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Entity Framework Introduction

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Outline

Goals of Entity Framework

Source: Gil Fink http://blogs.microsoft.co.il/blogs/gilf

The story so far…

The “Impedance Mismatch”

Relational Database

Conceptual / Business Model (Objects)

?

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Traditional way....

Make a persistency layer that maps from SQL to OOP

It is a better solution than accessing the db from many places in the application

But it still can raise some problems, e.g.:

Tight coupling between application and db(e.g. can not change tables in the db without making changes to domain classes in the application)

Hard to maintain (e.g. SQL queries, dependent classes etc. might be spread in many places)

Application and db domains are probably not the same

Etc.

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ORM’s provides a way to map

ORM is:• (Wikipedia) A programming technique for converting data

between incompatible type systems (such as DBMS) in relational databases and object-oriented programming languages.

• It does the plumbing work for you to aggregate, modify and save your data back to its storage in Object Oriented manner (easy to understand, maintain and extend)

Source: Bishoy Demian

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ORM external tools for .Net (Wikipedia)

NHibernate, open source

.netTiers, open source, based on commercial code generation tool (Codesmith)

Developer Express, eXpress Persistent Objects (XPO)

LLBLGen, open source drivers, commercial

TierDeveloper, free ORM and code generation tool

Subsonic, open source

Source: Bishoy Demian

Source: Gil Fink http://blogs.microsoft.co.il/blogs/gilf

Entity Framework is also a ORM

Data access framework

Supports data-centric applications and services

Enables programming against a conceptual application model

Enables independency of any data storage engine or relational schema

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8Source: Gil Fink http://blogs.microsoft.co.il/blogs/gilf

Programming Against a Model

EF uses a model called an Entity Data Model (EDM)

EDM is a client-side data model

EDM is an abstraction layer on top of the data storage

Remove the pain of

Interacting with the data storage

Translating the data into objects

Source: Jeff Derstadt

Entity Framework in a Nutshell

Goal: Simple and seamless data access for the .NET platform

Better layering

Better re-use of existing knowledge and assets

EDM – Entity Data Model

An abstract model for defining entities and relationships

Includes schema and mapping

Store Schema Definition (SSDL)

Conceptual Schema Definition (CSDL)

Mapping Schema between the two (MSL)

Entity Framework

An implementation of EDM and an ORM layer on top

A framework for using entities over data

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10Source: Jeff Derstadt

Data Store

The Entity Framework has no knowledge of the database that stores the data

It connects and interacts with the database through a provider that is usually declared in the configuration file

As in ADO.NET the providers are supplied by the dbms vendors

Some supported dbms’es:

SQLServer ;-)

Oracle

MySQL

postgreSQL

db2

...

Source: Jeff Derstadt

Getting Started

DB ModelCode

DB ModelCode

DB ModelCode

Design time

Design time

Design time

Design time

Runtime Runtime

Model First (VS 2010 and .NET 4.0)

Code First (Entity Framework Feature CTP3)

why? it already exists, or you want low level control over the database

why? you want separation from code and database in a declarative format

why? primarily focused on code shape, database is an implementation detail

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Database First (VS 2008 and .NET 3.5 SP1)

Source: Jeff Derstadt

What’s New in .NET 4.0

There’s more!Self-tracking entitiesSQL generation improvementsObjectStateManager controlWPF designer integrationSPROC import improvementsModel defined functionsCode-Only development (Feature CTP)

Model-first developmentAutomatic pluralizationLazy loadingPOCO class supportT4 Code GenerationTemplate customizationObjectSet/IObjectSetForeign keys in modelsVirtual SaveChangesExecuteStoreQueryExecuteStoreCommandMore LINQ operator support 12

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Demo: Database first

How to create an entity model from an existing database

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Create a new entity model, step 1

Create a class library project

Add new item, select the ADO.NET Entity Data Model template

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Create a new entity model, step 2

When database first, select ”Generate From Database”

Select the database connection

Select the database entities that shall be visibleImportant: enable Pluralize... and foreign keys

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Create a new entity model, step 3

Adjust the entities, mapping etc.

Source: Gil Fink http://blogs.microsoft.co.il/blogs/gilf

The Designer Window

Graphical representation of an EDM and its members

Enables adding more features to the model

Enables properties configuration

Enables updating from the data store

Enables model validation

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Entity in the designer

Singularization and pluralization:

The ‘many’ part is named in plural, e.g. Orders

The ‘one’ part is named in single, e.g. Customer

Navigation Properties:

Make it possible to navigate from one entity (in code an object) to another.

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The EDM Document consists of 3 parts

The Entity Data Model (EDM) is a schema language for entities. It allows for the definition of entities, relationships between entities and logical sets of related entities

Storage Metadata Schema (SSDL) is a formal description of the database that persists data for an application built on the Entity Data Model (EDM). The entities and associations declared in this schema are the basis for mapping entities and associations in the conceptual schema to the corresponding entities in the storage model.

Mapping Specification (MSL) is used to connect the types declared in conceptual schema definition language (CSDL) to database metadata that persists data.

