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Nate Clark

Nate Clark. 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. A little history…. Origins of Databases and Objects. DBMS technology invented late 1960’s

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Nate Clark

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  1. Nate Clark

  2. 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

  3. A little history…

  4. Origins of Databases and Objects • DBMS technology invented late 1960’s • OO technology invented late 1960’s • Independent evolution for over 20 years • Different audiences • DBMS: corporate information systems • OO: academic and scientific

  5. 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++

  6. Object Meets Relational • Corporate IT already deeply invested in RDBMS • “We’ll just connect up our new client server applications to these databases!” • Efforts to marry the two technologies • Discovery of the “O/R Impedance Mismatch”

  7. The object/relational conflict

  8. 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

  9. 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

  10. 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

  11. 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

  12. 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

  13. 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

  14. O/R Mapping Tools • Invented in early 1990’s • 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

  15. [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

  16. 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

  17. 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

  18. 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

  19. 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

  20. 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)

  21. 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…. • You’re done!

  22. Questions ?

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