Active Records in Rails: Simplifying Database Access

1. Active Records

2. What’s Active Records? • O-R Mapping layer • To make database access almost a non-issue • Relies heavily on convention over configuration • some say this is simple, others say this is simplistic • Tables map to classes • Rows map to objects • Columns map to attributes • You write minimal code • Quite a bit of code is synthesized behind the scene

3. Accessing Data • You derive your class from ActiveRecord::Base • That’s almost all you need • You are provided with methods you can readily use • Attributes are inferred based on column names in schema • You may have additional attributes • Like having a clear text password in memory while the password column is encrypted • Has primary key id by convention

4. Locking • Supports Pessimistic or Optimistic Locking • Optimistic if your table has an integer column named lock_version • Active Records take care of rest • You can control this by ActiveRecord::Base.lock_optimistically = false

5. Classes to Tables • Active Record assumes the table name is plural form of your model class • Multiword class name transorms to words separated by underscores • Controlled by global glag in environment.rb • ActiveRecord::Base.pluralize_table_names = false • Common plurans and then some weird ones • people (Person), journals (Journal), children (Child), dear_friends (DearFriend) • You can break away from the convention (for legacy database) using set_table directive

6. MySql • Create mysql database • mysql • create database csalum_development; • Create a create.sql script for creating table • Run script • mysql –p –r root csalum_development < create.sql

7. A Model Class

8. Exploring Active Records • Columns() method tells us

9. SQL Type to Ruby Type Mapping • Standard SQL to Ruby type mapping • int, integer -> Fixnum • decimal, numeric, float, double -> Float • interval, date -> Date • clob, blob, text, char, varchar, string -> String • datetime, time -> Time • Money? – decimal to float may lead to rounding errors • For currency you may use • units of cents • aggregate Money objects

10. Accessing Attributes • Active Records converts column values to appropriate Ruby types • You can get raw value of an attribute as well by appending attribute name with _before_type_cast • Use caution with boolean types • No consistent represenation in databases • Use the ? form of method to get correct value • Still poses problems for i18n

11. CRUD • Active Records is based on the simple notion that you mostly need basic operations on tables • Create, Read, Update, and Delete • Methods for these are synthesized behind the scene • new • finders • save • update • Delete

12. Creating A Row • Use new to create object • Remember to save

13. Ways to create

14. create() Method • Combines new and save method • create(hash) => object • create(array_of_hash) => objects

15. Reading • find takes primary key or an array of primary keys • Gets fancier than that as well • Can take other parameters • :first specified to return first matching row • :all specified to return all matching rows • :condition helps send parameters to SQL where clause

16. Reading No guarantee on ordering unless you specify order by

17. Specifying Criteria • Unsafe way – SQL Injection problem • Don’t try this at home

18. Specifying Criteria – Better Ways

19. Specialized finds • You can search based on column values • Methods synthesized for you

20. Update/Save • Save updates existing row or inserts a new row • save returns true if model is valid and can be saved • save! raises a RecordInvalid exception if objects can’t be saved • You may also use update_attribute or update_attributes • Class method update allows you to update a row without reading it • update_all is like SQL update with SET and WHERE clause

21. Deleting • destroy allows you to delete a row based on id or condition • Once deleted, object in memory is frozen • destroy_all is a classes methods that destroys all objects that meet given condition