Entity Data Model 101: Part 1

[アーティクル]
01/30/2007

Most developers are familiar with the Object/Relational Mapping (ORM) problem: databases use the abstraction of rows in tables, but application programs use the abstraction of classes and objects. Existing ORM frameworks tend to address this mismatch by allowing programming classes to be annotated in order to relate them to the database.

The intent with ADO.NET is more ambitious: We view the ORM problem as just one of a number of services we want to build on the database. Other services include reporting, synchronization, backup, and so on. In order to cover all of these services, we have designed a data model that is similar to the object-oriented idiom that programmers use, while remaining independent of any particular programming language or programming platform. This data model is the Entity Data Model (EDM).

The EDM is a conceptual model for designing the data layer of applications. Entities and relationships in the domain of an application are specified in XML syntax. Both the programming classes used in applications and the storage schema map to this conceptual model.

Entities and Associations

The EDM is an implementation of the entity-relationship (ER) model. An EDM entity type is a specification for something in the application domain that must be represented by data. Examples of entity types can be found in a typical line of business (LOB) application and might include employees, customers, orders, order-lines, suppliers, shippers, products, invoices, and so on.

Logical connections between entities are called relationships: for example, the logical association between an employee and his/her contact information or between a merchandise order and the customer who makes the order. An EDM relationship type describes the possible associations between entities, and any constraints on multiplicity.

Schemas

The EDM provides an XML syntax named conceptual schema definition language (CSDL). A CSDL schema is used to specify entities and relationships in the domain of an application.

The CSDL schema will become the core data model of the application. It is usually referred to simply as the “conceptual schema”. A CSDL schema declares and defines a Namespace that contains entities and associations. An Alias can be assigned to save typing in the syntax that will follow. A schema is declared using the following XML:

<?xml version="1.0" encoding="utf-8"?>

<Schema Namespace="AdventureWorksModel"

Alias="AdventureWorksModel"

xmlns="https://schemas.microsoft.com/ado/2006/04/edm">

…

</Schema>

Entities

If the application requires a data type to represent an employee, the schematic representation of the employee type in CSDL looks something like this:

<Property Name="LoginID" Type="String" Nullable="false"

MaxLength="256" />

<Property Name="Title" Type="String" Nullable="false"

MaxLength="50" />

<Property Name="HireDate" Type="DateTime"

Nullable="false"/>

<Property Name="SalariedFlag" Type="Boolean"

Nullable="false" />

<Property Name="VacationHours" Type="Int16"

Nullable="false" />

<Property Name="SickLeaveHours" Type="Int16"

Nullable="false" />

<Property Name="CurrentFlag" Type="Boolean"

Nullable="false" />

<NavigationProperty Name="Contact"

Relationship="AdventureWorksModel.FK_Employee_Contact_ContactID"

FromRole="Employee" ToRole="Contact" />

<NavigationProperty Name="Managed_Employees"

Relationship="AdventureWorksModel.FK_Employee_Employee_ManagerID"

FromRole="Employee" ToRole="ManagedEmployee" />

</EntityType>

You don’t have to understand everything in this syntax to see that it declares an entity type named Employee and that it contains properties of various data types.

Properties of Entities

The first two lines of the example include declaration of the Name and Key attributes of the entity. When an instance of the Employee entity is created, the Key property is assigned an integer value that uniquely identifies the instance for all operations in which it is used.

The Key attribute can be any sequence of properties. Using an integer is only one possibility. The Key could also be a GUID or the combined sequence of one or more properties separated by a space in the attribute declaration.

Each property of the Employee type has a simple data type and several facets that further constrain the data that can be assigned to the property. For example, the Employee BirthDate property is of type DateTime. It has one designated facet: Nullable=”false”. The Nullable facet specifies that the BirthDate property cannot be null. The Employee entity also contains a LoginID property. This property is of type String. The facets of the String data include Nullable=”false” and MaxLength=”256”.

Attributes declared as Type=”x” are EDM types, each of which has a corresponding CLR type and corresponding SQL type. (The actual SQL type depends on the underlying database.)

The properties of this Employee entity type align with an Employee table in the Adventureworks sample database that ships with Microsoft® SQL Server™ 2005. Using the schemas in this article, you can build a class library mapped to data in Adventureworks. This class library would contain an Employee class to represent the Employee entity type.

The last six lines of the CSDL syntax shown above declare two navigation properties on the Employee type. We’ll return to these later.

To make our example more interesting, we also declare a Contact entity type.

Contact entities contain properties that will be assigned the various data needed to identify and locate a person or business.

