Change jobseekers email in db to Citext. Part 1.

To achieve a smooth non-case sensitive "search user by email", while
keeping the indexed search instead of table scanning, the ideal
solution seems to be using citext as the DB type, allowing postgresql to
do the non-case sensitive matches.

Changing the column type directly would lock the Jobseekers table. So,
following "Online Migrations" guidance (and its useful helpers), we're
doing the change in stages to avoid causing service downtime.

The first stages to be deployed are:

1 - Create a new email column of citext type that is kept in sync with
email column using DB triggers.

2 - Backfill this column for all the existing data.

After deploying and testing this, we will follow up with copying the
constraints, indexes, foreign keys, etc and swapping the columns.

Once that is deployed we will remove the copy trigger and the old
column.

Reference:

https://github.com/fatkodima/online_migrations?tab=readme-ov-file#bullettrain_side-concrete-steps-for-active-record

db/migrate/20241011112641_initialize_change_jobseekers_email_type.rb

@@ -0,0 +1,7 @@
+class InitializeChangeJobseekersEmailType < ActiveRecord::Migration[7.1]
+  def change
+    enable_extension("citext")
+
+    initialize_column_type_change :jobseekers, :email, :citext
+  end
+end

db/migrate/20241011114125_backfill_change_jobseekers_email_type.rb

@@ -0,0 +1,11 @@
+class BackfillChangeJobseekersEmailType < ActiveRecord::Migration[7.1]
+  disable_ddl_transaction!
+
+  def up
+    backfill_column_for_type_change :jobseekers, :email
+  end
+
+  def down
+    # no op
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.1].define(version: 2024_09_24_144355) do
+ActiveRecord::Schema[7.1].define(version: 2024_10_11_114125) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "btree_gist"
   enable_extension "citext"
@@ -320,6 +320,7 @@
     t.text "last_sign_in_ip_ciphertext"
     t.string "account_merge_confirmation_code"
     t.datetime "account_merge_confirmation_code_generated_at"
+    t.citext "email_for_type_change", default: "", null: false
     t.index ["confirmation_token"], name: "index_jobseekers_on_confirmation_token", unique: true
     t.index ["email"], name: "index_jobseekers_on_email", unique: true
     t.index ["reset_password_token"], name: "index_jobseekers_on_reset_password_token", unique: true