Windows Phone Mango Local Database(SQL CE): Database mapping

published on: 8/25/2011 | Views: N/A | Tags: LocalDB Mango

by WindowsPhoneGeek

This is the 7th post from the "Windows Phone Mango Local Database(SQL CE)" series of short posts that will cover all you need to know in order  to get started using a Local Database in Windows Phone 7.1 Mango.  This time I am going to talk about mapping classes to tables in the database, or in other words - the mapping between your object model and database schema.

Here is what else is included in this series:

• Windows Phone Mango Local Database(SQL CE): Introduction
• Windows Phone Mango Local Database(SQL CE): Linq to SQL
• Windows Phone Mango Local Database(SQL CE): [Table] attribute
• Windows Phone Mango Local Database(SQL CE): [Column] attribute
• Windows Phone Mango Local Database(SQL CE): [Association] attribute
• Windows Phone Mango Local Database(SQL CE): [Index] attribute
• Windows Phone Mango Local Database(SQL CE): Database mapping
• Windows Phone Mango Local Database(SQL CE): DataContext
• Windows Phone Mango Local Database(SQL CE): Connection Strings
• Windows Phone Mango Local Database(SQL CE): Creating the Database
• Windows Phone Mango Local Database(SQL CE): Database Queries with LINQ
• Windows Phone Mango Local Database(SQL CE): How to Insert data
• Windows Phone Mango Local Database(SQL CE): How to Update data
• Windows Phone Mango Local Database(SQL CE): How to Delete data

What is Database mapping?

To create a local database, you must first define the entity classes. These classes define your object model and its mapping to the schema of the database. The object-relational capabilities of LINQ to SQL depend on these mapping details to create a relational database that maps to the corresponding data context.

For each entity, mapping details are specified by using LINQ to SQL mapping attributes. These attributes specify database-specific features such as tables, columns, primary keys, and indexes. For more information, see Attribute-based Mapping (LINQ to SQL).

For reference you can also take a look at the MSDN documentation.

Understanding the Database mapping

Mapping types to tables and properties to columns in the database schema is done using attributes:

• Entity classes are attributed with a [Table] attribute, in order to map them to tables in the database
• The [Index] attribute can be used (on classes) to define an index
• Entity properties are attributed with a [Column] attribute, in order to map them to columns in the database 
  [Column( IsPrimaryKey = true )] is used to specify the primary key property

Associations are specified using the [Association] attribute, which allows you to configure a relation between two entities / tables in the database mapping. The Association attribute has the following important properties:

• OtherKey - the name of the property that corresponds to the id of the object at the other end of the association
• ThisKey - the name of the property that corresponds to the primary key for this type
• Storage - the backing variable for the property

The code-first approach to defining the database schema is preferred in this version of Windows Phone 7.1 Mango. I.e. at the moment there is no visual designer that can help developers in mapping and configuring their classes to work with the database. Or at least it is not supported officially.

This means that you will have to:

• Configure the mapping with attributes:
• Write all classes on your own
• Write the code that creates the database if it does not exists.
• If you want to have functionality that upgrades an old version of the database to a new one you will again have to write the code on your own.

In this article I talked about mapping your classes to database tables when working with Windows Phone Mango Local Database. Stay tuned for the rest of the posts.

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