I have a model with an embeds_many relation, on which I perform several lookups for business logic calculations. I noticed the lookup is slow, which is quite unexpected because the main document is already completely loaded from the database. I tried to look up using the Ruby Array select method and surprisingly it's much faster (simple benchmark on an embedded collection with 520 items):

[15] pry(main)> Benchmark.measure { 10.times { p.positions.where(:start_date.lte => Date.new(2015, 9, 1)).map(&:start_amount) } }.real
=> 2.168851
[16] pry(main)> Benchmark.measure { 10.times { p.positions.select { |pp| pp.start_date < Date.new(2015, 9, 1) }.map(&:start_amount) } }.real
=> 0.050369

What is the reason for this? Is there a way to make the where fast, or should I replace it to select everywhere?