Retrieve a list of polymorphic relations in Rails 4 - sql

Question

I have a Rails4 app with the following models:
1. Systems (has many devices, has many parameters through devices)
2. Devices (belongs to a system, has many parameters)
3. Parameters (belongs to a Device)
4. Events (polymorphic - Systems, Devices and Parameters can have events)
When an event is created, a boolean field (on the event) is assigned a value. False indicated a failure.
I have a scope on my events, to only show failing events:
scope :failing, -> { where(okay: false).order('created_at desc') }
I can retrieve events as follows:
System.events.failing
Device.events.failing
Parameter.events.failing
I am trying to return a list of Systems where either:
1. the most recent event for the system has failed
2. the most recent event for any of it's devices has failed
3. the most recent event for any parameters of it's devices have failed
I have written this (horrible) SQL query which when executed in the console, returns the systems as an array:
"SELECT * FROM Systems WHERE Systems.id IN (SELECT Devices.system_id FROM Devices WHERE Devices.id IN (SELECT Parameters.device_id FROM Parameters JOIN EVENTS ON Parameters.id=Events.eventable_id WHERE Events.okay ='f')) OR Systems.id IN (SELECT Devices.system_id FROM Devices JOIN Events ON Devices.id=Events.eventable_id WHERE Events.okay='f')")
I need to either define a scope on the System model or a class method to return a list of 'failing' systems. Can you help?

Answer 1

You could mix joins and merges (to merge the WHERE clauses of a scope):
class System
# 1. the most recent event for the system has failed
scope :with_failing_events, -> { joins(:events).merge Event.failing }
# 2. the most recent event for any of it's devices has failed
scope :with_failing_devices, -> { joins(devices: :events).merge Event.failing }
# 3. the most recent event for any parameters of it's devices have failed
scope :with_failing_parameters, -> { joins(devices: { parameters: :events }).merge Event.failing }
end
Notice that passing a hash to joins enables a multiple join, or at least that seems to work in the app I'm working now at (Rails 4.0.5 postgresql).
To filter just the latest event (warning does not work in postgres, untested on other adapters), you could append to this queries:
System.joins(:events).merge(Event.failing).where events: { updated_at: 'MAX("events"."updated_at")' }
In any case, you can merge all these scopes with OR like this:
class System
scope :failing, -> do
where(
[
with_failing_events,
with_failing_devices,
with_failing_parameters
].map(&:where_clauses).join(' OR ')
)
end
end

Answer 2

One option, which would simplify things a bit, would be to add the foreign key, system_id, to the events table. You could then do the following:
Event.where(system_id: [system_id], okay: false).order('created_at DESC')
NOTE: I wouldn't define any relations for this new foreign key, I would simply use it to filter the events table.