Railsドキュメント(v6.0.2.1)

belongs_to(関連モデル名 [, scope, オプション])

belongs_to :user

belongs_to :admin_user, class_name: "User"

belongs_to :user, conditions: "user_id = 2"

belongs_to :user, order: "id DESC"

belongs_to :user, dependent: :destroy

belongs_to :user, foreign_key: "blog_id"

belongs_to :account, default: -> { company.account }

* [GitHub](https://github.com/rails/rails/blob/f33d52c95217212cbacc8d5e44b5a8e3cdc6f5b3/activerecord/lib/active_record/associations.rb#L1657)

has_one(関連モデル名 [, scope ,オプション])

class CreateUsersProjectsTable < ActiveRecord::Migration
  def self.up
    create_table :users_projects, id: false do |t|
      t.integer :user_id
      t.integer :project_id
    end
  end
  def self.down
    drop_table :users_projects
 vend
end

class User < ActiveRecord::Base
  has_one :project
end

class Project < ActiveRecord::Base
  belongs_to :user
end

has_one :admin_user, class_name: "User"

has_one :user, conditions: "user_id = 2"

has_one :user, order: "id DESC"

has_one :user, dependent: :destroy

has_one :user, foreign_key: "blog_id"

has_many(関連モデル名 [, scope, オプション])

class CreateUsersProjectsTable < ActiveRecord::Migration
  def self.up
    create_table :users_projects, id: false do |t|
      t.integer :user_id
      t.integer :project_id
    end
  end
  def self.down
    drop_table :users_projects
  end
end

class User < ActiveRecord::Base
  has_many :projects
  accepts_nested_attributes_for :projects
end

class Project < ActiveRecord::Base
  belongs_to :user
  accepts_nested_attributes_for :user
end

has_many :admin_user, class_name: "User"
accepts_nested_attributes_for :admin_user

has_many :user, conditions: "user_id = 2"
accepts_nested_attributes_for :user

has_many :user, order: "id DESC"
accepts_nested_attributes_for :user

has_many :users, group: "category"
accepts_nested_attributes_for :users

has_many :user, foreign_key: "blog_id"
accepts_nested_attributes_for :user

has_and_belongs_to_many(関連モデル名 [, scope, オプション])

class CreateUsersProjectsTable < ActiveRecord::Migration
  def self.up
    create_table :users_projects, id: false do |t|
      t.integer :user_id
      t.integer :project_id
    end
  end
  def self.down
    drop_table :users_projects
  end
end

class User < ActiveRecord::Base
  has_and_belongs_to_many :projects
end

class Project < ActiveRecord::Base
  has_and_belongs_to_many :users
end

$ rails generate model AttachmentImage attachable_id:integer attachable_type:string

class CreateAttachmentImage < ActiveRecord::Migration
  def self.up
    create_table :attachment_images do |t|
      t.integer :attachable_id
      t.string :attachable_type

    t.timestamps
  end
end

class AttachmentImage < ActiveRecord::Base
  belongs_to :attachable, polymorphic: true
end

class Blog < ActiveRecord::Base
  has_many :entries
  has_one :attachment_image, as: :attachable
end

class Entry < ActiveRecord::Base
  belongs_to :blog
end

def self.down
  drop_table :attachment_images
end

belongs_to(:親モデル, counter_cache: 親モデルのカラム)

def self.up
  add_column :entries, :comments_count, :integer, default: 0
end

belongs_to :entry, counter_cache: :comments_count