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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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
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());