activerecord: Use a simpler query condition for aggregates with one mapping

[dylanahsmith]

Problem

When using an aggregate with one mapping, e.g.

class Customer < ActiveRecord::Base
  composed_of :balance, class_name: "Money", mapping: %w(balance amount)
end

The SQL query from Customer.where(balance: (1..50).map { |amount| Money.new(amount) }) would be something like

SELECT "customers".* FROM "customers" WHERE ((((((((((((((((((((((((((((((((((((((((((((((((("customers"."balance" = 1 OR "customers"."balance" = 2) OR "customers"."balance" = 3) OR "customers"."balance" = 4) OR "customers"."balance" = 5) OR "customers"."balance" = 6) OR "customers"."balance" = 7) OR "customers"."balance" = 8) OR "customers"."balance" = 9) OR "customers"."balance" = 10) OR "customers"."balance" = 11) OR "customers"."balance" = 12) OR "customers"."balance" = 13) OR "customers"."balance" = 14) OR "customers"."balance" = 15) OR "customers"."balance" = 16) OR "customers"."balance" = 17) OR "customers"."balance" = 18) OR "customers"."balance" = 19) OR "customers"."balance" = 20) OR "customers"."balance" = 21) OR "customers"."balance" = 22) OR "customers"."balance" = 23) OR "customers"."balance" = 24) OR "customers"."balance" = 25) OR "customers"."balance" = 26) OR "customers"."balance" = 27) OR "customers"."balance" = 28) OR "customers"."balance" = 29) OR "customers"."balance" = 30) OR "customers"."balance" = 31) OR "customers"."balance" = 32) OR "customers"."balance" = 33) OR "customers"."balance" = 34) OR "customers"."balance" = 35) OR "customers"."balance" = 36) OR "customers"."balance" = 37) OR "customers"."balance" = 38) OR "customers"."balance" = 39) OR "customers"."balance" = 40) OR "customers"."balance" = 41) OR "customers"."balance" = 42) OR "customers"."balance" = 43) OR "customers"."balance" = 44) OR "customers"."balance" = 45) OR "customers"."balance" = 46) OR "customers"."balance" = 47) OR "customers"."balance" = 48) OR "customers"."balance" = 49) OR "customers"."balance" = 50)

instead of the simpler query that we would get from not using composed_of (i.e. querying using Customer.where(balance: (1..50)))

SELECT "customers".* FROM "customers" WHERE "customers"."balance" IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50)

https://stackoverflow.com/questions/782915/mysql-or-vs-in-performance shows this could have a performance impact, at least in older versions of mysql. Mostly I just want to avoid a database regression by using composed_of to introduce an abstraction.

Solution

The code already had a special case for a single mapping, so I just separated this code path more to avoid building multiple conditions joined with OR.

[kamipo]

I'd prefer using the same variable names in both path.

Suggested change

	column_name, aggr_attr = mapping.first
	field_attr, aggregate_attr = mapping.first

[dylanahsmith]

That causes "shadowing outer local variable" ruby warnings

[kamipo]

oh... I see...