Can someone better explain what 'Projections' are in nHibernate?

a 夏天 提交于 2019-11-29 21:23:29
Nicholas Piasecki

Here's a practical example.

Let's say that you have an online store and one of your domain classes is a Brand like "Samsung". This class has a boatload of properties associated with it, perhaps an integer Identity, a Name, a free-text Description field, a reference to a Vendor object, and so on.

Now let's say that you want to display a menu with a list of all the brands offered on your online store. If you just do session.CreateCriteria<Brand>().List(), then you are indeed going to get all of the brands. But you'll also have sucked all of the long Description fields and references to Vendors from the database, and you don't need that to display a menu; you just need the Name and the Identity. Performance-wise, sucking all of this extra data down from the database slows things down and is unnecessary.

Instead, you can create a "projection" object that contains just the Identity and the Name calling it, say, NameIdentityPair:

public class NameIdentityPair
{
    public int Identity { get; set; }
    public string Name { get; set; }
}

And you could tell NHibernate to only select the data that you really need to perform the task at hand by telling it to transform the result set onto your projection:

var brandProjections = this.session.CreateCriteria<Brand>()
    .SetProjection(Projections.ProjectionList()
        .Add(Projections.Property("Name"), "Name")
        .Add(Projections.Property("Identity"), "Identity"))
    .SetResultTransformer(Transformers.AliasToBean<NameIdentityPair>())
    .List<NameIdentityPair>();

foreach (var brandProjection in brandProjections)
{
    Console.WriteLine(
        "Identity: {0}, Name: {1}", 
        brandProjection.Identity, 
        brandProjection.Name);
}

Now you don't have a list of Brands but instead a list of NameIdentityPairs, and NHibernate will have only issued a SQL statement like SELECT b.Identity, b.Name from dbo.Brand b to obtain this projection, as opposed to a massive SQL statement that grabs everything necessary to hydrate a Brand object (e.g., SELECT b.Identity, b.Name, b.Description from dbo.brand b left join dbo.vendor v ....).

Hope this helps.

If you're familiar with SQL, a projection is the SELECT clause of a query, used to select which fields from the available results to return.

For example, assume you have a Person with FirstName, LastName, Address, and Phone fields. If you want a query to return everything, you can leave off the projection, which is like SELECT * FROM Person in SQL. If you just want the first and last names, you would create a projection with FirstName and LastName -- which would be SELECT FirstName, LastName FROM Person in SQL terms.

You can use projections to call sql functions like SUM, COUNT... or select single fields without return an entity.

"...Retrieving only properties of an entity or entities, without the overhead of loading the entity itself in a transactional scope. This is sometimes called a report query; it’s more correctly called projection." [NHibernate in Action]

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