如何在Rails中编写代码以在SQL中运行所需的查询

Question

I'm making a chat room app in Rails and I have 3 tables as follows:

• Users id , name
• Rooms id , user_id , title
• Participants id , room_id , user_id

users and rooms have the following associations.

# user.rb
has_many :rooms

# room.rb
belongs_to :user

I want to be able to show the list of names of participants by writing:

# rooms/show.html.erb
<% @participants.each do |participant| %>
  <%= participant.user.name %>
<% end %>

In my controller, I have tried:

# rooms_controller.rb
def show
  @participants = Participant.includes(:users).where(room_id: 1)
end

But I get an error saying Association named 'users' was not found on Participant; perhaps you misspelled it? Association named 'users' was not found on Participant; perhaps you misspelled it? .
I have a feeling that setting associations between users and participants would might solve the problem, but I'm not sure what their relations are. (It's not a simple has_many / belongs_to relation, is it...?)

Overall, I want to run a SQL query that's something like SELECT * FROM participants INNER JOIN users ON participants.user_id = users.id WHERE participants.room_id = 1 . How can I write in Rails to run this query?

Answer 1

I think the relation between User and Room is a many-to-many. Then, use :has_and_belongs_to_many :

https://guides.rubyonrails.org/association_basics.html#the-has-and-belongs-to-many-association

But if you want to set the relation between User and Participant as many-to-many, use option :through :

https://guides.rubyonrails.org/association_basics.html#the-has-many-through-association

I think, a table Participant is not required, just a temporary variable participants seems ok.

Answer 2

Your participant model does have an association name users .

You can set up a has many through association like this:

# room.rb
belongs_to :user
has_many :participants
has_many :users, through: :participants

so in your controller you can get users of a room:

room = Room.find(1)
users = room.users