Database classes

Defining database classes

We can define the schema of a Starcounter database table in an application by defining public abstract C# classes decorated with the DatabaseAttribute attribute. We call these types database classes. The names and namespaces we give to these C# classes will be the names and namespaces of the corresponding database tables.

using System;
using Starcounter.Database;

[Database]
public abstract class Person
{
    public abstract string Name { get; set; }
}

All instances of these database classes created with the IDatabaseContext.Insert<T>() method are stored in the Starcounter database persistently.

Constructors

Database classes support default constructors, which is useful for adding logic that is run each time an instance of the database class is created. Public default constructors of database classes are called whenever a new instance of the class is created with IDatabaseContext.Insert<T>().

For exaαmple, here is our Person class with a custom default constructor:

using System;
using Starcounter.Database;

[Database]
public abstract class Person
{
    public abstract string Name { get; set; }
    public abstract DateTime CreatedAtUtc { get; set; }

    public Person()
    {
        CreatedAtUtc = DateTime.UtcNow;
    }
}

Fields and properties

Table columns are defined in database classes by abstract instance auto-implemented properties with public get and set accessors and with one of the supported data types. The Person database class below defines two such columns, Name and CreatedAtUtc:

using System;
using Starcounter.Database;

[Database]
public abstract class Person
{
    public abstract string Name { get; set; }
    public abstract DateTime CreatedAtUtc { get; set; }
}

Database classes can also define calculated properties, which are properties that do not expose an instance field. Commonly, we use them to reflect an aspect of the state of a database class instance. Let's say we want a shorthand for calculating the length of the Name string of a Person. We could do this by introducing a calculated property. These computed properties do not have the same restrictions as column properties, since they are transient and not bound to the corresponding database table.

using System;
using Starcounter.Database;

[Database]
public abstract class Person
{
    public abstract string Name { get; set; }
    public abstract DateTime CreatedAtUtc { get; set; }

    public int NameLength => Name.Length;
}

Note: If a database class definition of a Starcounter application contains any non-computed instance properties that are not declared as abstract auto-implemented properties with public get and set accessors and with one of the supported data types, an exception will be thrown when the application starts.

Database queries in computed properties

Sometimes it's very useful to make database queries inside the get accessors of computed properties of database classes. This way we can extend object-oriented principles like encapsulation onto the persistent nature of database objects. Let's expand our Person class with a Mother one-to-many relational property:

using System;
using Starcounter.Database;

[Database]
public abstract class Person
{
    public abstract string Name { get; set; }
    public abstract DateTime CreatedAtUtc { get; set; }
    public abstract Person Mother { get; set; }

    public IEnumerable<Person> Children
    {
        get
        {
            var db = DbProxy.GetContext(this);
            return db.Sql<Person>("SELECT p FROM Person p WHERE p.Mother = ?", this);
        }
    }

    public int NameLength => Name.Length;
}

We can then introduce a computed property Children with the purpose of listing, for each Person m, all Person instances p in the database where m is the value of the Mother property for p.

To implement this, we need access to the database context that the given Person instance lives in. We can achieve this by calling the static method DbProxy.GetContext(this).

Indexing

To achieve the full performance potential of the Starcounter database, it's crucial to register appropriate indexes for database classes. Database indexes can be defined with CREATE INDEX SQL queries. Both unique and not unique indexes are supported. Since CREATE INDEX is a DDL statement, we use the IDdlExecutor to perform it, outside an active transaction. We can obtain the IDdlExecutor from the service provider.

var ddlExecutor = services.GetRequiredService<IDdlExecutor>();

ddlExecutor.Execute("CREATE INDEX IX_Person_FirstName ON Person (FirstName)");

A single property index can also be registered for a column by decorating its associated C# property with the IndexAttribute attribute:

using System;
using Starcounter.Database;

[Database]
public abstract class Person
{
    [Index]
    public abstract string Name { get; set; }
}

Note: DDL statements can only be performed from IDdlExecutor.

Relations

One-to-many relations

It's recommended to model one-to-many relations using references both ways, with the child entity having a reference to the parent and the parent having an instance method or computed property that selects all the children (like in the example with the Mother/Children relation above).

Many-to-many relations

Many-to-many relations are best modelled using an association class.

