How to avoid N+1 queries in scope with parameter in Rails - ruby-on-rails

Question

I met a N+1 issue in this situation:
Library has many Programs. Now I want to get all the programs located a certain country, so I have a code:
country = "US"
programs = #libraries.includes(:programs).map do |library|
library.programs.where(country: country)
end
But now there is N+1 problem:
Program Load (0.8ms) SELECT "programs".* FROM "programs" WHERE "programs"."library_id" = $1 AND "programs"."country" = $2 [["library_id", 15], ["country", "US"]]
Program Load (0.4ms) SELECT "programs".* FROM "programs" WHERE "programs"."library_id" = $1 AND "programs"."country" = $2 [["library_id", 73], ["country", "US"]]
Program Load (0.5ms) SELECT "programs".* FROM "programs" WHERE "programs"."library_id" = $1 AND "programs"."country" = $2 [["library_id", 27], ["country", "US"]]
Program Load (0.3ms) SELECT "programs".* FROM "programs" WHERE "programs"."library_id" = $1 AND "programs"."country" = $2 [["library_id", 177], ["country", "US"]]
Program Load (0.3ms) SELECT "programs".* FROM "programs" WHERE "programs"."library_id" = $1 AND "programs"."country" = $2 [["library_id", 38], ["country", "US"]]
Program Load (0.4ms) SELECT "programs".* FROM "programs" WHERE "programs"."library_id" = $1 AND "programs"."country" = $2 [["library_id", 51], ["country", "US"]]
Program Load (0.6ms) SELECT "programs".* FROM "programs" WHERE "programs"."library_id" = $1 AND "programs"."country" = $2 [["library_id", 18], ["country", "US"]]
Program Load (0.3ms) SELECT "programs".* FROM "programs" WHERE "programs"."library_id" = $1 AND "programs"."country" = $2 [["library_id", 20], ["country", "US"]]
Program Load (0.5ms) SELECT "programs".* FROM "programs" WHERE "programs"."library_id" = $1 AND "programs"."country" = $2 [["library_id", 42], ["country", "US"]]
Program Load (0.5ms) SELECT "programs".* FROM "programs" WHERE "programs"."library_id" = $1 AND "programs"."country" = $2 [["library_id", 39], ["country", "US"]]
Update:
My purpose is not to just filter the programs, but to use it. For example:
programs = #libraries.includes(:programs).each do |library|
if library.programs.where(country: country).size < 5
puts "US programs are less than 5 so you can still add"
end
end
Does anyone know how to solve the N+1 problem?

Answer 1

You can chain the where query to the includes like below
programs = #libraries.includes(:programs).where(programs: {country: country})
which should solve the N+1 problem.
See specifying-conditions-on-eager-loaded-associations
Update #1:
You can simply do it like this
programs = #libraries.includes(:programs).where(programs: {country: country}).size < 5 #returns true or false
if programs #true
puts "US programs are less than 5 so you can still add"
else #false
#your code
end
Update #2:
This should do
programs_size = #libraries.includes(:programs).where(programs: {country: country}).map { |library| library.programs.size }
Which would perform only one query and returns the size of each library.programs matching that condition as array something like below
=> [5, 4, 7, 4, 6, 2, 1]
Now you can iterate over the programs_size array and perform the logic
programs_size.each do |ps|
if ps < 5 #true
puts "US programs are less than 5 so you can still add"
else #false
#your code
end
end