Logistics

Make sure you check in with your TA!

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)

Standard has_many

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

What if we want something different?

Do it manually!

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: :User, foreign_key: :a_id
  • know it's class and that user's id is the a_id column
• belongs_to :user_b, class: :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)