Let's say we want to model many-to-many relation of share ownership, as understood as a relation between Person and Company entities such that a single person can hold multiple shares in multiple companies, and a single company can have multiple shareholders. For this, let's use the association class ShareOwnership. We can then make queries to this database class to calculate useful information in Person and Company that is exposed using computed properties. Lastly, we make sure that indexes are registered for the Owner and Equity properties of ShareOwnership since we will be quering them a lot.

using Starcounter.Database;

[Database]
public abstract class Person
{
    public abstract string Name { get; set; }

    public IEnumerable<Company> OwnsSharesInCompanies
    {
        get
        {
            var db = DbProxy.GetContext(this);
            return db.Sql<Company>("SELECT s.Equity FROM ShareOwnership s WHERE s.Owner = ?", this);
        }
    }
}

[Database]
public abstract class Company
{
    public abstract string Name { get; set; }
    public abstract Person Ceo { get; set; }

    public IEnumerable<Person> Shareholders
    {
        get
        {
            var db = DbProxy.GetContext(this);
            return db.Sql<Person>("SELECT s.Owner FROM ShareOwnership s WHERE s.Equity = ?", this);
        }
    }
}

[Database]
public abstract class ShareOwnership
{
    [Index]
    public abstract Person Owner { get; set; }

    [Index]
    public abstract Company Equity { get; set; }

    public abstract int Quantity { get; set; }
}

Inheritance

Inheritance is supported between database classes and has the same general semantics as class inheritence in C#.

using Starcounter.Database;

[Database]
public abstract class Customer
{
   public abstract string Name { get; set; }
}

public abstract class PrivateCustomer : Customer
{
   public abstract string Gender { get; set; }
}

public abstract class CorporateCustomer : Customer
{
   public abstract string VatNumber { get; set; }
}

The DatabaseAttribute attribute decoration is inherited from any base class to its subclasses, meaning that any class that directly or indirectly inherits a class that is decorated with the DatabaseAttribute attribute becomes a database class. In the example above, both PrivateCustomer and CorporateCustomer become database classes due to them inheriting Customer.

This also means that all rows in the CorporateCustomer table also are contained in the Customer table. The result of the SELECT C FROM Customer c SQL query will contain all rows from Customer as well as all rows from subclasses of Customer.

A database class cannot inherit from a class that's not a database class. It's also not possible to cast a non-database class to a database class.

Database object identity

Starcounter automatically assigns an unique integer identifier called Oid to each database object. The key is unique across all table rows in the entire database. Any id value is used only once, and not reused in the future, even if the original object was deleted. We can configure the range to use for oid's using the FirstObjectId and LastObjectId options in the database creation options.

The IDatabaseContext type defines methods for working with object Oid's. For the examples above, let's assume that db holds a reference to an instance of IDatabaseContext, recieved from an ITransactor when starting a new database transaction.

Get the Oid from a database object

var newObject = db.Insert<Product>();
ulong oid = db.GetOid(newObject);

Find an object in the database by its unique key

var oid = 459123UL;
var product = db.Get<Product>(oid);

Notes

• Zero (0) is not a valid Oid.

• It's not yet possible to insert a database object with a predefined Oid.

Comparing database objects

Database objects can be checked for equality with the object.Equals method or the Equals instance method for each database class instance. Comparing database objects with object.ReferenceEquals or the == operator always returns false if any of the objects are retrieved from the database.

We can also compare database objects by comparing their Oid's.

Example:

var transactor = services.GetRequiredService<ITransactor>();

transactor.Transact(db =>
{
    var firstProduct = db.Insert<Product>();
    var firstProductOid = db.GetOid(firstProduct);
    var secondProduct = db.Insert<Product>();
    var anotherFirstProduct = db.Get<Product>(firstProductOid);

    firstProduct.Equals(secondProduct); // false
    firstProduct.Equals(anotherFirstProduct); // true

    _ = firstProduct == secondProduct; // false
    _ = firstProduct == anotherFirstProduct; // false
    _ = firstProduct == firstProduct; // true

    object.ReferenceEquals(firstProduct, secondProduct); // false
    object.ReferenceEquals(firstProduct, anotherFirstProduct); // false
    object.ReferenceEquals(firstProduct, firstProduct); // true
});

Limitations

• Database classes can have a maximum of 112 properties for performance reasons. The limit applies to the total number of persistent properties (including all inherited) per class.

• Nested database classes are not supported in SQL queries.