Learning Ruby on Rails
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Ruby on Rails - Models
Model Creation:
Model.new # creates a new empty model
Model.create( :field => 'value', :other_field => 42 )
# creates an object with the passed parameters and saves it
Model.find_or_create_by_field( value )
# searches for a record where "field = value", creates
# a new record if not found
User.find_or_create_by_name_and_email( 'ramjoe', 'ram@example.com')
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Model Relations:
There are four ways of associating models. has_one, has_many, belongs_to and has_and_belongs_to_many. Assuming the following four entities:

def Order < ActiveRecord::Base
has_many :line_items
belongs_to :customer
end
def LineItem < ActiveRecord::Base
belongs_to :order
end
def Customer < ActiveRecord::Base
has_many :orders
has_one :address
end
def Address < ActiveRecord::Base
belongs_to :customer
end
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Consider following relaship:

def Category < ActiveRecord::Base
has_and_belongs_to_many :products
end
def Product < ActiveRecord::Base
has_and_belongs_to_many :categories
end
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Association Join Models:
Consider the following relationship now. This depicts how we can use joins while defining relationship.

class Author < ActiveRecord::Base
has_many :authorships
has_many :books, :through => :authorships
end
class Authorship < ActiveRecord::Base
belongs_to :author
belongs_to :book
end
class Book < ActiveRecord::Base
has_one :authorship
end
@author = Author.find :first
# selects all books that the author's authorships belong to.
@author.authorships.collect { |a| a.book }
selects all books by using the Authorship join model
@author.books
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Check the following link for more detail on Associations.
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