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RoR: Database Associations ---how to declare and use them
(from RoR Guide)
Why Associations between Models?
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common operations simpler and easier (look later)
What are the Associations you can declare in a Model (in relation to another Model)
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belongs_to
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has_one
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has_many
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has_many :through
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has_one :through
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has_and_belongs_to_many
Example of how to declare Associations in your Modles (has_many, belongs_to, etc.)
model for customers and a model for orders.
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Each customer can have many orders. Without associations, the model declarations would look like this:
class Customer < ActiveRecord::Base
has_many :orders, dependent: :destroy
end
class Order < ActiveRecord::Base
belongs_to :customer
end
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Special Tip: Notice when we use has_many we reference the model as PLURAL -- this is convention in RoR you MUST Adhere to. But, for belongs_to it is the singular.
Example of How easier with Associations
Without Associations
class Customer < ActiveRecord::Base
end
class Order < ActiveRecord::Base
end
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Add New Order:
@order = Order.create(order_date: Time.now, customer_id: @customer.id)
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Deleting a customer, and ensuring that all of its orders get deleted as well:
@orders = Order.where(customer_id: @customer.id)
@orders.each do |order|
order.destroy
end
@customer.destroy
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Looking up all of a Customer's orders
@customer = Customer.where(*****);
@orders = Order.where(customer_id: @customer.id);
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With Associations
...see above definition with Associations of model classes.
Add new Order is easier:
@order = @customer.orders.create(order_date: Time.now)
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Deleting a customer and all of its orders is much easier:
Looking up all of a Customer's orders (is a bit easier --at least if you were doing the second step in a view you wouldn't be contacting the Model class directly)
@customer = Customer.where(*****);
#this gets all orders associated with this customer
@orders = @customer.orders;
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