Object-Relational Mapping (ORM)

Lecture Notes for CS349W
Fall Quarter 2008
John Ousterhout

• Virtually all Web applications use relational databases for storing back-end data.
• Unfortunately, RDBMS'es don't fit naturally into Web applications:
  • SQL vs. objects
  • Database adapters different for every database
• Web application frameworks provide ORMs to reduce the mismatch:
  • Use objects to hold database records
  • Manage the movement of information between objects and the back-end database.
  • Manage relationships between databases (joins), turn into linked data structures.
• Example from Ruby on Rails:

  student = new Student()
  student.last_name = "Smith"
  student.email = "smith@stanford.edu"
  student.save;
  
  ...
  
  student = Student.find_by_name("Smith")
  courses = student.courses;
  

• Web application frameworks also provide abstraction layer on top of database adapters to hide differences between databases.