# 2. First Steps

Let's get started as simple as possible. We are going to demonstrate how to store, retrieve, update and delete instances of a single class that only contains primitive and String members. In our example this will be a Formula One (F1) pilot whose attributes are his name and the F1 points he has already gained this season.
First we create a class to hold our data. It looks like this:

`namespace com.db4o.f1.chapter1 { public class Pilot { string _name; int _points; public Pilot(string name, int points) { _name = name; _points = points; } public string Name { get { return _name; } } public int Points { get { return _points; } } public void AddPoints(int points) { _points += points; } override public string ToString() { return _name + "/" + _points; } } }`

Notice that this class does not contain any db4o-related code.

## 2.1. Opening the database

To access a db4o database file or create a new one, call Db4o.openFile(), providing the path to your file as the parameter, to obtain an ObjectContainer instance. ObjectContainer represents "The Database", and will be your primary interface to db4o. Closing the container with the #.close() method will close the database file and release all resources associated with it.

`[accessDb4o] ObjectContainer db=Db4o.openFile(Util.YapFileName); try { // do something with db4o } finally { db.close(); }`

For the following examples we will assume that our environment takes care of opening and closing the ObjectContainer automagically, and stores the reference in a variable named 'db'.

## 2.2. Storing objects

To store an object, we simply call set() on our database, passing any object as a parameter.

`[storeFirstPilot] Pilot pilot1 = new Pilot("Michael Schumacher", 100); db.set(pilot1); Console.WriteLine("Stored " + pilot1);`

OUTPUT: Stored Michael Schumacher/100

We'll need a second pilot, too.

`[storeSecondPilot] Pilot pilot2 = new Pilot("Rubens Barrichello", 99); db.set(pilot2); Console.WriteLine("Stored " + pilot2);`

OUTPUT: Stored Rubens Barrichello/99

## 2.3. Retrieving objects

db4o supplies three different quering systems, Query by Example (QBE), Native Queries (NQ) and the SODA Query API (SODA). In this first example we will introduce QBE. Once you are familiar with storing objects, we encourage you to use Native Queries, the main db4o querying interface.
When using Query-By-Example, you create a prototypical object for db4o to use as an example of what you wish to retrieve. db4o will retrieve all objects of the given type that contain the same (non-default) field values as the example. The results will be returned as an ObjectSet instance. We will use a convenience method 'listResult' to display the contents of our results:

`public static void listResult(ObjectSet result) { Console.WriteLine(result.size()); while (result.hasNext()) { Console.WriteLine(result.next()); } }`

To retrieve all pilots from our database, we provide an 'empty' prototype:

`[retrieveAllPilotQBE] Pilot proto = new Pilot(null, 0); ObjectSet result = db.get(proto); listResult(result);`

OUTPUT: 2 Michael Schumacher/100 Rubens Barrichello/99

Note that we specify 0 points, but our results were not constrained to only those Pilots with 0 points; 0 is the default value for int fields.
db4o also supplies a shortcut to retrieve all instances of a class:

`[retrieveAllPilots] ObjectSet result = db.get(typeof(Pilot)); listResult(result);`

OUTPUT: 2 Michael Schumacher/100 Rubens Barrichello/99

For there also is a generics shortcut, using the query method:
To query for a pilot by name:

`[retrievePilotByName] Pilot proto = new Pilot("Michael Schumacher", 0); ObjectSet result = db.get(proto); listResult(result);`

OUTPUT: 1 Michael Schumacher/100

And to query for Pilots with a specific number of points:

`[retrievePilotByExactPoints] Pilot proto = new Pilot(null, 100); ObjectSet result = db.get(proto); listResult(result);`

OUTPUT: 1 Michael Schumacher/100

Of course there's much more to db4o queries. They will be covered in more depth in later chapters.

## 2.4. Updating objects

Updating objects is just as easy as storing them. In fact, you use the same set() method to update your objects: just call set() again after modifying any object.

`[updatePilot] ObjectSet result = db.get(new Pilot("Michael Schumacher", 0)); Pilot found = (Pilot)result.next(); found.AddPoints(11); db.set(found); Console.WriteLine("Added 11 points for " + found); retrieveAllPilots(db);`

OUTPUT: Added 11 points for Michael Schumacher/111 2 Michael Schumacher/111 Rubens Barrichello/99

Notice that we query for the object first. This is an importaint point. When you call set() to modify a stored object, if the object is not 'known' (having been previously stored or retrieved during the current session), db4o will insert a new object. db4o does this because it does not automatically match up objects to be stored, with objects previously stored. It assumes you are inserting a second object which happens to have the same field values.
To make sure you've updated the pilot, please return to any of the retrieval examples above and run them again.

