Trying to avoid an N+1 query when we only want the first record of an ordered ActiveRecord association.

has_one_of_a_has_many.rb

class Organization < ActiveRecord::Base
  has_many :people, dependent: :destroy, inverse_of: :organization
  has_many :locations, dependent: :destroy
  has_many :activity_items, -> { order(last_activity_at: :desc) }, dependent: :destroy
  has_one :last_activity, -> { order(last_activity_at: :desc) }, class_name: 'ActivityItem'
end

class ActivityItem < ActiveRecord::Base
  belongs_to :organization
  belongs_to :user
  belongs_to :item, polymorphic: true
end

# item is one of Email::Message, PhoneCall, or Note
module Email
  class Message < ActiveRecord::Base
    belongs_to :thread, class_name: 'Email::Thread'
    belongs_to :user
    has_many :activity_items, as: :item, dependent: :destroy
  end
end

class PhoneCall < ActiveRecord::Base
  belongs_to :user
  has_many :activity_items, as: :item, dependent: :destroy
end

class Note < ActiveRecord::Base
  belongs_to :user
  has_many :activity_items, as: :item, dependent: :destroy
end

# We need to load a list of Organizations, and each org's: Locations, People + ContactMethod, and last ActivityItem + Item (email, note, call).
# The killer here is that `last_activity` relation, which is just the most recent `ActivityItem` plus its associated Item.

# The N+1 ends up being the `ActivityItem#item` relationship.
organizations = current_dealership.organizations.
  order(id: :desc).
  includes(:locations, :last_activity, people: :contact_methods).

Defining the offending has_one association as this seems to do the trick

class Organization < ActiveRecord::Base
  # other associations and scopes...
  has_one :last_activity, -> {
    select('DISTINCT ON (organization_id) *').order(:organization_id, created_at: :desc)
  }, class_name: 'ActivityItem'
  # ... other associations and scopes
end

This works by ordering the activity_items table first by organization_id and then breaks any ordering ties using created_at desc. Then Postgres limits the result set such that organization_id is distinct, which leaves only the "top" result for each organization.