Calling a Static Method from a Controller and Updating instance of that class - Getting undefined method for AR::Association::HasOneAssociation

My Question:

What should the correct structure be? I have tried redesign this several times but keep getting tight-coupling issues.

Relevant Info:

I am writing a new endpoint for some third party software. It will receive a payload that I am then passing into a static method of my Subscription class. From there I want to do a look-up of the payload related subscription, establish an instance of it, then perform an update based off the rest of my information on my class. I am running into an error that is saying the following: undefined method update for ActiveRecording::Association::HasOneAssociation

EndPointController:

class Api::V2::EndpointController < Api::V5::BaseController
    def connect
        data = decode(request.body.read)
        Subscription.static_method(data)
    end
end

Subscription Model:

class Subscription < ActiveRecord::Base

    def self.static_method(data)
        @subscription = Subscription.find_by_id(data.subscription_id)
        @subscription.update(data)
    end
end

Subscription Controller:

class SubscriptionsController < ApplicationController
    def update
        #execute update
    end
end