I am struggling to get a counter cache working on a has_one :through and running into some weird behaviour. I want to have a counter_cache on the Company model that caches the number of associated Location (through the BusinessUnit model).

Objects:

class Company < ApplicationRecord
  has_many :business_units, class_name: :BusinessUnit, dependent: :destroy
  has_many :locations, through: :business_units, dependent: :destroy, counter_cache: :locations_count
end

class BusinessUnit < ApplicationRecord
  belongs_to :company, counter_cache: :locations_count
  has_many :locations, dependent: :destroy
end

class Location < ApplicationRecord
  belongs_to :business_unit
  has_one :company, through: :business_unit
end

Schema:

ActiveRecord::Schema[7.1].define(version: 2025_04_03_203549) do
  enable_extension "plpgsql"

  create_table "business_units", force: :cascade do |t|
    t.string "name"
    t.bigint "company_id", null: false
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.index ["company_id"], name: "index_business_units_on_company_id"
  end

  create_table "companies", force: :cascade do |t|
    t.string "name"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.integer "locations_count", default: 0, null: false
  end

  create_table "locations", force: :cascade do |t|
    t.string "name"
    t.bigint "business_unit_id", null: false
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.index ["business_unit_id"], name: "index_locations_on_business_unit_id"
  end

  add_foreign_key "business_units", "companies"
  add_foreign_key "locations", "business_units"
end

My understanding based on these SO answers (1, 2) is that this should work. It's not well documented and a bit counterintuitive how Rails automatically knows to refer to the locations count, but indeed it does seem to have some rails magic and calling Company.reset_counters(1, :locations) correctly sets the counter to match the number of locations.

HOWEVER, the counter doesn't update automatically (up or down) when locations are added or removed, and if a BusinessUnit with N locations is updated and assigned to another company the counter only increments/decrements by 1 (instead of by the expected N locations). This behaviour is present on both Rails 8.0.1 and 7.1.3 (which I accidentally used to build my MRE).

Is there something obvious I'm missing that can make the counter_cache work correctly?