A little history

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Nate Clark
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• An Object/Relational Mapping tool
• Free and open source
• Simplifies storage of object data in a relational
  database
• Removes the need to write and maintain SQL queries

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A little history
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Origins of Databases and Objects

• DBMS technology invented late 1960s
• OO technology invented late 1960s
• Independent evolution for over 20 years
• Different audiences
• DBMS corporate information systems
• OO academic and scientific

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The Rise of Client/Server

• Corporate IT in early 90s
• High cost of mainframe hardware
• Low cost of the IBM PC-based platform
• Desire to move away from COBOL
• The hot languages were OO
• Smalltalk
• C

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Object Meets Relational

• Corporate IT already deeply invested in RDBMS
• Well just connect up our new client server
  applications to these databases!
• Efforts to marry the two technologies
• Discovery of the O/R Impedance Mismatch

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The object/relational conflict
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DBMS Representation

• Based on the concept of a Relation
• A set of data values that go together
• Represented as a Row in a Table
• No two rows should be duplicates
• Useless to state the same fact twice
• Very useful for flexible set manipulation
• SQL queries

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Object-Oriented Representation

• An object is a model of something in the real
  world
• Public interfaces
• How an object interacts with the rest of the
  world
• Private members
• How an object behaves internally

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Object-Oriented Representation

• Merges data and behavior
• Allows an object to initiate interactions and
  respond flexibly to them
• Important for modeling the real world
• Effective OO models allow for very high reuse
• Parcel program logic and data into self-defined
  and self-consistent bundles

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Object-Oriented Representation

• Each object (instance) is unique
• even if all data members are equal
• Object types inherit from other types
• allows for the creation of rich, powerful models
• allows opportunities for reuse

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O/R Impedance Mismatch

• Impedance matching
• from electronics theory
• matching a power source with a power load
• Mismatch between object and relational
  technologies
• Different theoretical foundations
• Relations are facts about data relatedness
• Objects are models of reality

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O/R Impedance Mismatch

• Mismatched uniqueness rules
• Rows containing identical data are the same
• Considered undesirable
• Objects containing identical data are different
• Common and useful
• equal, but not
• Mismatched data model topologies
• Relational tables are flat
• Object types are hierarchical

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O/R Mapping Tools

• Invented in early 1990s
• One of the first came from Cleveland
• Raleigh Systems ObjectPM
• Shield the OO application from the database
  paradigm
• Mappings to tables and columns are localized in
  configuration files

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Enter Hibernate

• Initially developed for Java
• created in late 2001 by Gavin King
• absorbed by the JBoss Group / Red Hat
• Ported to .NET 1.1 and 2.0
• Resulting product called NHibernate
• All popular databases supported
• XML-based configuration files

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What NHibernate Does

• Takes as input
• a C class with properties that need to be stored
  in a database and retrieved later
• an XML file containing mapping information
• Outputs automatically generated SQL
• when you tell NHibernate to save an object
• when you ask NHibernate to search/load objects
• can also create all database tables if desired

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How to Use NHibernate

• Step 1 create an object oriented business model
• Start simple one or two small classes
• Note which properties will save in DBMS
• Implement the model using C
• Best kept in a separate C project

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Step 2 The Mapping File

• One per business model class
• Named classname.hbm.xml
• Tells NHibernate where data is to reside
• Name of table to store data for this class
• Name of a column for each property in class
• Also describes relationships between classes
• One-to-many, many-to-one, many-to-many
• Generated SQL is based on this information

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Step 3 Configuration File

• XML-based file for global configuration
• Usually named hibernate.cfg.xml
• Application-wide settings
• Database connection string
• Database dialect
• Specifies database product and version
• Allows NHibernate to use non-standard features
  where appropriate
• Logging options

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Step 4 User Interface Tasks

• Create business model objects
• Create an NHibernate session object
• Insert/Update to DBMS
• Session.save(object)
• Retrieve business model data from the DBMS as
  business model objects
• Session.load(type, id)
• Session.find(query)

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Step 5 Unit Test and Debug

• Unit tests can be created for each persistent
  class if desired
• VSTS unit testing support
• NUnit
• Run, test and debug in the usual fashion
• Sit back and relax.
• Youre done!

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Questions
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