I would just add a query string parameter to whatever url you are redirecting to after signup. For example:

def after_sign_up_path_for(resource)
  root_path(show_welcome_popup: true)
end

See How To: Redirect to a specific page on successful sign up (registration)

Then you can just check:

<%= if params[:show_welcome_popup] %>
  # ...
<% end %>

After you have displayed the popup use history.replaceState to modify the browser history:

// path/to/some/file.js
history.replaceState(null, window.title, window.location.pathname)

This will prevent the popup from reappearing if the user hits the back button which is a nice touch.

I solved with the following code line.

 <% if current_user.sign_in_count == 1  %>
  <div class="modal fade in" id="myModal0" role="dialog">
    <div class="modal-dialog modal-lg">
      <div class="modal-content" style="width: 90%" >
        <div class="modal-header">
          <button type="button" class="close" data-dismiss="modal">&times;</button>
        </div>
        <div class="modal-body">
          <h3 style="color: black; position: center">¡Welcome! <%=current_user.name%>.</h3>
          <iframe src="/uploads/pdfs/5.pdf" width="700px" height="650px"></iframe>
        </div>
        <div class="modal-footer">
          <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
        </div>
      </div>
    </div>
  </div>
  <%=current_user.increment!(:sign_in_count) %>
<%end %>

what the next line does <%=current_user.increment!(:sign_in_count) %>
is to officially increase an attribute, in this case :sign_in_count it is the attribute to increase.

Cheers.

I am trying to put a modal for new users, that says “Welcome” (or other message), I am using “sign_in_count” to record the number of logins.
At the moment I have something like this:

<% if current_user.sign_in_count == 1  %>
  <div class="modal fade in" id="myModal0" role="dialog">
    <div class="modal-dialog modal-lg">
      <div class="modal-content" style="width: 90%" >
        <div class="modal-header">
          <button type="button" class="close" data-dismiss="modal">&times;</button>
        </div>
        <div class="modal-body">
          <h3 style="color: black; position: center">¡Welcome! <%=current_user.name%>.</h3>
          <iframe src="/uploads/pdfs/5.pdf" width="700px" height="650px"></iframe>
        </div>
        <div class="modal-footer">
          <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
        </div>
      </div>
    </div>
  </div>
<%end %>

obviously, if the user enters for the first time, sign_in_count will always be equal to 1 and the modal will appear every time the user reloads the page, unless the user disconnects and re-enters (that is not the idea); and if sign_in_count+=1 there is no increase as such. Can there be a way where sign_in_count is 2 at the first login? The modal must appear when the user is new; or any solution you can offer? I really appreciate the help.
Cheers and thanks!