If you’re a user of NHibernate, I hope you enjoy using polymorphic queries. It’s one of the most powerful features of this ORM allowing you to write queries spanning against several hierarchies. Finally, you can even query for the derivations of object to get all the objects from db (don’t try this at home:P). The most important fact is, that having a nicely implemented dialect NH can do it in one DB call, separating specific queries by semicolon (in case of MS SQL)
var allObjects = session.QueryOver<object>().List();
Although the feature is powerful, you can find a small problem. How to count entities returned by the query without getting’em all to the memory. Setting a simple projection Projections.RowCount() will not work. Why? ‘Cause it’s gonna query each table with COUNT demanding at the same time that IDataReader should contain only one, unique result. It won’t happen, and all you’ll be left with it’ll be a nice exception. So, is it possible to count entities in a polymorphic way? Yes! You can define in a extension method and use it every time you need a polymorphic count.
private static int GetPolymorphicCount(ISession s, Type countedType)
var factory = s.GetSessionImplementation().Factory;
var implementors = factory.GetImplementors(countedType.FullName);
.Select(i => s.CreateCriteria(i)
.ToArray() // to eagerly create all the future values for counting
.Aggregate(0, (count, v) => count + v.Value); // sum up counts
As i it is stated in the official documentation, the column family can be compared to a table in the relational database and as with table, the main target of creating one is to hold the same type of objects together to create a better model, allow querying, etc. Speaking about Cassandra it has one more advantage: the whole column family is stored on one server (the data are consistently hashed by key), so you may consider a column family as a collection of items frequently used together, for instance: the surname and the name, or the user name and the password.
In NHibernate you can embed those values within component to make it look like a whole, but they’re still stored in the same table. Speaking about Deiphobus, it can be easily configured to match the needs of grouping properties (simple properties, and referencing other entities in many-to-one or one-to-one way) by implementing a convention interface:
public interface IPropertyFamilyConvention
/// Gets the Cassandra family name.
/// <param name="mappedType">The mapped type.</param>
/// <param name="propertyInfo">The info of a mapped property.</param>
/// <returns>The family name.</returns>
FamilyName GetFamilyName(Type mappedType, PropertyInfo propertyInfo);
It’s worth to mention, that by default all the properties are mapped to one column family called ‘Entity’. Ok, you know how it influences the storage from the Cassandra point of view, but what about Deiphobus? As it was implemented, every time you access previously not loaded property of an entity mapped with Deiphobus, the whole column family, containing the specified property is loaded from the database (actually a bit more data is retrieved, but for the sake of simplicity it can be omitted in here). It means, that once you started using a property which is strongly connected with others, all the needed properties will be loaded in one db hit. Simple and powerful, isn’t it?
Last time, when an identity map of the Deiphobus was described, one user entity of was asked about several times. We already know, that the same object was returned, but what about hitting the Cassandra DB? Consider the following code:
var user = session.Load<IUser>(5);
// some other loads and operations
var model = user.Login; // here goes db hit!
As you can see, the database is not hit till one of the properties is queried. It’s default and only mode of loading entities with Deiphobus. Is allows a great reduction of db calls, if your code is structured in a right way (query for data first, then operate). The question is, what if a user holds a few massive, in terms of bytes transported, properties. Will of them will be loaded at once, even if they’re unneeded? The answer will be revealed in a very next post.