Preventing a Ruby on Rails notification daemon from emailing the same information more than once.

Question

Part of a real estate application I'm writing will allow a user to subscribe to a location and if a new property becomes available in that location then the user will receive an email notifying them of this.

I plan on runnning a background process once every few hours to handle the matching.

Right now I have a model called location and the plan is to add another model called notification. The location model has a latitude and longitude and so will the notification model.

Something like:

create_table "locations", :force => true do |t|
    t.decimal  "lat",            :precision => 15, :scale => 10
    t.decimal  "lng",            :precision => 15, :scale => 10
end

create_table "notifications", :force => true do |t|
    t.decimal  "lat",            :precision => 15, :scale => 10
    t.decimal  "lng",            :precision => 15, :scale => 10
    t.string   "user_name"
    t.string   "user_email"
end

The obvious thing to do is loop through the list of locations and if one matches that in a notification then send a mail to the user_email defined in the notification model.

What I'm trying to avoid is sending the same email about a location to the same user more than once. What with the process running every few hours.

I thought of adding another field to the notification model called "has_been_mailed" which would be set to true once a mail is sent but then that means the won't get any future updates of other locations added.

Does anyone have any suggestions as to the best way to implement this?

Answer 1

Without changing your model much, you should have the timestamp fields created_at and updated_at in both tables.

With that, you could query only for locations and notifications that have been updated since the last time you ran your mailer script.