SELECT * FROM user AS u
INNER JOIN user_group AS ug ON u.user_group_id=ug.user_group_id
INNER JOIN user_group_info AS ugi ON ug.user_group_id=ugi.user_group_id
WHERE ugi.language_id = 1

class User < ApplicationRecord
    self.table_name = "user"
    self.primary_key = "user_id"
    belongs_to :user_group
end

class UserGroup < ApplicationRecord
    self.table_name = "user_group"
    self.primary_key = "user_group_id"
    has_many :user
    belongs_to :user_group_info
end

class UserGroupInfo < ApplicationRecord
    self.table_name = "user_group_info"
    has_many :user_group
end