This (should) considerably improve traces like
https://app.datadoghq.com/apm/trace/917632173599137280?spanID=3163385094622710144&env=production&sort=time&colorBy=service&spanViewType=metadata&graphType=flamegraph&shouldShowLegend=true
by fixing the following 3 N+1s
```
user: root
GET /admin/customers.json?enterprise_id=4
USE eager loading detected
Customer => [:enterprise]
Add to your query: .includes([:enterprise])
Call stack
/usr/src/app/app/serializers/api/admin/customer_with_calculated_balance_serializer.rb:24:in `balance_value'
/usr/src/app/app/serializers/api/admin/customer_with_calculated_balance_serializer.rb:9:in `balance'
/usr/src/app/app/controllers/admin/customers_controller.rb:20:in `block (2 levels) in index'
/usr/src/app/app/controllers/admin/customers_controller.rb:17:in `index'
user: root
GET /admin/customers.json?enterprise_id=4
USE eager loading detected
Spree::Address => [:state]
Add to your query: .includes([:state])
Call stack
/usr/src/app/app/serializers/api/address_serializer.rb:14:in `state_name'
/usr/src/app/app/controllers/admin/customers_controller.rb:20:in `block (2 levels) in index'
/usr/src/app/app/controllers/admin/customers_controller.rb:17:in `index'
user: root
GET /admin/customers.json?enterprise_id=4
USE eager loading detected
Spree::Address => [:country]
Add to your query: .includes([:country])
Call stack
/usr/src/app/app/serializers/api/address_serializer.rb:10:in `country_name'
/usr/src/app/app/controllers/admin/customers_controller.rb:20:in `block (2 levels) in index'
/usr/src/app/app/controllers/admin/customers_controller.rb:17:in `index'
```
This popped up after improving the balances calculation. Now, that it's
fast, it's clear that are more performance problems on that endpoint.
We'll see if there are any left after this.
- Test that the unit price wrapper is here
- Click on the question mark icon and display the tooltip
- Click outside the question mark icon and hide the toolip
This moves them to a more unit-like style where the mailer methods are
the subjects. However, I did so only for the methods that show order
balance and thus we want to be extra sure of their coverage.
If these are unit tests, it's much easier to find a `describe` with the
method under test and putting all the tests exercising that method
together.
It turns out that `#update_payment_state` is by far the method that we
test the most which leads me to think:
a) this class might be doing too many things.
b) other methods might not be that well covered.
