Sony Arouje

I was working on a small Invoicing application for my brother and I uses db4o for data persistence. Again I used db4o here because of it’s simplicity and ease of doing future updates.

I started with POCO entities and db4o handles the insertions and deletion quiet well. I started experiencing trouble when I try to update object graphs. For e.g. I created a SaleOrder with two Items then I retrieve the order and added one more item and save the changes. db4o will never save the newly added Item, it always shows the items that added initially. Because db4o couldn’t make out the changes in the object graph. There are two ways to let db4o handle this scenario.

UpdateDepth

There is a problem with UpdateDepth, we have to define a depth say 5, 10, etc and db4o will traverse the graph to identify the changes, that means we should forecast our object graph depth. This also introduce performance issues. So I decided not to use it.

Transparent Persistence

With Transparent Persistence (TP) the object will tell which all the fields got modified and db4o will update the changed fields efficiently.

To enable Transparent Persistence we have to do some changes in

Let’s start with repository, I used the same kind of repository I created for iTraveller with some changes in creating the db4o context/container. The change is as shown below.

public IObjectContainer Context
{
    get
    {
        if (_context == null)
            this.CreateContext();
        return _context;
    }
}

public void CreateContext()
{
    if (_context == null)
    {
        IEmbeddedConfiguration config = Db4oEmbedded.NewConfiguration();
        config.Common.Add(new TransparentActivationSupport());
        config.Common.Add(new TransparentPersistenceSupport());
        _context = Db4oEmbedded.OpenFile(config, DBNAME);
    }
}

It’s the time to do some modification in Entity level.

using System;
usingDb4objects.Db4o.Activation;
usingDb4objects.Db4o.Collections;

usingDb4objects.Db4o.TA;
namespaceNet.Sales.Manager.Domain
{
    public classSaleOrderHeader : IActivatable
   {
        [System.NonSerialized]
        privateIActivator _activator;

        publicSaleOrderHeader ()
        {
           
            this.OrderDate = DateTime.Now;
            this.SaleOrderItems = newArrayList4<SaleOrderItem>();
        }

        public int OrderId { get; set; }

        private IList<SaleOrderItem> _salesOrderItems;
        public IList<SaleOrderItem> SaleOrderItems 
        {
            get 
            {
                this.Read();
                return this._salesOrderItems;
            }
            private set
            {
                this._salesOrderItems = value;
                this.Write();
            }
        }

        private double _discount;
        public double Discount
        {
            get
            {
                this.Read();
                return _discount;
            }
            set
            {
                this.Write();
                this._discount = value;
            }
        }

        public void With(SaleOrderItem saleOrderItem)
        {
            if(this.SaleOrderItems==null)
                this.SaleOrderItems = new ArrayList4<SaleOrderItem>();
            this.Read();
            this.SaleOrderItems.Add(saleOrderItem);
        }

        public void Activate(ActivationPurpose purpose)
        {
            if (this._activator != null)
                _activator.Activate(purpose);
            
        }

        public void Bind(IActivator activator)
        {
            if (_activator == activator)
                return;
            
            if (activator != null && null != _activator)
                throw new System.InvalidOperationException();
            
            _activator = activator;
        }

        public void Read()
        {
            this.Activate(ActivationPurpose.Read);
        }

        public void Write()
        {
            this.Activate(ActivationPurpose.Write);
        }

    }
}

For simplicity I removed some functions and properties from the above class. As you can see the SalesOrderHeader is implemented IActivatable. All classes that implements IActivatable is a candidate for Transparent Activation and Transparent Persistence. IActivatable will add two methods Activate and Bind. Make sure you implement the functions as shown above. I also added two helper methods Read and Write to ease the use of Activate function. Have a look at the property Discount and see how we are using it.

private double _discount;
public double Discount
{
    get
    {
        this.Read();
        return _discount;
    }
    set
    {
        this.Write();
        this._discount = value;
    }
}

It is as simple as that.

If we a have collection and that needs TA and TP enabled then we have to use ArrayList4. It’s inherited from IList. In the above e.g. SaleOrderItem is a collection, see how we implemented it.

private IList<SaleOrderItem> _salesOrderItems;
public IList<SaleOrderItem> SaleOrderItems 
{
    get 
    {
        this.Read();
        return this._salesOrderItems;
    }
    private set
    {
        this._salesOrderItems = value;
        this.Write();
    }
}

Now we have to implement IActivatable in SaleOrderItem entity as well, the approach is same as mentioned above so skipping the code.

We are done, now db4o can handle Transparent Activation and Transparent Persistence and no need to provide any graph depth. Even if we provide Update or Activate depth db4o will ignore it.

Note

We don’t need to implement IActivatable in top level class if it’s not dealing with a collection. Say for e.g. we have Customer and Address class and Customer has an instance of Address. Then implement IActivatable in Address no need to implement it in Customer as by default db4o will update top level object.

Query TA/TP enabled objects

You can see that in my Generic Repository I query data using Lambda expression, and the query is created as shown below.

public IList<TEntity> Find<TEntity>(Func<TEntity, bool> criteria) where TEntity : class
{
    return this.Query<TEntity>().Where<TEntity>(criteria).ToList<TEntity>();
}

private IList<TEntity> Query<TEntity>() where TEntity : class
{
    return  this.Context.Query<TEntity>();
}

_repository.Find<SaleOrderHeader>(soh => soh.OrderId == salesOrderNumber);

This Query model is not working if we enable TA and TP. First or Find of SaleOrder always return null. I think I need to check with db4o devs.

The alternative is use Query by Example (QBE) as shown below.

public IList<TEntity> Find<TEntity>(TEntity criteria) where TEntity : class
{
    return this.Context.QueryByExample(criteria).Cast<TEntity>().ToList<TEntity>();
}

public SaleOrderHeader getSalesOrder(int salesOrderNumber)
{
    return _repository.First<SaleOrderHeader>(new SaleOrderHeader(salesOrderNumber));
}

You can find more details from db4o online help.

I will be releasing the entire source to Github soon.