Rails 查询：根据最新的子记录获取所有父记录

Question

An order has_many order_events.

An order_event is a record of the state of the order, eg, pending_approval , confirmed , etc.

Changes to an order are tracked by creating order_events .

I usually get the current state of an order by doing something like: order.order_events.last.try(:state) .

I would like a query to get all of the orders with a given current state, eg, all orders where the current state is cancelable .

I initially had a scope in the OrderEvent model:

scope :cancelable, -> { where('state = ? OR state = ?', 'pending_approval', 'pending_confirmation') }

and then a scope in the Order model:

scope :with_dates_and_current_state_cancelable, -> { with_dates.joins(:order_events).merge(OrderEvent.cancelable) }

and simply used the latter for other purposes.

The problem here is that it returns all orders that are currently or have in the past satisfied the condition.

What is the best way to get all of the orders that currently satisfy the condition?

Answer 1

I ended up using a query like this:

scope :with_dates_and_current_state_cancelable, -> {
  with_dates
  .joins(:order_events)
  .where('order_events.created_at = (SELECT MAX(order_events.created_at) FROM order_events WHERE order_events.order_id = orders.id)')
  .where('order_events.state = ? OR order_events.state = ?', 'pending_approval', 'pending_confirmation')
  .group('orders.id')
}

A bit hard to read, but it seems to work.

Answer 2

A classic solution here would be to use Rails enum .

Add this to your order model:

class Order
  enum status: [ :pending_approval, :confirmed, etc.. ]
  ...
end

The status can be changed by doing the following:

# order.update! status: 0
order.pending_approval!
order.pending_approval? # => true
order.status  # => "pending_approval"

No need for the order_events model.

To query all the orders that are pending approval:

Order.where(status: :pending_approval)

Edit: Alternate solution when order_event has necessary columns.

Add a column to the order_event called archived which can either be set to 1 or 0. Set the default scope in the order_event model to this:

default_cope where(:archived => 0)

Assuming 0 is not archived.

Now, when you create a new order event set the old event to 1.

old_event = OrderEvent.find(params[:order_id])
old_event.update(archived: 1)
new_event = OrderEvent.create(...archived: 0)

Whenever you query for pending review like so:

OrderEvent.where(:status => pending_approval)

Only events that are not archived will be shown.

Answer 3

I think I figured out a query that might work. I didn't turn it in to ActiveRecord methods, but here it is:

SELECT t.order_id
FROM
  (SELECT MAX(created_at) AS created, order_id
  FROM order_events
  GROUP BY order_id) as t
INNER JOIN order_events
ON t.order_id = order_events.order_id AND
   t.created = order_events.created_at
WHERE order_events.state = 'whatever_state_you_want'