ChatSpace DB設計 usersテーブル Column Type Options name string null: false, unique: true, add_index: true email string null: false, unique: true password string null: false Association has_many :groups_users has_many :groups, through: groups_users has_many :messages messeagesテーブル Column Type Options text text image string user_id integer null: false, foreign_key: true group_id integer null: false, foreign_key: true Association belongs_to :group belongs_to :user groupsテーブル Column Type Options name string null: false, unique: true Association has_many :groups_users has_many :users, through: groups_users has_many :messages groups_usersテーブル Column Type Options user_id integer null: false, foreign_key: true group_id integer null: false, foreign_key: true Association belongs_to :group belongs_to :user