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Joins

How to do more complicated relationships we didn't cover.

Heroku

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Joins

• We learned has_one, belongs_to, and has_many
• What if we do something that is not standard?
  • Example modeling friendships(a join of a user and  user)
  • What if a pokemon belongs to more than one trainer?

Standard has_many

• has_many :quits
  • Within user model
• Inferred that
  • quits table exists
  • users_id on quits refers to the owning user's id

Join Tables

• Use pets as an example
• If a pet had a single owner we can simple have
  • User
    • has_many :pets
  • Pet
    • belongs_to :user
    • pet has a user_id column

What if a pet is owned by multiple users?

• What about creating multiple rows in the pet table, with multiple entries for the same pet, each with a different user_id? Problems?
• Any other ideas?

Create a Join Table

• A users_pets table
• Each row in this table represents a pet ownership
• The users_pets table contains a user_id and a pet_id
• In user.rb just have
  • has_and_belongs_to_many :pets
• In pet.rb just have
  • has_and_belongs_to_many :users
• Rails will infer a users_pets table and get the associated records

Self Join For Users: Friendships

• Initial Problem, we can't say belongs_to :user
  • reference two users
• Separate them into two references
  • belongs_to :user_a
  • belongs_to :user_b
• Problem: it will infer tables user_as and user_bs

Use foreign_key

• belongs_to :user_a, class_name: :User, foreign_key: :a_id
  • know it's class and that user's id is the a_id column
• belongs_to :user_b, class_name: :user, foreign_key: :a_id
  • now we can distiguish

Listing the users friendships

• has_many :friendships
  • assumes friendships table and friendship class
  • is the user user_a or user_b?
• has_many :friendships, foreign_key: :a_id
  • specifies the user calling the method has the id a_id

Listing all the users friends

• One way is to define a function
  • def friends
    • friendships.map(:user_b).flatten
• Can define as a relationship
  • has_many :friends, through: :friendships, source: :user_b
    • through - another relationship we want to reference
    • source - method called on each
    • in the end flattened

Heroku

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• Currently free (not in the near future)