## 2.5. Deleting objects

Objects are removed from the database using the delete() method.

`[deleteFirstPilotByName] ObjectSet result = db.get(new Pilot("Michael Schumacher", 0)); Pilot found = (Pilot)result.next(); db.delete(found); Console.WriteLine("Deleted " + found); retrieveAllPilots(db);`

OUTPUT: Deleted Michael Schumacher/111 1 Rubens Barrichello/99

Let's delete the other one, too.

`[deleteSecondPilotByName] ObjectSet result = db.get(new Pilot("Rubens Barrichello", 0)); Pilot found = (Pilot)result.next(); db.delete(found); Console.WriteLine("Deleted " + found); retrieveAllPilots(db);`

OUTPUT: Deleted Rubens Barrichello/99 0

Please check the deletion with the retrieval examples above.
As with updating objects, the object to be deleted has to be 'known' to db4o. It is not sufficient to provide a prototype object with the same field values.

## 2.6. Conclusion

That was easy, wasn't it? We have stored, retrieved, updated and deleted objects with a few lines of code. But what about complex queries? Let's have a look at the restrictions of QBE and alternative approaches in the next chapter .

## 2.7. Full source

`namespace com.db4o.f1.chapter1
{
using System;
using System.IO;
using com.db4o;
using com.db4o.f1;
public class FirstStepsExample : Util
{
public static void Main(string[] args)
{
File.Delete(Util.YapFileName);
accessDb4o();
File.Delete(Util.YapFileName);
ObjectContainer db = Db4o.openFile(Util.YapFileName);
try
{
storeFirstPilot(db);
storeSecondPilot(db);
retrieveAllPilots(db);
retrievePilotByName(db);
retrievePilotByExactPoints(db);
updatePilot(db);
deleteFirstPilotByName(db);
deleteSecondPilotByName(db);
}
finally
{
db.close();
}
}
public static void accessDb4o()
{
ObjectContainer db=Db4o.openFile(Util.YapFileName);
try
{
// do something with db4o
}
finally
{
db.close();
}
}
public static void storeFirstPilot(ObjectContainer db)
{
Pilot pilot1 = new Pilot("Michael Schumacher", 100);
db.set(pilot1);
Console.WriteLine("Stored " + pilot1);
}
public static void storeSecondPilot(ObjectContainer db)
{
Pilot pilot2 = new Pilot("Rubens Barrichello", 99);
db.set(pilot2);
Console.WriteLine("Stored " + pilot2);
}
public static void retrieveAllPilotQBE(ObjectContainer db) {
Pilot proto = new Pilot(null, 0);
ObjectSet result = db.get(proto);
listResult(result);
}
public static void retrieveAllPilots(ObjectContainer db) {
ObjectSet result = db.get(typeof(Pilot));
listResult(result);
}
public static void retrievePilotByName(ObjectContainer db)
{
Pilot proto = new Pilot("Michael Schumacher", 0);
ObjectSet result = db.get(proto);
listResult(result);
}
public static void retrievePilotByExactPoints(ObjectContainer db)
{
Pilot proto = new Pilot(null, 100);
ObjectSet result = db.get(proto);
listResult(result);
}
public static void updatePilot(ObjectContainer db)
{
ObjectSet result = db.get(new Pilot("Michael Schumacher", 0));
Pilot found = (Pilot)result.next();
found.AddPoints(11);
db.set(found);
Console.WriteLine("Added 11 points for " + found);
retrieveAllPilots(db);
}
public static void deleteFirstPilotByName(ObjectContainer db)
{
ObjectSet result = db.get(new Pilot("Michael Schumacher", 0));
Pilot found = (Pilot)result.next();
db.delete(found);
Console.WriteLine("Deleted " + found);
retrieveAllPilots(db);
}
public static void deleteSecondPilotByName(ObjectContainer db)
{
ObjectSet result = db.get(new Pilot("Rubens Barrichello", 0));
Pilot found = (Pilot)result.next();
db.delete(found);
Console.WriteLine("Deleted " + found);
retrieveAllPilots(db);
}
}
}
`

--
generated by Doctor courtesy of db4objects Inc.