Add migration for existing volunteers

db/migrate/20240418144517_add_volunteer_team_to_volunteers.rb

@@ -0,0 +1,30 @@
+class AddVolunteerTeamToVolunteers < ActiveRecord::Migration[7.1]
+  class VolunteerTeam < ActiveRecord::Base
+    has_and_belongs_to_many :volunteers
+  end
+
+  class Volunteer < ActiveRecord::Base
+    belongs_to :volunteer_team
+    has_and_belongs_to_many :volunteer_teams
+  end
+
+  def change
+    add_reference :volunteers, :volunteer_team, foreign_key: true
+
+    reversible do |dir|
+      dir.up do
+        transaction do
+          Volunteer.all.find_each do |volunteer|
+            volunteer.update(volunteer_team: volunteer.volunteer_teams.first)
+          end
+        end
+      end
+
+      dir.down do
+        # no-op
+      end
+    end
+
+    change_column_null :volunteers, :volunteer_team_id, true
+  end
+end