Work around race condition on last_seen_at update

[glebm]

There is race condition in code like:

# Say this doesn't find a user and initializes a new one:
user_detail = Thredded::UserDetail.find_or_initialize_by(user_id: user_id)
# A user gets created by another thread / process in-between these two lines:
user_detail.update!(last_seen_at: now)
# ActiveRecord::RecordNotUnique is raised

Our two options here are to either rescue the exception and retry, or use upsert. Unfortunately, there is no suitable upsert gem for Ruby that I could find.

https://github.com/seamusabshere/upsert is the most promising one but it currently has a number of show-stopping issues, namely it doesn't let us specify "insert but not update nor select" columns manually, and has potential thread-safety issues. May revisit this in the future.

[timdiggins]

Hi Gleb 👋 -- for what it's worth this must have been a real observable problem -- I know because I've got an issue upgrading a real database to this version of Thredded, because the index can't be created 😔.

I've made a crappy little addition to my migration to delete the offending unique keys which I post here in case useful to anyone else / me later on.

    subselect = <<-SQL
    SELECT tmu.id
    FROM thredded_messageboard_users tmu, thredded_messageboard_users tmu2
    WHERE tmu.thredded_user_detail_id = tmu2.thredded_user_detail_id AND tmu.thredded_messageboard_id = tmu2.thredded_messageboard_id AND tmu.id > tmu2.id
    SQL
    loop do
      break unless Thredded::MessageboardUser.where("id in (#{subselect})").delete_all > 0
    end

(based on https://stackoverflow.com/a/35990071/109175 -- should work on mysql as well as postgresql. I only encountered the problem with Thredded::MessageboardUser (probably because there are many more of them for me -- few users, many messageboards)

[glebm]

Hi! I also had this error when running the migration! I used slightly different code to delete the duplicates: https://github.com/thredded/thredded/releases/tag/v0.15.0