Preload has_many associations with dynamic conditi

问题:

I have a Place model and an Event model. Places can have events that take place on a specific date.

How can I set up my associations and finders to load all places including (eager loading) their events at a specific date without N+1 query problem?

What I've tried:

class Place
    has_many :events
end

Place.all.preload(:events).where("events.start_date > '#{time_in_the_future}'")
#ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR:  missing FROM-clause entry for table "events".

Place.all.includes(:events).where("events.start_date > '#{time_in_the_future}'").references(:event)
# only loads places that have an event at the specific date and not all places including their events (if there are any events).

I successfully came up with an association that does what I want but is not dynamic (does not accept parameters)

class Place
    has_many :events, -> {where("events.start_date > '#{Time.now}'")}
end

Place.all.preload(:events)
# perfect: executes two queries: One to get all 'places' and one to get all 'events' that belong to the places and merges the 'events' into the 'place' objects. 
# But I can't pass time as a parameter, so time is always Time.now (as specified in the has_many association). 
# Place.all.preload(:events).where(xyz) gives wrong results like the examples above.

The problem for me is that I can't find a way to preload/eager load with dynamic conditions. Because preload and includes expect the association name as a parameter and can´t be refined with parameters. At least I found no way to do this.

回答1:

This seems to be the only solution that works:

# 1st query: load places
places = Place.all.to_a

# 2nd query: load events for given places, matching the date condition
events = Event.where(place: places.map(&:id)).where("start_date > '#{time_in_the_future}'")
events_by_place_id = events.group_by(&:place_id)

# 3: manually set the association
places.each do |place|
  events = events_by_place_id[place.id] || []

  association = place.association(:events)
  association.loaded!
  association.target.concat(events)
  events.each { |event| association.set_inverse_instance(event) }
end

It's a bit hacky but it's quite easy to adapt to any situation where you might want to load an association using a separate query and then attach it to an existing object.

All credit goes to https://mrbrdo.wordpress.com/2013/09/25/manually-preloading-associations-in-rails-using-custom-scopessql/

回答2:

To solve the dynamic date problem, have you considered:

class Event < ActiveRecord::Base

  belongs_to :place

  scope :on_date, lambda {|the_date| where(start_date: the_date) }
  scope :on_or_after, lambda {|the_date| where('start_date >= ?', the_date) }
end

You could then do this:

@place = Place.find(params[:id]) # let's say...
@place.events.on_date(params[:chosen_date])

You can incorporate the eager loading stuff that others have mentioned too.

回答3:

As I understand, you want to fetch all the places that has at least one event satisfying some condition, but places should be fetched with all events list even those which doesn't satisfy condition. You can't figure this is out with one simple query, but if you will use suquery then issue will done. Here is the solution:

Place.includes(:events).where(id: Place.joins(:events).where("events.start_date > '#{time_in_the_future}'")).references(:events)

There is one complex query will be constructed, but it do the things right.

回答4:

I have mentioned in some cases includes doesn't properly select eager loading method. There is an explanation of how this method work http://blog.arkency.com/2013/12/rails4-preloading/ . You can directly call eager_load(:events) an I think it will load your AR objects without n+1 problem.