mongoid scope find the wrong conversation

Question

I have the following scope on my model:

scope :between, -> (sender_id,recipient_id) do
    where(sender_id: sender_id, recipient_id: recipient_id).or(sender_id: recipient_id, recipient_id: sender_id).exists?
  end

this try to find the conversation between two persons, the on my controller I have the following action:

def create
    if Conversation.between(params[:sender_id],params[:recipient_id]).exists?
      @conversation = Conversation.between(params[:sender_id],params[:recipient_id]).first
      redirect_to dashboard_conversation_path(@conversation.id)
    else
      @conversation = Conversation.create!(conversation_params)
      redirect_to dashboard_conversation_path(@conversation.id)
    end
  end

And here's my problem:

I have 3 o more user:

Users, A, B, C, D...no conversations between anyone. I create a conversation between user A and User B. The conversation does not exists, so is created, then if User A wants to start a conversation with User C, the model return false, because the conversation does not exists, son my controller need to create a new one, but, instead of that, the controller is open the conversation between user A and User B, but it have to create a new conversation between user A and User C, and open this conversation.

What I'm doing wrong?? I tried on different browsers and clean my cache.

---

UPDATE:

after create the first conversation, it always show the first conversation. I mean:

First Conversation

• User A - User B

Trying to create other conversations between the following users:

• User A - User C, show conversation User A, B
• User C - User B, show conversation User A, B
• User C - User D, show conversation User A, B

Answer 1

The or method doesn't work the way you think it does. If you look at the underlying selector:

Conversation.where(sender_id: s, recipient_id: r).or(sender_id: r, recipient_id: s).selector

you'll see this:

{
  "sender_id" => s,
  "recipient_id" => r,
  "$or" => [
    {
      "sender_id" => r,
      "recipient_id" => s,
    }
  ]
}

Calling or on a query doesn't mean "whatever is in the query already or this extra condition", it simply means "and any of these conditions".

The selector you want is:

{
  "$or" => [
    { "sender_id" => s, "recipient_id" => r },
    { "sender_id" => r, "recipient_id" => s }
  ]
}

and that would be built with a single or call:

or([
  { sender_id: s, recipient_id: r },
  { sender_id: r, recipient_id: s }
])

Presumably the trailing exists? call in your scope isn't there in reality. If it is then you're abusing scope to create a plain old class method and saying def self.between(sender_id, recipient_id) would be a better idea.