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1 Entity Framework Introduction

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Entity Framework Introduction. Outline. Goals of Entity Framework. The story so far…. The “Impedance Mismatch”. ?. Relational Database . Conceptual / Business Model (Objects). Traditional way. Make a persistency layer that maps from SQL to OOP - PowerPoint PPT Presentation


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1Entity Framework IntroductionOutlineGoals of Entity Framework

2The story so farThe Impedance Mismatch

Relational Database Conceptual / Business Model (Objects)?3Source: Gil Fink http://blogs.microsoft.co.il/blogs/gilf3Traditional way....Make a persistency layer that maps from SQL to OOPIt is a better solution than accessing the db from many places in the applicationBut 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 sameEtc.


ORMs provides a way to mapORM 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 5ORM 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, commercialTierDeveloper, free ORM and code generation toolSubsonic, open source

Source: Bishoy Demian

6Entity Framework is also a ORMData access frameworkSupports data-centric applications and services Enables programming against a conceptual application modelEnables independency of any data storage engine or relational schema

7Source: Gil Fink http://blogs.microsoft.co.il/blogs/gilf7Programming Against a ModelEF uses a model called an Entity Data Model (EDM)EDM is a client-side data modelEDM is an abstraction layer on top of the data storage Remove the pain ofInteracting with the data storageTranslating the data into objects

8Source: Gil Fink http://blogs.microsoft.co.il/blogs/gilf8Entity Framework in a NutshellGoal: Simple and seamless data access for the .NET platformBetter layeringBetter re-use of existing knowledge and assetsEDM Entity Data ModelAn abstract model for defining entities and relationshipsIncludes schema and mappingStore Schema Definition (SSDL)Conceptual Schema Definition (CSDL)Mapping Schema between the two (MSL)Entity FrameworkAn implementation of EDM and an ORM layer on topA framework for using entities over data9Source: Jeff DerstadtPresentation_titledd/mm/yyyy, author_alias9Data StoreThe Entity Framework has no knowledge of the database that stores the dataIt connects and interacts with the database through a provider that is usually declared in the configuration fileAs in ADO.NET the providers are supplied by the dbms vendorsSome supported dbmses:SQLServer ;-)OracleMySQLpostgreSQLdb2... 10Source: Jeff DerstadtGetting StartedDBModelCodeDBModelCodeDBModelCodeDesign timeDesign timeDesign timeDesign timeRuntimeRuntimeModel 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 databasewhy? you want separation from code and database in a declarative formatwhy? primarily focused on code shape, database is an implementation detail11Database First (VS 2008 and .NET 3.5 SP1)Source: Jeff DerstadtPresentation_titledd/mm/yyyy, author_alias11Whats New in .NET 4.0Theres 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

12Source: Jeff DerstadtPresentation_titledd/mm/yyyy, author_alias12Demo: Database firstHow to create an entity model from an existing database

13Create a new entity model, step 1Create a class library projectAdd new item, select the ADO.NET Entity Data Model template14

Create a new entity model, step 2When database first, select Generate From DatabaseSelect the database connectionSelect the database entities that shall be visibleImportant: enable Pluralize... and foreign keys15

Create a new entity model, step 3Adjust the entities, mapping etc.16

The Designer WindowGraphical representation of an EDM and its membersEnables adding more features to the modelEnables properties configurationEnables updating from the data storeEnables model validation

17Source: Gil Fink http://blogs.microsoft.co.il/blogs/gilf17Entity in the designerSingularization and pluralization: The many part is named in plural, e.g. OrdersThe one part is named in single, e.g. CustomerNavigation Properties:Make it possible to navigate from one entity (in code an object) to another.18

The EDM Document consists of 3 partsThe 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 entitiesStorage 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).

19The relationships are declared in XMLThe output of the designer is an XML document that consists of 3 parts:


Source: Julia Lerman

Code GenerationEntity classes are automatically generated from the EDMThe classes consists primary of properties and eventsSo it is possible to get and set values, and to get notified when an event occurs, e.g. an update21

Use the generated codeYou 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.entityAnd add using for your EDM in the code where the context class is usedThe 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

22Demo: Access EDM assembly Access it from a console applicationDo a simple LINQ query and write the result on screen23Querying the Model

Queries are built against a data modelEDM query transform into data storage queryQuery results materialize into model entities

The image is taken from Julia Lermans bookProgramming Entity Framework, 1st EditionSource: Gil Fink http://blogs.microsoft.co.il/blogs/gilf24QueryingYou can query the model in different ways:Using LINQUsing Entity SQLWith Object ServicesWith Entity Client25Querying with LINQStraight forward LINQ syntaxIntelliSense support

26 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);Entity SQL with Object ServicesT-SQL-like query languageEF translates Entity SQL into storage-specific queriesLazy queriesNo IntelliSense support

27String queryString = @"SELECT VALUE c FROM Customers AS c WHERE c.Country='UK'";var customers = Context.CreateQuery(queryString);

foreach (Customer c in customers) Console.WriteLine(c.CompanyName);Entity SQL with client queriesWorks like traditional PODA: Get a set of records from the data storeE.g. like SQLClient or OracleClient No IntelliSense support

28 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)); } } }MutationChanges to entities are stored in memory:Every entity in ObjectContext has a ObjectStateEntryObjectContext uses ObjectStateEntries to track entity changesThe context.SaveChanges() method is used to update the database according to ObjectStateEntriesRollback (in memory) can be done with context.SaveChanges(false)

The normal mutations insert, update and delete are possible

29InsertCreate a new instance (entity) of the class. Evt. set references to related objects (as normal in oop)Add the entity to the context.30Console.WriteLine("Create new order");var order = new Order { CustomerID = "SEVES", EmployeeID = 1 };context.AddToOrders(order);context.SaveChanges();UpdateJust get a reference to the entityMake the changesAnd call context.SaveChange()31 Console.WriteLine("Changing order"); var order = from o in context.Orders where o.OrderID == 11078 select o; order.Single().OrderDate = DateTime.Now; context.SaveChanges();DeleteGet a reference to the entityCall context.DeleteObject(theEntity)Call context.SaveChanges()

32 Console.WriteLine("Deleting order"); var order = from o in context.Orders where o.OrderID == 11078 select o; context.DeleteObject(order.Single()); context.SaveChanges();InherianceTo types of inherianceTable-per-Hierarchy InheritanceTable-per-Type Inheritance

These covers two different situations in SQL databases without O/RIn the first situation youll have one type table where different subtypes are listed.In the second situation youll have a specialized table pr. type33Demo: Table-per-Hierarchy InheritanceExample based on School database from msdnSource: http://mosesofegypt.net/post/Inheritance-and-Associations-with-Entity-Framework-Part-1.aspx34

Demo: Table-per-Type InheritanceExample based on School database from msdnSource: http://mosesofegypt.net/post/Inheritance-and-Associations-with-Entity-Framework-Part-2.aspx35

Association no need to carry the full loadFields that are not used so frequently might be extracted to another entity with a 1-1 relation36

Get the picture ;-)37 //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());