<Property Name="FirstName" Type="String" Nullable="false"

MaxLength="50" />

<Property Name="LastName" Type="String" Nullable="false"

MaxLength="50" />

<Property Name="PasswordHash" Type="String" Nullable="false"

MaxLength="128" />

<Property Name="PasswordSalt" Type="String" Nullable="false"

MaxLength="10" />

</EntityType>

Associations and Navigation Properties

An Association between Employee and Contact entities is declared and defined in the following CSDL syntax.

<End Role="Contact" Type="AdventureWorksModel.Contact"

Multiplicity="1..1" />

<End Role="Employee" Type="AdventureWorksModel.Employee"

Multiplicity="1..*" />

</Association>

Recall that the definition of the Employee entity includes the NavigationProperty between the Employee entity and the Contact entity. This NavigationProperty makes it possible to follow an instance of an Employee entity according to the Association above to an instance of a related Contact entity or vice versa. The Association above declares the End elements of the Association and defines Role, Type, and Multiplicity attributes of these End elements.

Comments

Anonymous
February 02, 2007
You can find the first part of what promises to be a very educational series over here: http://blogs.msdn.com/adonet/archive/2007/01/30/entity-data-model-part-1.aspx
Anonymous
February 02, 2007
If you are interested in ADO.Net Orcas and the Entity Framework, then you should check out this new posting
Anonymous
February 02, 2007
Mike Dodaro, a Microsoft programming writer for ADO.NET, has published Part I of what promises to be an interesting series of technical articles about the forthcoming ADO.NET 3.0 Entity Data Model (EDM). ... New Entity Data Model 101 Documentation Series http://oakleafblog.blogspot.com/2007/02/new-entity-data-model-101-documentation.html
Anonymous
February 04, 2007
Does anybody know of an expected release date for the entit framework? I'm eagerly anticipating it and trying to determine if an application I am developing can wait for this to be released. Thanks.
Anonymous
February 09, 2007
Here are a few good links to Orcas material (some old and some new). There is a lot of good and bad material
Anonymous
February 11, 2007
Winter has finally set in with single digit temps and minus degrees wind chills but still no snow. WPF/Avalon
Anonymous
February 12, 2007
The ADO.Net team has published a serries of posts about the Entity Data Model (EDM) which is part of
Anonymous
February 14, 2007
Here are a few interesting blog posts. Eric White's Functional Programming Tutorial is a few months old
Anonymous
February 15, 2007
Jason, At the moment, we do not have an expected release date for the Entity Framework. We do however expect it to ship with the Visual Studio "Orcas" release in the second half of the year.
Anonymous
February 26, 2007
Now that you’re getting accustomed to the Entity Framework and Entity Data Model ( Entity Data Model
Anonymous
May 10, 2007
Continuing along with posts on the essentials of Entity Framework mapping (see Mapping 101: Part 1 ),
Anonymous
July 31, 2007
The comment has been removed
Anonymous
August 09, 2007
Note: Some of the posts referenced in this post (!) are likely to be more out of date than others but...
Anonymous
September 05, 2007
There are two new data access technologies coming out of Microsoft in the coming year that have particular
Anonymous
September 11, 2007
So far for me LINQ to SQL has been perfectly adequate and has just worked without any intervention needed.However,
Anonymous
September 11, 2007
So far for me LINQ to SQL has been perfectly adequate and has just worked without any intervention needed.However,
Anonymous
September 17, 2007
Poslední reinkarnace LINQu, o které jsem se zmínil v přehledovém článku , je LINQ to ADO.NET Entities,
Anonymous
September 17, 2007
Poslední reinkarnace LINQu, o které jsem se zmínil v přehledovém článku , je LINQ to ADO.NET Entities,
Anonymous
October 02, 2007
SQL Server 2008 is at the door and the same stands for Visual Studio 2008. The data access strategy is
Anonymous
May 06, 2008
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Anonymous
May 14, 2008
Great info - however I've not been able to find any in-depth tutorials on the EDM. Does anyone have a link to some good source of info? Thanks!
Anonymous
May 19, 2008
Most developers are familiar with the Object/Relational Mapping (ORM) problem: databases use the abstraction of rows in tables, but application programs use the abstraction of classes and objects. Existing ORM frameworks tend to address this mismatch
Anonymous
June 05, 2008
The comment has been removed
Anonymous
September 21, 2008
Unable to see another kind of database provide while i am trying to create Entity model using VS2008 Designer
Anonymous
July 20, 2009
Hi All, Can anybody recommend good book on EDM
Anonymous
July 20, 2009
Hi All, Can anybody recommend good book on EDM. Thanks, Alex

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Entity Data Model 101: Part 1

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