builder interview preparation

polymorphic association :: picture product employee

has_and_belongs_to_many :: assemblies parts

has_many_through association patients physicians appointments

has one_through association  supplier account and account history

self join   employee with its subordinate and manager 

Polymorphic Association

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Class Picture < Application Record

 belongs_to :imageable,polymorphic: true

end

Class Product <Application Record

 has_many :pictures ,as: :imageable

end

Class Employee< Application Record

 has_many :pictures ,as: :imageable

end

has_and_belongs_to_many

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Class Assembly < Application Record

 has_and_belongs_to_many :parts

end

Class Part <Application Record

 has_and_belongs_to_many :assemblies

end

The has_many :through Assocication :: many to many association

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this association is often used to set up a many -to-many connection with another model , this association indicates that the declaring model can be matched with zero or more instances of another model by proceeding through a third model 

patient         appointment              physician  

Class Patient < Application Record

   has_many :appointments

   has_many :physicians,through: :appointments

end

Class Appointment < Application Record

  belongs_to :patients

  belongs_to :physicians

end

Class Physician < Application Record

  has_many :appointments

  has_many :patients,through: :appointments

end

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The has_one :through Assocication     ::  one to one association

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A has one thorugh association sets up a one to one connection with another model. this association indicates that the declaring model can be matched with one instance of another model by procceeding through third model 

Ex:: this example is with supplier , account and account history

supplier      account        account-history

if each supplier has one account and each account has one account history then the supplier model could look like this ::

Class Supplier < Application Record

  has_one :account

  has_one :accounthistory,through: :account

end

Class Account < Application Record

  belongs_to:supplier

  has_one:account_history

end

Class AccountHistory < Application Record

  belongs_to:account 

end

Self Joins ::

sometimes in desigining a data model we find a model that should have a relation to itself

Self join is the join method in which a table makes the relation with itself in this a table creates its two copies and one table use the foreign key as the primary key with the other table.

For ex :: when we have the employee table and we have the manager and subordinates then we can make the two copies of the table and makes the relationship between manager and subordinates as employee with the help of self join method of the association.

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Class Employee < Application Record

has_many :subordinates, class_name : "Employee"

                        foreign_key : "manager_id"

    belongs_to :manager, class_name: "Employee", optional:true            

end

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In this we use the class_name and foreign key :: in this we take the class_name as the table name and we take the foreign key inside has_many association.

we can retreive ::

@employee.subordinates and @employee.manager