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Entity Framework Introduction
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OutlineGoals 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 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.
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ORM’s 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
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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, commercialTierDeveloper, free ORM and code generation toolSubsonic, open source
Source: Bishoy Demian
Source: Gil Fink http://blogs.microsoft.co.il/blogs/gilf
Entity 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
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8Source: Gil Fink http://blogs.microsoft.co.il/blogs/gilf
Programming 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 of
Interacting with the data storageTranslating the data into objects
Source: Jeff Derstadt
Entity Framework in a NutshellGoal: Simple and seamless data access for the .NET platform
Better layeringBetter re-use of existing knowledge and assets
EDM – Entity Data ModelAn abstract model for defining entities and relationshipsIncludes schema and mapping
Store 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 data
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10Source: Jeff Derstadt
Data 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 dbms’es:
SQLServer ;-)OracleMySQLpostgreSQLdb2...
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
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Demo: Database firstHow to create an entity model from an existing database
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Create a new entity model, step 1Create a class library projectAdd new item, select the ADO.NET Entity Data Model template
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Create a new entity model, step 2When database first, select ”Generate From Database”Select the database connectionSelect the database entities that shall be visibleImportant: enable Pluralize... and foreign keys
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Create a new entity model, step 3Adjust the entities, mapping etc.
Source: Gil Fink http://blogs.microsoft.co.il/blogs/gilf
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
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Entity in the designerSingularization and pluralization:
The ‘many’ part is named in plural, e.g. OrdersThe ‘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 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).
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The relationships are declared in XMLThe 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 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 update
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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
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Demo: Access EDM assembly Access it from a console applicationDo 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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QueryingYou can query the model in different ways:
Using LINQUsing Entity SQL
With Object ServicesWith Entity Client
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Querying with LINQStraight forward LINQ syntaxIntelliSense 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 ServicesT-SQL-like query languageEF translates Entity SQL into storage-specific queriesLazy queriesNo 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 queriesWorks like traditional PODA: Get a set of records from the data storeE.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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MutationChanges to entities are stored in memory:
Every entity in ObjectContext has a ObjectStateEntryObjectContext uses ObjectStateEntries to track entity changes
The 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
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InsertCreate 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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UpdateJust get a reference to the entityMake the changesAnd 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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DeleteGet a reference to the entityCall 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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InherianceTo types of inheriance
Table-per-Hierarchy InheritanceTable-per-Type Inheritance
These covers two different situations in SQL databases without O/RIn 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 InheritanceExample 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 InheritanceExample 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 loadFields 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());