Transcript UML Models
Data Source Architectural Patterns – Hiding SQL Ralph Johnson 1 SQL Another language Standard – sort of Not object-oriented Can have duplication Can be slow CS327 16-2 SQL Tables Rows Queries Select Project Sum Update … CS327 16-3 SQL Database connection Socket to a DBMS Send SQL and get rows String sql = “SELECT * FROM person WHERE id = ?”; IDbCommand comm = new OleDbCommand(sql, DB.Connection); Comm.Parameters.Add(new OleDbParameter(“key”,key)); IDataReader reader = comm.Execute(); CS327 16-4 How to implement domain model Transaction script – Table Data Gateway Row Data Gateway Domain model – Active Record Data Mapper Table module – Table Data Gateway CS327 16-5 Table Data Gateway Put all SQL about a table in the same class, the Table Data Gateway. One instance of that class. Instance variable: database connection Operations: findAll, findPerson, findWithAge, … insert, delete, … CS327 16-6 Row Data Gateway Put all SQL about a table in the same class, the Row Data Gateway. One instance for each row. Methods contain only SQL Instance variables just row fields “Find” operations Static methods In a builder object CS327 16-7 Active Record Like Row Data Gateway except Some methods are domain logic Some variables not stored in database Designed more for domain model than for database CS327 16-8 Data Mapper Moves data between domain objects and database Domain objects know little about data mapper Database design independent of domain model Might look more like Table Data Gateway CS327 16-9 Data Mapper Persistent object knows data mapper Has operation like “store” Persistent object does not know d.m. personMapper.store(aPerson) One mapper for each class, or one for entire application CS327 16-10 Transaction script Embedded SQL Move SQL into class for the table Table Data Gateway Make an instance for each row Data Row Gateway Move behavior into row, encapsulating data Active Record CS327 16-11 Why Table Data Gateway? Separate SQL from logic Can change DBMS easier Oracle to DB2 or Access SQL to XML CS327 16-12 Why Row Data Gateway? Reduce duplication Make it easier to add/remove fields CS327 16-13 Why Active Record? Reduce duplication Transaction scripts can become very simple, just descriptions of use case CS327 16-14 Why Data Mapper Separate SQL from domain logic Make it easier to change database CS327 16-15 Conclusion Think of these patterns are refactorings Start simple, become complex as needed CS327 16-16