Active Record and file: How do i write Json file with my data?

The problem is that you assign a value to @eventJson in a loop so the previous values are lost. You should use an array:

def write_json
  events_json = []
  Event.all.each do |event|
    event_json = {
      "id" => event.id,
      "start" => event.start_at,
      "end" => event.end_at,
      "title" => event.name,
      "body" => event.event_description,
      "status" => event.status
    } 
    events_json << event_json
  end
  File.open("public/event.json","w") do |f|
    f.write(events_json.to_json)
  end 
end

In this case, you might want to use map instead of each -- it's much cleaner. Given that you said the method is in the model, this is how it would look.

class Event < ActiveRecord::Base
    ...

    def self.write_json
      record_json = self.all.map{ |record| { self.name => record.attributes } }.to_json
      File.open("#{Rails.root}/#{(self.name.underscore)}.json", "w") do |f|
        f.write record_json
      end 
    end
end