Rails Association Guide

rails_associations_guide.rb

#// RAILS ASSOCIATIONS

class Author
  has_one :book   
    # indicates that one other model has a reference to this model.
    # Rails assumes that Books.author_id is a foreign key to Author.id in this table.
end

class Book < AR
  belongs_to :author
    # tells Rails that books.author_id is linked to Author.id
    # RAILS INFERS THE CLASS NAME FROM THE ASSOCIATION NAME.
end

# corresponding migration
class CreateBooks < ActiveRecord::Migration[7.0]
  def change
    create_table :books do |t|

      t.belongs_to :author  , [foreign_key: true]     
          # belongs_to does not enforce FK constraint, so we add DB level foreign key constraint.
          # Each book knows their author, but author does not know its books.

      # OR
      t.references :author    ||   t.integer :author_id
        # creates a field called author_id in books table

      t.datetime :published_at
      t.timestamps
    end
  end
end

class CreateAuthors < ActiveRecord::Migration[7.0]

  def change
    create_table :authors do |t|
      t.string :name
      t.timestamps
    end
  end
end

#* ______________________________________________________


#// For a has_one association the method to create a new associated object would be user.create_post.
#// For a has_many association the method to create a new associated object would be user.posts.create.

```ruby
a = Author.new(name:"Daniel Silva")
a.create_book(book_name:"The Last Heist")   # creates both book, and associates it with the author
```
#* ______________________________________________________

# without has_one association in the model, if we run
a1 = Author.first
a1.book => gives error: method_missing': undefined method `book' for #<Author:0x000000010e9c7778>
  #* basically has_[one|many] association creates a method with the same name as the attribute
  #* so has_one adds a method to the AR oject which encapsulates this query:
  ```sql
  SELECT "books".* FROM "books" WHERE "books"."author_id" = ? LIMIT ?  [["author_id", 1], ["LIMIT", 1]]                                                    
  ```
  # direction => moves from author to book in the database.


#without belongs_to association in the model, if we run
b1 = Book.first
b1.author => gives error: method_missing': undefined method `author' for #<Book:0x000000010b7a31e8>
   ```sql
   SELECT "authors".* FROM "authors" WHERE "authors"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]             
  ```
  # direction => moves from book to author in the database.   

#* ______________________________________________________

#// MANY-TO-MANY ASSOCIATIONS

#! p = Patient.first
#! p.appointments.create(location:"NY")
  `raise_validation_error': Validation failed: Physician must exist (ActiveRecord::RecordInvalid)  
  # this ensures that all belongs_to for a model are met.


#! p.patients = all_p
```sql
  Patient Load (0.1ms)  SELECT "patients".* FROM "patients" INNER JOIN "appointments" ON "patients"."id" = "appointments"."patient_id" WHERE "appointments"."physician_id" = ?  [["physician_id", 1]]
  TRANSACTION (0.0ms)  begin transaction                    
  Appointment Create (0.3ms)  INSERT INTO "appointments" ("created_at", "updated_at", "location", "physician_id", "patient_id") VALUES (?, ?, ?, ?, ?)  [["created_at", "2022-04-24 13:47:50.003907"], ["updated_at", "2022-04-24 13:47:50.003907"], ["location", nil], ["physician_id", 1], ["patient_id", 2]]                       
  Appointment Create (0.0ms)  INSERT INTO "appointments" ("created_at", "updated_at", "location", "physician_id", "patient_id") VALUES (?, ?, ?, ?, ?)  [["created_at", "2022-04-24 13:47:50.005083"], ["updated_at", "2022-04-24 13:47:50.005083"], ["location", nil], ["physician_id", 1], ["patient_id", 3]]                       
  TRANSACTION (0.9ms)  commit transaction   
```

#! patient.physicians
```sql
  Physician Load (0.4ms)  SELECT "physicians".* FROM "physicians" INNER JOIN "appointments" ON "physicians"."id" = "appointments"."physician_id" WHERE "appointments"."patient_id" = ?  [["patient_id", 2]]

#* ______________________________________________________


#// SETTING UP FOREIGN KEYS
# By convention, Rails assumes that the column used to hold the foreign key on this model
# is the name of the # association with the suffix _id added. The :foreign_key option lets you
# set the name of the foreign key # directly. 

#// IF CLASS NAME IS NOT THAT OBVIOUS FOR AN ASSOCIATION
# If the name of the other model cannot be derived from the association name, you can use the 
# :class_name # option to supply the model name.
# class_name is used when the name of the model cannot be derived from the association name, therefore we
# use class name to supply the model name.

``` ruby
class Book < ApplicationRecord
  belongs_to :author, class_name: "Patron",
                      foreign_key: "patron_id"
end
```

# we can also set the foreign key on the has_many side, if the foreign key column is something other
# than the name of the model (with has_many/has_one) with the suffix _id added.

``` ruby
class Author < ApplicationRecord
  has_many :books, foreign_key: "cust_id"
end
```

#* ______________________________________________________

#// ADDING A SELF JOIN ASSOCIATION AFTER THE TABLE HAS BEEN CREATED
# in migration script to add mentor_id field on the authors table, add the link:
  add_reference :authors, :mentor, references: :author, foreign_key: { to_table: :authors }
  # this creates a reference to the mentor_id field on the authors table, and references itself using foreignKey as authors_id

# in the model itself:
``` ruby
  has_many :subordinates, class_name: "Author", foreign_key: "mentor_id"
  belongs_to :mentor, class_name: "Author"
```