将实例变量保存到数据库Rails

Question

I have been trying to save an instance variable and then increment a value on that instance variable, but it always resets when I reload.

Model looks like this

class Waiting < ApplicationRecord
def increment
    self.number_waiting += 1
    save!
end
def decrement
    self.number_waiting -= 1
    save!
end
def reset
    self.number_waiting = 0
    save!
end
end

Controller looks like this

class IndexController < ApplicationController
def index
    @waiting ||= Waiting.new(number_waiting: 0)
    @waiting.increment
    @waiting.save!
end
end

Html looks like this:

<p id="waiting_counter">
        <% @waiting.increment %>
        <%= @waiting.number_waiting %>
        <% @waiting.save! %>
    </p>

For some reason it is always at 2 and never changes... Been looking for a solution to this simple problem for several hours, please help.

Answer 1

Walkthrough your code and it should be obvious why it is always 2.

I assume you are navigating to IndexController#index (probably through indexes_path). This creates a completely new Waiting object initialised to 0 which is on the next line incremented by 1. Then you are saving it.

Then (assuming that html is from index.html.erb) rails will render the html during which the @waiting object is incremented once more, before displaying the value which is now 2. Then that object is again saved.

It's behaving exactly as it should, however I think the real issue is that you may have a misconception about how rails works. When calling index action on the controller the result of calling that action (by default) is that the page index.html.erb will be rendered during which time the embedded ruby is evaluated. This all happens server side and then the resulting html is sent back to the client. You have nothing client side to initiate triggering the increment.

It's unclear what you are actually trying to achieve here, and in what way the increment is expected be invoked.

After reviewing your question it has occurred to me that maybe you are expecting the instance variable to be available to all requests once initialised. That isn't the case - each request will get a new instance of the controller, so each time you reload the page @instance will initially be nil and will result in the creation of a new Waiting object. What you should do instead is grab the Waiting object from the database and then increment that, eg:

def index
    @waiting = Waiting.first
    @waiting ||= Waiting.new(number_waiting: 0)
    @waiting.increment
    @waiting.save!
end

Incidentally by convention you would normally display a list of existing objects through index action.