How to handle derived columns

Hello,

Consider the following entities:

namespace Sample
{
    public class Blob : Entity<long>
    {
        public string ContentType 
        {
            get;
            set;
        }

        public long ContentLength
        {
            get;
            set;
        }

        public string SHA512Hash
        {
            get;
            set;
        }

        // more properties
    }

    public class BlobReference : Entity<long>
    {
        // a blob has many references and a reference has one blob
        public Blob Blob 
        {
            get;
            set;
        }
    }

    public class Document : Entity<long>
    {
        // A document has a single reference
        public BlobReference BlobReference
        {
            get;
            set;
        }

        // this needs to be mapped to BlobReference.Blob.ContentType
        public string ContentType 
        {
            get;
        }

        // this needs to be mapped to BlobReference.Blob.ContentLength
        public long ContentLength
        {
            get;
        }

        // this needs to be mapped to BlobReference.Blob.SHA512Hash
        public string SHA512Hash
        {
            get;
        }
    }
}

How does one map the properties on Document such that it would return the equivalent properties on the "Blob"? For example, I'd expect the ContentType, ContentLength, SHA512Hash to populated when performing UnitOfWork.Query, UnitOfWork.Find, UnitOfWork.FindOne. Ideally I'd only like one SELECT statement to be sent to the database. However, I should not be able to set the values on the Document class (as these things are calculated when a blob is created the very first time).

I don't use the designer.

Thanks, Werner

You cant map them in such a way that they would be loaded in when querying for that entity directly (an entity with properties spanning multiple tables) where only one SELECT statement would be generated. You could however look at eager loading BlobReference and have BlobReference eager load its Blob and then have the properties refer to the associated properties across those associations, this would keep it to one batch of statements.

e.g.

    // this needs to be mapped to BlobReference.Blob.ContentType
    public string ContentType 
    {
        get { return BlobReference.Blob.ContentType; }
    }

Jeremy

I'm already doing that.

Its possible for me to write an expression as follows, which gets the content type for a given document and it spans multiple tables.

Query query  = Query(typeof(Document), Entity.Attribute("Id") == 1);
query.Projection.Add("BlobReference.Blob.ContentType");
var results = unitOfWork.Project<string>(query);  
if (results.Count == 1)
{
    return results[0];
}
else
{
    return null;
}

So I am curious whether there is anything I can do to "assist" LightSpeed, perhaps by writing a query (or expression) which may result in a query similar than the one above, but include all the "derived" properties.

Thanks, Werner

Unfortunately not. In the Find and FindOne cases (when we are loading a whole entity rather than a projection) we are expecting to load the fields which are specified in the model and there is no way for loading a field from across an association in the model in the same manner that you specify them in the projection syntax above.

Jeremy