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.
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:
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
:
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.
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:
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.
A single property index can also be registered for a column by decorating its associated C# property with the IndexAttribute
attribute:
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.
Inheritance
Inheritance is supported between database classes and has the same general semantics as class inheritence in C#.
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
Oid
from a database objectFind an object in the database by its unique key
Notes
Zero (
0
) is not a validOid
.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:
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.
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