|
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 Db4objects.Db4o.Tutorial.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 string.Format("{0}/{1}", _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 Db4oFactory.OpenFile() and provide the path to your database file as the parameter, to obtain an IObjectContainer instance. IObjectContainer represents "The Database", and will be your primary interface to db4o. Closing the IObjectContainer with the #Close() method will close the database file and release all resources associated with it.
// accessDb4o
IObjectContainer db = Db4oFactory.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 IObjectContainer 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 {0}", 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 {0}", 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 IObjectSet instance. We will use a convenience method #ListResult() to display the contents of our result IObjectSet :
public static void ListResult(IObjectSet result)
{
Console.WriteLine(result.Count);
foreach (object item in result)
{
Console.WriteLine(item);
}
} |
To retrieve all pilots from our database, we provide an 'empty' prototype:
// retrieveAllPilotQBE
Pilot proto = new Pilot(null, 0);
IObjectSet 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
IObjectSet 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);
IObjectSet 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);
IObjectSet 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
IObjectSet 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 {0}", 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
IObjectSet result = db.Get(new Pilot("Michael Schumacher", 0));
Pilot found = (Pilot)result.Next();
db.Delete(found);
Console.WriteLine("Deleted {0}", found);
RetrieveAllPilots(db); |
OUTPUT: Deleted Michael Schumacher/111
1
Rubens Barrichello/99
|
Let's delete the other one, too.
// deleteSecondPilotByName
IObjectSet result = db.Get(new Pilot("Rubens Barrichello", 0));
Pilot found = (Pilot)result.Next();
db.Delete(found);
Console.WriteLine("Deleted {0}", 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
using System;
using System.IO;
using Db4objects.Db4o.Query;
using Db4objects.Db4o.Tutorial;
namespace Db4objects.Db4o.Tutorial.F1.Chapter1
{
public class FirstStepsExample : Util
{
public static void Main(string[] args)
{
File.Delete(Util.YapFileName);
AccessDb4o();
File.Delete(Util.YapFileName);
IObjectContainer db = Db4oFactory.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()
{
IObjectContainer db = Db4oFactory.OpenFile(Util.YapFileName);
try
{
// do something with db4o
}
finally
{
db.Close();
}
}
public static void StoreFirstPilot(IObjectContainer db)
{
Pilot pilot1 = new Pilot("Michael Schumacher", 100);
db.Set(pilot1);
Console.WriteLine("Stored {0}", pilot1);
}
public static void StoreSecondPilot(IObjectContainer db)
{
Pilot pilot2 = new Pilot("Rubens Barrichello", 99);
db.Set(pilot2);
Console.WriteLine("Stored {0}", pilot2);
}
public static void RetrieveAllPilotQBE(IObjectContainer db)
{
Pilot proto = new Pilot(null, 0);
IObjectSet result = db.Get(proto);
ListResult(result);
}
public static void RetrieveAllPilots(IObjectContainer db)
{
IObjectSet result = db.Get(typeof(Pilot));
ListResult(result);
}
public static void RetrievePilotByName(IObjectContainer db)
{
Pilot proto = new Pilot("Michael Schumacher", 0);
IObjectSet result = db.Get(proto);
ListResult(result);
}
public static void RetrievePilotByExactPoints(IObjectContainer db)
{
Pilot proto = new Pilot(null, 100);
IObjectSet result = db.Get(proto);
ListResult(result);
}
public static void UpdatePilot(IObjectContainer db)
{
IObjectSet 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 {0}", found);
RetrieveAllPilots(db);
}
public static void DeleteFirstPilotByName(IObjectContainer db)
{
IObjectSet result = db.Get(new Pilot("Michael Schumacher", 0));
Pilot found = (Pilot)result.Next();
db.Delete(found);
Console.WriteLine("Deleted {0}", found);
RetrieveAllPilots(db);
}
public static void DeleteSecondPilotByName(IObjectContainer db)
{
IObjectSet result = db.Get(new Pilot("Rubens Barrichello", 0));
Pilot found = (Pilot)result.Next();
db.Delete(found);
Console.WriteLine("Deleted {0}", found);
RetrieveAllPilots(db);
}
}
}
|
--
generated by Doctor courtesy of db4objects Inc.