In this day we talked about a very cool design pattern called ActiveRecord. We also talked about Rails model associations, validations and callbacks. We finally talked about the rails query interface.

1. Ruby on rails
course
Day 3

2. M for Model

3. Agenda
ActiveRecord Basics
Associations
Validations
Callbacks
Query Interface

4. Active record and
MVC
What’s ActiveRecord ?
What to expect from ActiveRecord Model.
associations.
inheritance.
validations.
OO DB operations.
Convention over configuration.

5. Active Record
An object that wraps a
row in a database table,
encapsulates the database
access and adds domain
logic on that data.

6. What to expect from ActiveRecord
Model?
associations.
Relations to different models to simulate one2many, many2one self
joins, etc..
inheritance.
also is considered as relation however it’s parent-child relationship
between models.
validations.
validation logic
OO DB operations.
Business logic example:
ahmed = User.last
ahmed.joined_course(“math”)

7. Convention over
configuration
Model class name
and table name
Foreign and primary
key convention:
FK:
<table_name>_id
PM:
id

8. Rails g model
<model_name>
[field:type, ]
Creates model file
Creates model test
Created migration
file.

9. Associations
belongs_to
has_one
how is that different
than belongs_to ?
through
has_many
through
has_and_belongs_to_many

10. belongs_to
One2One
connection.
single model
belongs_to :depart
ment

11. has_one
has_one is One2One
connection.
single model
has_one <model>
No FK required in
model, it’s pointed in
the other model.
through is the
intermediate junk table.

12. has_many
pointing to me.

13. has_many through :model
VS. has_and_belongs_to_many

14. Self Join
class_name
foreign_key
class Employee < ActiveRecord::Base
has_many :subordinates, class_name:
"Employee",foreign_key: "manager_id"

15. Validations
Built-in
validates :name, presence: true, length:
{minimum: 3}
Class Validator, EachValidator
Template design pattern
Methods
on:

16. Call Backs
Before, after and
around
->Save
Save<-
->Save<-
When
Create, update,
destroy
Save, validate
Executionorder
Destroy
Update
Create

17. Query interface
find(id or array(id))
take, first, last
find_by(key: value)*
where
? for sql injection
hash conditions
Ordering
Selecting
Group and Having
includes*
limit and offset
*find_each will be used with huge amount of records
*includes will eager load all associated models for less queries.

18. Thank you!