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Active records before_type_cast

thehoagie
thehoagie

Covers the before_type_cast method provided by ActiveRecord::Base, and when you would want to use it

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The Calm Before the Cast
 Understanding ActiveRecord's Casting
What is Casting?
Casting is a manufacturing process by which a
liquid material is usually poured into a mold

In computer science it is the explicit changing
of the data type of a given entity

e.g, casting a string into an array:
Array("foo") # => ["foo"]
ActiveRecord's Casting
Extending the ActiveRecord::Base class
creates attribute accessors that map to a given
database table's columns

The column type in the database dictates the
data type of the returned value

Database data types != Ruby data types
ActiveRecord's Type Casting
System
bundle open activerecord

Database specific adapters read column
definitions (extend abstract):
lib/active_record/connection_adapters/postgres
ql_adapter.rb|74

These are cast to Ruby classes:
lib/active_record/connection_adapters/abstract/
schema_definitions.rb|70
CREATE TABLE foo (
                            bar integer,
                            baz character varying(255),
                            qux timestamp without time zone
                          );

                          SELECT * FROM foo WHERE id=1;




Casting occurs here
                      {   Abstract adapter




                          Postgres adapter




                          Foo.find(1) # =>
                          #<Foo bar: 1, baz "a", qux: "2012-03-
                          06 12:00:00">
What happens when this implicit
type casting fails?
# == Schema Information
# bar :integer
# qux :datetime
class Foo < ActiveRecord::Base; end


$ Foo.new(:bar => 'a', :qux => 'cheezburger')
#<Foo bar: 0, qux: nil>
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Active records before_type_cast

  • 1. The Calm Before the Cast Understanding ActiveRecord's Casting
  • 2. What is Casting? Casting is a manufacturing process by which a liquid material is usually poured into a mold In computer science it is the explicit changing of the data type of a given entity e.g, casting a string into an array: Array("foo") # => ["foo"]
  • 3. ActiveRecord's Casting Extending the ActiveRecord::Base class creates attribute accessors that map to a given database table's columns The column type in the database dictates the data type of the returned value Database data types != Ruby data types
  • 4. ActiveRecord's Type Casting System bundle open activerecord Database specific adapters read column definitions (extend abstract): lib/active_record/connection_adapters/postgres ql_adapter.rb|74 These are cast to Ruby classes: lib/active_record/connection_adapters/abstract/ schema_definitions.rb|70
  • 5. CREATE TABLE foo ( bar integer, baz character varying(255), qux timestamp without time zone ); SELECT * FROM foo WHERE id=1; Casting occurs here { Abstract adapter Postgres adapter Foo.find(1) # => #<Foo bar: 1, baz "a", qux: "2012-03- 06 12:00:00">
  • 6. What happens when this implicit type casting fails? # == Schema Information # bar :integer # qux :datetime class Foo < ActiveRecord::Base; end $ Foo.new(:bar => 'a', :qux => 'cheezburger') #<Foo bar: 0, qux: nil>
  • 8. What About Validations? ActiveRecord has some validations, such as Integer's "validates :numericality => true" (is this even a real word?!) But not all data types have validations e.g. DateTime A nil value for datetime after the type cast may silently pass validation, losing a user's input without notice
  • 9. Before the Cast: ActiveRecord provides dynamic methods that hold a value before and after the type cast: a = Foo.new(:bar => 'a', :qux => 'cheezburger') a.qux # nil a.qux_before_type_cast # "cheezburger"
  • 10. Lets Write our Own Validator class Foo < ActiveRecord::Base validate :datetime_value_is_castable def datetime_value_is_castable begin DateTime.parse(qux_before_type_cast) rescue ArgumentError errors.add(:qux, "is not a valid date format") end end end
  • 11. And The Exciting Conclusion a = Foo.new(:qux => 'cheezburger') a.valid? # => false a.errors # => { :qux => ["is not a valid date format" ]} Now the user can be alerted that their value is invalid, without it being silently discarded
  • 12. Questions/Comments Ben Simpson ben@mojotech.com http://mrfrosti.com @mrfrosti