Rails iterating through model and changing values doesn't work - ruby-on-rails

Question

I have a PiggyBank model where one of the attributes is interest_modifier(format: decimal). I wrote the following method to change the interest_modifier to a random number:
def set_random_interest_modifier(min = 0.5, max = 1)
self.interest_modifier = rand(min..max).round(2)
self.save!
end
It works. Now, I want a method which iterates over all PiggyBanks and changes the interest_modifier, so i wrote this:
def self.change_interest_modifiers
find_each do |pb|
pb.set_random_interest_modifier
end
end
It iterates over all records but doesn't change the values. Heres the log
[12] pry(main)> PiggyBank.change_interest_modifiers
PiggyBank Load (1.4ms) SELECT "piggy_banks".* FROM "piggy_banks" ORDER BY "piggy_banks"."id" ASC LIMIT 1000
(0.3ms) BEGIN
SQL (1.0ms) UPDATE "piggy_banks" SET "interest_modifier" = $1, "updated_at" = $2 WHERE "piggy_banks"."id" = 7 [["interest_modifier", "0.58"], ["updated_at", "2015-06-02 11:53:33.981262"]]
(3.1ms) COMMIT
(0.4ms) BEGIN
SQL (0.7ms) UPDATE "piggy_banks" SET "interest_modifier" = $1, "updated_at" = $2 WHERE "piggy_banks"."id" = 8 [["interest_modifier", "0.53"], ["updated_at", "2015-06-02 11:53:33.990404"]]
(0.4ms) COMMIT
(0.2ms) BEGIN
SQL (0.7ms) UPDATE "piggy_banks" SET "interest_modifier" = $1, "updated_at" = $2 WHERE "piggy_banks"."id" = 9 [["interest_modifier", "0.78"], ["updated_at", "2015-06-02 11:53:33.994231"]]
(0.4ms) COMMIT
(0.2ms) BEGIN
SQL (0.7ms) UPDATE "piggy_banks" SET "interest_modifier" = $1, "updated_at" = $2 WHERE "piggy_banks"."id" = 10 [["interest_modifier", "0.68"], ["updated_at", "2015-06-02 11:53:33.997966"]]
(0.5ms) COMMIT
(0.2ms) BEGIN
SQL (0.7ms) UPDATE "piggy_banks" SET "interest_modifier" = $1, "updated_at" = $2 WHERE "piggy_banks"."id" = 11 [["interest_modifier", "0.71"], ["updated_at", "2015-06-02 11:53:34.002414"]]
(0.5ms) COMMIT
(0.1ms) BEGIN
(0.1ms) COMMIT
(0.1ms) BEGIN
SQL (0.5ms) UPDATE "piggy_banks" SET "interest_modifier" = $1, "updated_at" = $2 WHERE "piggy_banks"."id" = 13 [["interest_modifier", "0.91"], ["updated_at", "2015-06-02 11:53:34.007100"]]
(0.5ms) COMMIT
(0.1ms) BEGIN
SQL (0.6ms) UPDATE "piggy_banks" SET "interest_modifier" = $1, "updated_at" = $2 WHERE "piggy_banks"."id" = 14 [["interest_modifier", "0.96"], ["updated_at", "2015-06-02 11:53:34.010488"]]
(0.4ms) COMMIT
(0.2ms) BEGIN
SQL (0.6ms) UPDATE "piggy_banks" SET "interest_modifier" = $1, "updated_at" = $2 WHERE "piggy_banks"."id" = 15 [["interest_modifier", "0.8"], ["updated_at", "2015-06-02 11:53:34.014190"]]
(0.5ms) COMMIT
(0.3ms) BEGIN
SQL (0.6ms) UPDATE "piggy_banks" SET "interest_modifier" = $1, "updated_at" = $2 WHERE "piggy_banks"."id" = 16 [["interest_modifier", "0.71"], ["updated_at", "2015-06-02 11:53:34.018658"]]
(0.5ms) COMMIT
(0.2ms) BEGIN
SQL (0.6ms) UPDATE "piggy_banks" SET "interest_modifier" = $1, "updated_at" = $2 WHERE "piggy_banks"."id" = 18 [["interest_modifier", "0.67"], ["updated_at", "2015-06-02 11:53:34.023011"]]
(0.5ms) COMMIT
=> nil
Now, I have no idea why this happens. No errors are raised. Please, help:)
After a few experiments with reloading in console:
p = PiggyBank.first
p.interest_modifier.to_s => "0.69"
PiggyBank.change_interest_modifiers
p.interest_modifier.to_s => "0.69"
reload!
p.interest_modifier.to_s => "0.69"
But, if i do the following, it works:
p = PiggyBank.first
p.interest_modifier.to_s => "0.69"
PiggyBank.change_interest_modifiers
p.interest_modifier.to_s => "0.69"
p.reload
p.interest_modifier.to_s => "0.57"
Why would that be?

Answer 1

This is what's happening:
p = PiggyBank.first
p.interest_modifier.to_s => "0.69"
so we've got an object in memory, p, which we've created and in the act of creation, loaded it's data out of the database.
PiggyBank.change_interest_modifiers
This will iterate through all of the PiggyBank records, creating an object for each. These objects are all different objects to the one that you made already. So, effectively, there have been two different objects made from the same record. The object inside the class method will save the new data to the database, behind p's back if you like.
These objects do not have a "live" connection to the database - that is to say, if the database is changed by a different method, the object in memory does not magically update itself with the new data.
So, while you have p sitting there, something else has changed the data in the database. When you say p.interest_modifier, you're just accessing an instance variable inside the object, you're not accessing the database.
When you then go on to say
p.reload
what you are actually doing is something like this:
p = PiggyBank.find(p.id)
ie, re-setting the contents of the p pointer to point to a new object, created out of the database record. This object now has the up to date data from the database.

Answer 2

Well, the method worked from the start, just forgot to reload the resource :O