1 対 1

# user model
 has_one :item

# item model
 belongs_to :user

@user = User.new
@item = @user.build_item

1 対　多

# user model
 has_many :item

# item model
 belongs_to :user

@user = User.new
@item = @user.items.build

多対多

# user model
  has_many :group_users
  has_many :groups, through: :group_users

# group model
  has_many :group_users
  has_many :users, through: :group_users

# group_user model (中間テーブル)
  belongs_to :group
  belongs_to :user

@user = User.new
@group = @user.groups.build