basename + "vendors.txt";
    try
    {
        // Open all databases.
        MyDb inventoryDB(databaseHome, iDbName);
        MyDb vendorDB(databaseHome, vDbName);
        MyDb itemnameSDB(databaseHome, itemSDbName, true);
        // Associate the primary and the secondary
        inventoryDB.getDb().associate(NULL,
                                      &(itemnameSDB.getDb()),
                                      get_item_name,
                                      0);
        // Load the vendor database
        loadVendorDB(vendorDB, vendorFile);
        // Load the inventory database
        loadInventoryDB(inventoryDB, inventoryFile);
    } catch(DbException &e) {
        std::cerr << "Error loading databases. " << std::endl;
        std::cerr << e.what() << std::endl;
        return(e.get_errno());
    } catch(std::exception &e) {
        std::cerr << "Error loading databases. " << std::endl;
        std::cerr << e.what() << std::endl;
        return(-1);
    }
    return(0);
} // End main 
        
        Note that the order in which we instantiate our
        MyDb class instances is important. In general you
        want to close a secondary database before closing the primary with which
        it is associated. This is particularly true for multi-threaded or
        multi-processed applications where the database closes are not single
        threaded. Even so, it is a good habit to adopt, even for simple
        applications such as this one. Here, we ensure that the databases are
        closed in the desired order by opening the secondary database last.
        This works because our MyDb objects are on
        the stack, and therefore the last one opened is the first one closed.
    
        That completes our update to example_database_load.
        Now when this program is called, it will automatically index inventory
        items based on their names. We can then query for those items using the
        new index. We show how to do that in the next section.
    
            In Cursor Example we
            wrote an application that displays every inventory item in the
            Inventory database. In this section, we will update that example to
            allow us to search for and display an inventory item given a
            specific name. To do this, we will make use of the secondary
            database that example_database_load now
            creates.
        
            The update to example_database_read is
            relatively modest. We need to open the new secondary database
            in exactly the same way was we do for
            example_database_load. 
            We also need to add a command line parameter on
            which we can specify the item name, and we will need a new function
            in which we will perform the query and display the results.
        
To begin, we add a single forward declaration to the application, and update our usage function slightly:
// File: example_database_read.cpp
#include <iostream>
#include <fstream>
#include <cstdlib>
#include "MyDb.hpp"
#include "gettingStartedCommon.hpp"
// Forward declarations
int show_all_records(MyDb &inventoryDB, MyDb &vendorDB);
int show_item(MyDb &itemnameSDB, MyDb &vendorDB, std::string &itemName);
int show_vendor(MyDb &vendorDB, const char *vendor); 
        
        Next, we update main() to 
        open the new secondary database and
        accept the new command line switch.
        We also need a new variable to contain the item's name.        
    
        The final update to the main() entails a little bit
        of logic to determine whether we want to display all available inventory
        items, or just the ones that match a name provided on the
        -i command line parameter.
    
// Displays all inventory items and the associated vendor record.
int
main (int argc, char *argv[])
{
    // Initialize the path to the database files
    std::string databaseHome("./");
    std::string itemName;
    // Database names
    std::string vDbName("vendordb.db");
    std::string iDbName("inventorydb.db");
    std::string itemSDbName("itemname.sdb");
    // Parse the command line arguments
    // Omitted for brevity
    try
    {
        // Open all databases.
        MyDb inventoryDB(databaseHome, iDbName);
        MyDb vendorDB(databaseHome, vDbName);
        MyDb itemnameSDB(databaseHome, itemSDbName, true);
        // Associate the secondary to the primary
        inventoryDB.getDb().associate(NULL,
                                      &(itemnameSDB.getDb()),
                                      get_item_name,
                                      0);
        if (itemName.empty())
        {
            show_all_records(inventoryDB, vendorDB);
        } else {
            show_item(itemnameSDB, vendorDB, itemName);
        }
    } catch(DbException &e) {
        std::cerr << "Error reading databases. " << std::endl;
        std::cerr << e.what() << std::endl;
        return(e.get_errno());
    } catch(std::exception &e) {
        std::cerr << "Error reading databases. " << std::endl;
        std::cerr << e.what() << std::endl;
        return(-1);
    }
    return(0);
} // End main 
        
        The only other thing that we need to add to the application is the
        implementation of the 
             
            show_item() 
        function.
    
In the interest of space, we refrain from showing the other functions used by this application. For their implementation, please see Cursor Example. Alternatively, you can see the entire implementation of this application in:
DB_INSTALL/examples_cxx/getting_started
          
            where DB_INSTALL is the location where you
            placed your DB distribution.
        
// Shows the records in the inventory database that
// have a specific item name. For each inventory record
// shown, the appropriate vendor record is also displayed.
int
show_item(MyDb &itemnameSDB, MyDb &vendorDB, std::string &itemName)
{
    // Get a cursor to the itemname secondary db
    Dbc *cursorp;
    try {
        itemnameSDB.getDb().cursor(NULL, &cursorp, 0);
        // Get the search key. This is the name on the inventory
        // record that we want to examine.
        std::cout << "Looking for " << itemName << std::endl;
        Dbt key((void *)itemName.c_str(), itemName.length() + 1);
        Dbt data;
        // Position the cursor to the first record in the secondary
        // database that has the appropriate key.
        int ret = cursorp->get(&key, &data, DB_SET);
        if (!ret) {
            do {
                InventoryData inventoryItem(data.get_data());
                inventoryItem.show();
                show_vendor(vendorDB, inventoryItem.getVendor().c_str());
            } while(cursorp->get(&key, &data, DB_NEXT_DUP) == 0);
        } else {
            std::cerr << "No records found for '" << itemName
                      << "'" << std::endl;
        }
    } catch(DbException &e) {
        itemnameSDB.getDb().err(e.get_errno(), "Error in show_item");
        cursorp->close();
        throw e;
    } catch(std::exception &e) {
        itemnameSDB.getDb().errx("Error in show_item: %s", e.what());
        cursorp->close();
        throw e;
    }
    cursorp->close();
    return (0);
}
        
        This completes our update to
        example_inventory_read. Using this update, you
        can now search for and show all inventory items that match a particular
        name. For example:
    
example_inventory_read -i "Zulu Nut"