Rails有效地查询记录和最新的关联记录，包括吗？

Question

I'm having trouble with a rails activerecord query.

I have 2 tables.

One for Projects:

id | name | created_at | updated_at

One for Project Reports:

id | project_id | number | created_at | updated_at

The corresponding class for Project:

class Project < ActiveRecord::Base
  has_many :project_reports

  def latest_report_number
    project_reports.last.number
  end
end

And ProjectReports:

class ProjectReport < ActiveRecord::Base
  belongs_to :project
end

I'm attempting to write a query to get a list of the projects and the most recent report for the project.

I can do this in the following way, but this leads to a N+1 situation:

Project
  .select("projects.*, max(project_reports.created_at) as latest_report")
  .joins(:project_reports)
  .order("latest_report desc")
  .group("projects.id")
  .map { |p| "#{p.name}: #{p.latest_report_number}" }

I therefor wanted to use rails 'includes' feature so that I don't run into the N+1 situation:

Project
  .select("projects.*, max(project_reports.created_at) as latest_report")
  .joins(:project_reports)
  .order("latest_report desc")
  .group("projects.id")
  .includes(:project_reports)    # <----
  .map { |p| "#{p.name}: #{p.latest_report_number}" }

This causes my query to no longer work:

PG::GroupingError: ERROR:  column "project_reports.id" must appear in 
the GROUP BY clause or be used in an aggregate function

How can I write my query so that I both accomplish the goal and limit the number of times I hit the database (ideally with includes)?

Answer 1

In the case of a join query, includes asks for extra columns from the associated table (ie tasks ) in order to build the associated objects without an additional query. Normally, that's a welcome optimization, but in the case of a GROUP BY clause, you wind up with an invalid SELECT .

In your case, you want a completely separate query to load the associations, so preload is a better choice.

Project
  .select("projects.*, max(project_reports.created_at) as latest_report")
  .joins(:project_reports)
  .order("latest_report desc")
  .group("projects.id")
  .preload(:project_reports)
  .map { |p| "#{p.name}: #{p.latest_report_number}" }