undefined method `questions' for nil:NilClass

Question 1

Your @conversation variable is never set to anything, so it is nil. You need to initialize it to something, either by setting it to Converation.new or retrieving a conversation from the database (which appears to be what you want to do in this case).

Question 2

Why don't you use a before_create callback in your model:

#app/controllers/conversations_controller.rb
Class ConversationsController < ActiveRecord::Base
    def create
         @conversation = Conversation.new(conversation_params)
         @conversation.save
    end

    private

    def conversation_params
        params.require(:conversation).permit(:conversation, :params, questions: [])
    end
end


#app/models/conversation.rb
Class Conversation < ActiveRecord::Base
    before_create :build_questions

    private
    def build_questions
         self.questions.build
    end
end

Question 3

You have a @conversation that holds a nil value in your create action of your QuestionsController,because it not set to nothing. Try giving it like this

def create
    @conversation = Conversation.find(params[:id])
    @question = @conversation.questions.build(params[:question])
    if @question.save
      #scoping to the current user is the right thing to do here
      @message = current_user.messages.new(:subject => "You have a question from #{@question.sender_id}",
                             #Original code :sender_id
                             :notification_id => @question.sender_id,
                             #Original code :recipient_id
                             :receiver_id => @question.recipient_id,
                             :conversation_id => @cnversation.id
                             :body => @question.question)

      @question.message = @message
      @question.save
      redirect_to questions_path, notice: 'Your question was saved successfully. Thanks!'
    else
      render :new, alert: 'Sorry. There was a problem saving your question.'
    end
  end

Hope it helps!

Update

I believe you should have a Conversation model to relate the Question model.Create a Conversation model by generating this query

rails g model Conversation

Add a relation has_many questions to the Conversation model

Class Conversation < ActiveRecord::Base

has_many :questions

end

And also You should be adding a belongs_to conversation in your Question model.

Try updating your Question model

Class Question < ActiveRecord::Base

attr_accessible :answer, :question, :sender_id, :recipient_id

belongs_to :user

belongs_to :sender,:class_name => 'User',:foreign_key => 'sender_id'

belongs_to :recipient,:class_name => 'User',:foreign_key => 'recipient_id'

belongs_to :message

belongs_to :conversation

end

Question 4

As others have said, the error results from never setting the @conversation variable at all.

However, I believe no one who has answered so far realizes that Conversation is a model inside the mailboxer gem: Mailboxer::Conversation. Conversation instances are available from the result of the send_message call. You'd have to say:

receipt = send_message ....
conversation = receipt.conversation
# Now you can save the conversation of this message 
# in some useful place so it can be found later.

The other place conversations are available is in the messageable. I gather that this is your User model and appears as current_user.

You can get all the conversations of the current user with

current_user.mailbox.conversations

You can perform some filtering by choosing a single message queue instead, for example:

current_user.mailbox.inbox

Of course you probably want to get the last conversation. For this, you want

current_user.mailbox.conversations.last

So my wild guess is you might want

@conversation = current_user.mailbox.conversations.last

Another inconsistency in your code is that the basic Mailboxer::Conversation model created by the generator has no questions relation, yet you seem to be assuming there is one when you say

@conversation.questions

To relate conversations to questions, you will have to monkey patch with something like:

class Mailboxer::Conversation < ActiveRecord::Base
  has_many :questions
end

in your own file app/models/conversation.rb. Additionally, this makes sense only if there is a belongs_to :conversation and foreign key conversation_id in the Question model. You don't have either.

In summary, two observations:

You'll need to spend some time learning how mailboxer works by studying its source code because the documentation is sparse.
There is at least the fatal inconsistency in your models described above. Others are likely. You'll have to fix these before you have any hope of success. The way to do this is test the models before moving on to the controllers. Success in Rails is all about getting the models exactly right.

Question 5

From the mailboxer documentation, I can see that any message has a conversation_id. You claim that the question's conversation_id is not set, so I think your question model also has a link to a conversation?

Yet I do not see that belongs_to in your question model?

So I see a few options:

either you should access the conversation through the message, e.g. question.message.conversation
either you want the conversation_id of the question to be the same as that of the message, do something like

@message = current_user.messages.new(...) @question.message = @message @question.conversation_id = @message.conversation_id

You do save the @question but not the @message? Is that the problem?

either your @message is not created with a conversation, but maybe you should send it, like you do in your MessagesController :

@message = current_user.send_message(.... @question.update_attributes :message_id => @message.id

Hope this helps.