Conceptual Schema (CSDL) is a design template for the object model that will be used by applications built on the Entity Data Model (EDM).

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The relationships are declared in XML

The output of the designer is an XML document that consists of 3 parts:

Source: Julia Lerman

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Code GenerationEntity classes are automatically generated from the EDM

The classes consists primary of properties and events

So it is possible to get and set values, and to get notified when an event occurs, e.g. an update

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Use the generated code

You should place the EDM and therefore the generated code in a class library (dll-assembly).

Then you can access the EDM from your data-tier, business-tier, persistent-tier, etc.

When you access from another assembly remember to add a reference to your EDM assembly and to system.data.entity

And add ‘using’ for your EDM in the code where the context class is used

The connection string in the app.config must also be in the app-config for the exe-assembly.

But it is easier to see in a demo

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Demo: Access EDM assembly

Access it from a console application

Do a simple LINQ query and write the result on screen

Source: Gil Fink http://blogs.microsoft.co.il/blogs/gilf

Querying the Model

• Queries are built against a data model• EDM query transform into data storage query• Query results materialize into model entities

The image is taken from Julia Lerman’s

bookProgramming Entity Framework, 1st Edition

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Querying

You can query the model in different ways:

Using LINQ

Using Entity SQL

With Object Services

With Entity Client

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Querying with LINQ

Straight forward LINQ syntax

IntelliSense support

var context = new NWEntities(); var query = from c in context.Customers where c.Orders.Count()>25 select c;

foreach(Customer c in query) Console.WriteLine(c.CompanyName);

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Entity SQL with Object Services

T-SQL-like query language

EF translates Entity SQL into storage-specific queries

Lazy queries

No IntelliSense support

String queryString = @"SELECT VALUE c FROM Customers AS c WHERE c.Country='UK'";var customers = Context.CreateQuery<Customer>(queryString);

foreach (Customer c in customers) Console.WriteLine(c.CompanyName);

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Entity SQL with client queries

Works like traditional PODA: Get a set of records from the data store

E.g. like SQLClient or OracleClient

No IntelliSense support

using (var conn = new EntityConnection("name=NWEntities")) { conn.Open(); var qStr = @"SELECT VALUE c FROM Customers AS c WHERE c.Country='UK'"; var cmd = conn.CreateCommand(); cmd.CommandText = qStr; using (var rdr = cmd.ExecuteReader()){ while (rdr.Read()){ Console.WriteLine(rdr.GetString(1)); } } }

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Mutation

Changes to entities are stored in memory:

Every entity in ObjectContext has a ObjectStateEntry

ObjectContext uses ObjectStateEntries to track entity changes

The context.SaveChanges() method is used to update the database according to ObjectStateEntries

Rollback (in memory) can be done with context.SaveChanges(false)

The normal mutations insert, update and delete are possible

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Insert

Create a new instance (entity) of the class.

Evt. set references to related objects (as normal in oop)

Add the entity to the context.

Console.WriteLine("Create new order");var order = new Order { CustomerID = "SEVES", EmployeeID = 1 };context.AddToOrders(order);context.SaveChanges();

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Update

Just get a reference to the entity

Make the changes

And call context.SaveChange()

Console.WriteLine("Changing order"); var order = from o in context.Orders where o.OrderID == 11078 select o; order.Single().OrderDate = DateTime.Now; context.SaveChanges();

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Delete

Get a reference to the entity

Call context.DeleteObject(theEntity)

Call context.SaveChanges()

Console.WriteLine("Deleting order"); var order = from o in context.Orders where o.OrderID == 11078 select o; context.DeleteObject(order.Single()); context.SaveChanges();

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Inheriance

To types of inheriance

Table-per-Hierarchy Inheritance

Table-per-Type Inheritance

These covers two different situations in SQL databases without O/R

In the first situation you’ll have one ”type table” where different subtypes are listed.

In the second situation you’ll have a specialized table pr. type

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Demo: Table-per-Hierarchy Inheritance

Example based on School database from msdn

Source: http://mosesofegypt.net/post/Inheritance-and-Associations-with-Entity-Framework-Part-1.aspx

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Demo: Table-per-Type Inheritance

Example based on School database from msdn

Source: http://mosesofegypt.net/post/Inheritance-and-Associations-with-Entity-Framework-Part-2.aspx

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Association – no need to carry the full load

Fields that are not used so frequently might be extracted to another entity with a 1-1 relation

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Get the picture ;-)

//Get photo when needed AssoEntities assoContext = new AssoEntities(); var employee = (from e in assoContext.Employees select e).FirstOrDefault(); Console.WriteLine(employee.LastName); Console.WriteLine( employee.EmployeePhoto.Photo.Length.ToString());

//Get the photo together with the rest of Employee AssoEntities assoContext = new AssoEntities(); var employee = (from e in

assoContext.Employees.Include("EmployeePhoto") select e).FirstOrDefault(); Console.WriteLine(employee.LastName); Console.WriteLine(

employee.EmployeePhoto.Photo.Length.ToString());