Rails 4+，按孩子排序父记录列表

Question

I have a page that displays a lot of data and I'm working on optimizing it and I'm struggling with balancing reducing DB queries with getting the proper data in the order I need.

A Transaction is the top-level element, and each transaction has multiple workflows and each workflow has many milestones . I order the workflows and milestones by a position:integer attribute.

# Top-level element
class Transaction < ApplicationRecord
  has_many :workflows
  has_many :team_members
end

# Parent element for milestones
class Workflow < ApplicationRecord
  belongs_to :transaction
  has_many :milestones
end

# Unique individual
class Person < ApplicationRecord
  has_many :team_members
end

# Acts as a join between Transactions and Persons
class TeamMember < ApplicationRecord
  belongs_to :transaction
  belongs_to :person
  belongs_to :team_member_type
end

class TeamMemberType < ApplicationRecord
  has_many :team_members
end

In my controller this is what I have:

def show
  @transaction = Transaction.includes(
    team_members: [:person, :team_member_type],
    workflows: [:milestones]
  ).find(params[id])
end

This gathers all the data I need in 1 DB query, but the problem is when I iterate over the milestones for a workflow, I need it to be sorted by the position attribute.

I can grab the data from the DB and then sort it with Ruby, but that seems inefficient and I would prefer to get it all done in the DB properly (if possible).

If I were to write it with an N+1 issue, it would look like this:

<% @workflows.order(:position).each do |workflow| %>
  <%= workflow.name %>
  <% workflow.milestones.order(:position).each do |milestone| %>
    <%= milestone.name %>
  <% end %>
<% end %>

I think everyone agrees that's a bad idea, but I'm struggling to figure out how to sort the data and optimize my DB calls at the same time.

I also sort the team_members by the team_member_type.value attribute, but that's the same question as above.

<% @team_members.includes(:team_member_type, :person).order("team_member_types.value").each do |team_member| %>
  ...
<% end %>

My main objective is efficiency. If I need to use some Query objects to clean it up and raw SQL, I'm ok with that, I'm just not extremely proficient at SQL and would prefer using ActiveRecord if reasonable.

Answer 1

You can order by those two attributes in the controller method:

def show
  @transaction = Transaction.includes(
    team_members: [:person, :team_member_type],
    workflows: [:milestones]
  ).order('milestones.position, team_member_types.value').find(params[id])
end

Let me know if this helps.

Answer 2

You could create associations that include the order_by, but my experience has been that that can silently fail when eager loading, leaving you with an unordered result.

Although you believe, probably correctly, that ordering in Ruby would be less efficient, I would at least benchmark it. The major performance problem is in the n + 1 query situation, and I would be optimistic that you'll find sorting in Ruby to be performant enough.