Rails: Optimize querying maximum values from associated table - sql

Question

I need to show a list of partners and the maximum value from the reservation_limit column from Klass table.
Partner has_many :klasses
Klass belongs_to :partner
# Partner controller
def index
#partners = Partner.includes(:klasses)
end
# view
<% #partners.each do |partner| %>
Up to <%= partner.klasses.maximum("reservation_limit") %> visits per month
<% end %>
Unfortunately the query below runs for every single Partner.
SELECT MAX("klasses"."reservation_limit") FROM "klasses" WHERE "klasses"."partner_id" = $1 [["partner_id", 1]]
If there are 40 partners then the query will run 40 times. How do I optimize this?
edit: Looks like there's a limit method in rails so I'm changing the limit in question to reservation_limit to prevent confusion.

Answer 1

You can use two forms of SQL to efficiently retrieve this information, and I'm assuming here that you want a result for a partner even where there is no klass record for it
The first is:
select partners.*,
max(klasses.limit) as max_klasses_limit
from partners
left join klasses on klasses.partner_id = partners.id
group by partner.id
Some RDBMSs require that you use "group by partner.*", though, which is potentially expensive in terms of the required sort and the possibility of it spilling to disk.
On the other hand you can add a clause such as:
having("max(klasses.limit) > ?", 3)
... to efficiently filter the partners by their value of maximum klass.limit
The other is:
select partners.*,
(Select max(klasses.limit)
from klasses
where klasses.partner_id = partners.id) as max_klasses_limit
from partners
The second one does not rely on a group by, and in some RDBMSs may be effectively transformed internally to the first form, but may execute less efficiently by the subquery being executed once per row in the partners table (which would stil be much faster than the raw Rails way of actually submitting a query per row).
The Rails ActiveRecord forms of these would be:
Partner.joins("left join klasses on klasses.partner_id = partners.id").
select("partners.*, max(klasses.limit) as max_klasses_limit").
group(:id)
... and ...
Partner.select("partners.*, (select max(klasses.limit)
from klasses
where klasses.partner_id = partners.id) as max_klasses_limit")
Which of these is actually the most efficient is probably going to depend on the RDBMS and even the RDBMS version.
If you don't need a result when there is no klass for the partner, or there is always guaranteed to be one, then:
Partner.joins(:klasses).
select("partners.*, max(klasses.limit) as max_klasses_limit").
group(:id)
Either way, you can then reference
partner.max_klasses_limit

Answer 2

Your initial query brings all the information you need. You only need to work with it as you would work with a regular array of objects.
Change
Up to <%= partner.klasses.maximum("reservation_limit") %> visits per month
to
Up to <%= partner.klasses.empty? ? 0 : partner.klasses.max_by { |k| k.reservation_limit }.reservation_limit %> visits per month
What maximum("reservation_limit") does it to trigger an Active Record query SELECT MAX.... But you don't need this, as you already have all the information you need to process the maximum in your array.
Note
Using .count on an Active Record result will trigger an extra SELECT COUNT... query!
Using .length will not.

Answer 3

It generally helps if you start writing the query in pure SQL and then extract it into ActiveRecord or Arel code.
ActiveRecord is powerful, but it tends to force you to write highly inefficient queries as soon as you derail from the standard CRUD operations.
Here's your query
Partner
.select('partners.*, (SELECT MAX(klasses.reservation_limit) FROM klasses WHERE klasses.partner_id = partners.id) AS maximum_limit')
.joins(:klasses).group('partners.id')
It is a single query, with a subquery. However the subquery is optimized to run only once as it can be parsed ahead and it doesn't run N+1 times.
The code above fetches all the partners, joins them with the klasses records and thanks to the join it can compute the aggregate maximum. Since the join effectively creates a cartesian product of the records, you then need to group by the partners.id (which in fact is required in any case by the MAX aggregate function).
The key here is the AS maximum_limit that will assign a new attribute to the Partner instances returned with the value of the count.
partners = Partner.select ...
partners.each do |partner|
puts partner.maximum_limit
end

Answer 4

This will return max. limits in one select for an array of parthner_ids:
parthner_ids = #partners.map{|p| p.id}
data = Klass.select('MAX("limit") as limit', 'partner_id').where(partner_id: parthner_ids).group('partner_id')
#limits = data.to_a.group_by{|d| d.id}
You can now integrate it into your view:
<% #partners.each do |partner| %>
Up to <%= #limits[partner.id].limit %> visits per month
<% end %>