C# ADO.NET 

Entity Framework Basics

Intro


  • DB as abstraction of real world information using
    • tables, indexes, views, ....
  • Developer introduces a second abstraction
    • to map between DB data and App data
  • Entity Framework as ORM
  • ADO.NET extension
  • Create model of DB - some code is generated for the App

Entity


  • Data associated to some object from app point of view
  • Customer object
    • name, address, tel, company
  • Entity as the data necessary for the app
  • Three views on data:
  1. Physical view
  2. Logical view
  3. Conceptual view

Physical View


  • Tables, keys, indexes, views, etc. that hold the data
  • Optimized: DBMS handles operations efficiently
  • Single customer data could be split up to multiple tables
  • Maybe require multiple keys to create cohesive view
  • Physical storage is efficient but hard to understand for developer

Logical View


  • Combined elements required to define the data for a single object
  • From DB perspective the logical view combines data found in tables using keys and other DB elements
  • ADO.NET reduces amout of work by providing built-in objects, but developer has to understand the underlying physical structure

Conceptual View


  • Real world view on the data
  • As it applies to the object
  • Presents information in understandable manner
  • E.g. each customer is an entity
  • The customers as a group are also entities
  • Entity framework relies on model to visualize data

Entity Framework


  • Stores models in XML format
  • Works with any data source
    • flat file
    • hierarchical DBs
    • relational DBs
    • etc.
  • Reduces work overhead
  • Developer focuses on the entity not on the physical or logical structure

Entity Framework #2


  • Entity cotains properties
    • last name, first name, etc
  • Has a special key property
    • makes data unique
  • May have more than 1 key property, but needs at least 1
  • Similar to classes in C#
  • Properties may hold simple and complex data
  • Relationship between two entities through association

Associations

  • Association types define the specifics of the association
  • Similar to a DB level join operation
  • 1+ properties in each entity (association endpoints) define the relationship between two entities
  • Properties may define both single and multicolumn connections between entities (1-to-1, 1-to-many, many-to-many)
  • Associations are bidirectional
  • May exist although data lacks DB join specification
  • Association set: all association instances used to define an association

Associations #2

  • Navigation property: allows one entity to view the data provided by an associated entity
  • Allows to create a view of an entity from the perspective of another entity
  • E.g. Access all orders submitted by that customer (foreign key)
  • Derived entities
    • Customer can create an order
    • Derived entities exist in the same container as the order entity

Entity Container


  • Holds all of the entity information together
  • App creates entity container instance (context)
  • App accesses the entities within a particular context

Entity Framework Elements


  • relies on XML
  • These files perform 3 tasks
    • Define conceptual model
    • Define storage model
    • Create mappings between models and physical DB
  • Framework lets developer interact with the DB using one of 3 techniques

Entity Framework Approaches


  • DB first
    • Entity framework creates classes that reflect an existing DB design
  • Design first:
    • Define model of DB, Entity Framework creates DB on server
  • Code first:
    • Create app, Entity framework creates a model to create the DB later

Conceptual Model

  • Define how DB looks from app perspective
  • E.g.: C# uses Int32, underlying datatype maybe SQL int type
  • Conceptual model may refer to it as Int32, actual physical type maybe different
  • Create classes used to interact with DB
  • As the conceptual model changes
    • Entity framework automatically tracks changes
  • Conceptual model defines namespaces (similar to C# namespaces)
  • Mainly consists of entity and association definitions used to create the view of the DB

Storage Model

  • Part of Entity framework that defines how the DB looks from the DB manager's perspective
  • Similar to logical model
  • Provides logical view on DB that ultimately translates to physical DB
  • Also consists of entity and association definitions
  • Storage model includes actual DB data such as commands used to query the info within the DB
  • This info is used by ADO.NET to create connection and command objects automatically

Model Mappings


  • In order to make the two models work together, the Entity framework requires model mappings
  • These three models are stored in separate XML files
  • In Visual Studio 2012 there is a single .EDMX file
  • .CSDL contains XML for the conceptual model of the DB
  • .SSDL defines storage model
  • .MSL file creates relation between .CSDL and .SSDL file
  • Model mapping defines which conceptual model property translates to particular storage model property

Basic Example

  • Model first approach
  • Install SQL Server Express 2014
  • Install SQL Management Studio

  1. Create WPF App
  2. VIEW - OTHER WINDOWS - Data Sources
  3. ADD New Data Source
  4. Choose DB Data Source Type
  5. Choose Entity Data Model DB Model
  6. Choose Entity Model Content
  7. View Toolbox - Add Entity
  8. Right Click - Validate - Generate DB from model



References








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Entity Framework Basics

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Entity Framework Basics

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