Increase performance: avoid looking for the right element in a collection - ruby-on-rails

Question

I have this situation.
activity.rb
belongs_to :user
belongs_to :cause
belongs_to :sub_cause
belongs_to :client
def amount
duration / 60.0 * user.hourly_cost_by_year(date.year).amount rescue 0
end
user.rb
has_many :hourly_costs # one hourly_cost for year
has_many :activities
def hourly_cost_by_year(year = Date.today.year)
hourly_costs.find { |hc| hc.year == year }
end
hourly_cost.rb
belongs_to :user
I have a big report where I achieved good performance (the number of SQL queries is fixed) but I think I could do better. The query I use is
activities = Activity.includes(:client, :cause, :sub_cause, user: :hourly_costs)
And this is ok, it's fast, but I think is improvable because hourly_cost_by_year method. I mean, activity has a date and I can use that date to know which of those hourly costs I should use. Something like this in activity
def self.user_with_single_hourly_cost
joins('LEFT JOIN users u ON u.id = activities.user_id').
joins('LEFT JOIN hourly_costs hc ON hc.user_id = u.id AND hc.year = EXTRACT(year from activities.date)')
end
But I don't how integrate this in my query. Whatever I tried did not work. I could use raw SQL but I'm trying to use ActiveRecord. I even thought to use redis to cache every hourly cost by user and year, could work, but I think this query, with the extract part, should do the best job because I'd have a flat table.
Update: I try to clarify. Whatever query I use in my action at some point I have to do
activities.sum(&:amount)
and that method, you know, is
def amount
duration / 60.0 * user.hourly_cost_by_year(date.year).amount rescue 0
end
And I don't know how to pick directly the hourly_cost I want without search between hourly_costs. Is this possible?

Answer 1

You may consider using Arel for this. Arel is the underlying query assembler for rails/activerecord (so no new dependencies) and can be very useful when building complex queries because it offers far more depth than the high level ActiveRecord::QueryMethods.
Obviously with a broader API comes more verbosity (which actually adds quite a bit to the readability) and less syntactical sugar which takes some getting used to but has proven indispensable for me on multiple occasions.
While I did not take the time to recreate your data structure something like this may work for you
activities = Activity.arel_table
users = User.arel_table
hourly_costs = HourlyCost.arel_table
activity_users_hourly_cost = activities
.join(users,Arel::Nodes::OuterJoin)
.on(activities[:user_id].eq(users[:id]))
.join(hourly_costs,Arel::Nodes::OuterJoin)
.on(hourly_costs[:user_id].eq(users[:id])
.and(hourly_costs[:year].eq(Arel::Nodes::Extract.new(activities[:date],'year'))
)
)
Activity.includes(:client, :cause, :sub_cause).joins(activity_users_hourly_cost.join_sources)
This will add the requested join e.g.
activity_users_hourly_cost.to_sql
#=> SELECT
FROM [activities]
LEFT OUTER JOIN [users] ON [activities].[user_id] = [users].[id]
LEFT OUTER JOIN [hourly_costs] ON [hourly_costs].[user_id] = [users].[id]
AND [hourly_costs].[year] = EXTRACT(YEAR FROM [activities].[date])
Update
If you just want to add the "hourly_cost" this should work for you
Activity.includes(:client, :cause, :sub_cause)
.joins(activity_users_hourly_cost.join_sources)
.select("activities.*, activities.duration / 60.0 * ISNULL([hourly_costs].[amount],0) as hourly_cost_by_year")
Please note that this will only return Activity objects but they will now have a method called hourly_cost_by_year which will return the result of that calculation. Full SQL will look like
SELECT
[activities].*,
activities.duration / 60.0 * ISNULL([hourly_costs].[amount],0) as hourly_cost_by_year
FROM [activities]
-- Dependant upon WHERE Clause
LEFT OUTER JOIN causes ON [activities].[cause_id] = [causes].[id]
LEFT OUTER JOIN sub_causes ON [activities].[subcause_id] = [subcauses].[id]
LEFT OUTER JOIN clients [activities].[client_id] = [clients].[id]
--
LEFT OUTER JOIN [users] ON [activities].[user_id] = [users].[id]
LEFT OUTER JOIN [hourly_costs] ON [hourly_costs].[user_id] = [users].[id]
AND [hourly_costs].[year] = EXTRACT(YEAR FROM [activities].[date])
You could build the select portion in Arel too if you like but seems overkill for such a simple statement.