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

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++

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

The object/relational conflict

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

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

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

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

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

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

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

[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

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

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

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

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

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)

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!

Questions