counter_cache increments twice

Question

Rails 3.2.14

Ruby 2.1.7

When using counter_cache, the count column gets incremented by 2 instead of 1. For example:

class Finger < BaseModel
  belongs_to :hand, :counter_cache => true
end

class Hand < BaseModel
  has_many :fingers
end

So if I do:

finger = Finger.new(:hand_id => 1)
finger.save

I can see in my console:

(0.2ms)  BEGIN
SQL (4.3ms)  INSERT INTO "fingers" ("hand_id") VALUES ($1) RETURNING "id"  [["hand_id", 311222]
Hand Load (0.4ms)  SELECT "hands".* FROM "hands" WHERE "hands"."id" = 311222 LIMIT 1
SQL (0.7ms)  UPDATE "hands" SET "fingers_count" = COALESCE("fingers_count", 0) + 1 WHERE "hands"."id" = 311222
(8.2ms)  COMMIT
=> true

So as you can see, COALESCE only gets called once.

But then:

Hand[1].fingers_count
> 2

But:

Hand[1].fingers.count
> 1

In my application, this is all done with out-of-the-box rails forms that create the new Finger related to the Hand.