Methods

class Author < ApplicationRecord
  has_many :books
end
  • author.books
  • author.books.delete
  • author.books.destroy
  • author.books << book
  • author.books.delete book remove one or more books from the author by settings their foreign_key to NULL 
    irb(main):010:0> author.books.delete book
 (0.1ms)  begin transaction
SQL (1.3ms)  UPDATE "books" SET "author_id" = NULL WHERE "books"."author_id" = ? AND "books"."id" = 1  [["author_id", 1]]
 (5.7ms)  commit transaction
=> [#<Book id: 1, author_id: 1, name: "Your life", created_at: "2017-05-11 10:50:36", updated_at: "2017-05-11 10:50:36">]

    In case

    class Author < ApplicationRecord
has_many :books, dependent: :destroy
end 

    irb(main):005:0> a.books.delete b
 (0.1ms)  begin transaction
SQL (0.4ms)  DELETE FROM "books" WHERE "books"."id" = ?  [["id", 2]]
 (6.3ms)  commit transaction
=> [#<Book id: 2, author_id: 1, name: "Your name 2", created_at: "2017-05-11 11:08:50", updated_at: "2017-05-11 11:08:50">]
  • author.books.destroy book remove one or more books form author.books by running destroy on each other.
  • author.book_ids return array of ids of objects in collection
  • author.books.clear removes all objects from collection according to the strategy specified by the dependent option. If no option is defined, it allows the default strategy. The default strategy for has_many :through is delete_all, and for has_many associations is set the foreign_key to NULL 
    class Author < ApplicationRecord
has_many :books 
end

     a.books.clear
SQL (0.8ms)  UPDATE "books" SET "author_id" = NULL WHERE "books"."author_id" = ?  [["author_id", 1]]
=> #<ActiveRecord::Associations::CollectionProxy []>

    class Author < ApplicationRecord 
has_many :books, dependent: :destroy
end 

    irb(main):002:0> a.books.clear
SQL (0.4ms)  DELETE FROM "books" WHERE "books"."author_id" = ?  [["author_id", 1]]
=> #<ActiveRecord::Associations::CollectionProxy []>

class Author < ApplicationRecord
  has_many :books, dependent: :delete_all
end 

irb(main):002:0> a.books.clear
  SQL (0.2ms)  DELETE FROM "books" WHERE "books"."author_id" = ?  [["author_id", 1]]
=> #<ActiveRecord::Associations::CollectionProxy []>
  • books.empty?
  • books.size
  • books.find(...)
  • books.where(...)