如何在Rails活动记录查询中执行条件计数？

Question

In a Rails 3.2 app I'm trying to construct a query that will return an array with two calculated counts. The app has three models: a User has many Events, and a Location has many Events. I need to return an array for a user that contains the number of events they have at each location, as well as the number of active events.

eg, [#<Location id: 1, name: "Location Name", total_events_count: 4, active_event_count: 4>]>

I can get the total_event_count

user = User.find(params[:id])
user.locations
  .select("locations.id, locations.name, count(events.id) AS total_events_count")
  .joins(:events)
  .group("locations.id")

Given that my Event model has a string status field that can take a value active , how would I also include an active_events_count in this query?

EDIT

After some useful suggestions from xdazz and mu, I'm still struggling with this. My problem appears to be that the count is counting all events, rather than events that belong to the location AND the user.

I'm going to try to rephrase my question, hopefully someone can help me understand this.

Events belong to both a User and a Location (ie User has_many :locations, through: :events )

Events have several fields, including status (a string) and participants (an integer).

In a User show view, I'm trying to generate a list of a User's locations. For each location, I want to display a "success rate". The success rate is the total number of a User;s events with participants, divided by the total number of that User's events. ie, if User1 has 4 events at LocationA, but only two of those events had participants, User1's success rate at LocationA is 0.5 (or 50%).

The way I though to achieve this is via a select query that also includes calculated counts for total_events_count and successful_events_count . (there may be a better way?)

So I do something like:

user = User.find(params[:id])
user.locations
  .select("locations.id, locations.name, count(events.id) AS total_events_count, sum(case events.marked when NOT 0 then 1 else 0 end) AS successful_events_count")
  .joins(:events)
  .group("locations.id")

This is returning an array with the correct keys, but the values are not correct. I am getting the total number of all events (and all successful events) for that location, not just a count of those events that belong to the current user.

I have been looking at this problem for so long that I'm getting myself very confused. Would be very grateful for a fresh perspective and ideas!!

EDIT2

After a break and fresh eyes, I have managed to get the result I need using the following code. It seems quite convoluted. If there is a better way, please let me know. Otherwise I will tidy up this question in case anyone else runs into the same problem.

class User
def location_ratings
  events = self.events
  successful_events = events.where('events.participants > 0')
  user_events_by_location = Event.
    select("events.location_id AS l_id, count(events.id) AS location_event_count").
    where( id: events.pluck(:id) ).
    group("l_id").
    to_sql
  user_successful_events_by_location = Event.
    select("events.location_id AS l_id, count(events.id) AS location_successful_events_count").
    where( id: successful_events.pluck(:id) ).
    group("l_id").
    to_sql
  Location.
    joins("JOIN (#{user_events_by_location}) AS user_events ON user_events.l_id = location.id").
    joins("JOIN (#{user_successful_events_by_location}) AS successful_user_events ON successful_user_events.l_id = location.id").
    select('location.id, location.name, user_events.location_events_count, successful_user_events.location_successful_events_count').
    order("user_events.location_events_count DESC")
end

Answer 1

You could use sum(events.marked='active') to get it:

user.locations
  .select("locations.id, locations.name, count(events.id) AS total_events_count, sum(events.marked='active') AS marked_event_count")
  .joins(:events)
  .group("locations.id")

Update:

If you are using postgresql, then you have to case boolean to int before using SUM function.

user.locations
  .select("locations.id, locations.name, count(events.id) AS total_events_count, sum((events.marked='active')::int) AS marked_event_count")
  .joins(:events)
  .group("locations.id")