Memoize Database Field Names For Faster Fields Access

[bomberby]

Background: While working on improving performance on our app, I've noticed that the database_field_name is consistently taking long than expected when calling the same field repeatedly(very common when iterating over tens of thousands of objects or in inner loops) While debugging I've noticed that Mongoid::Fields#database_field_name is taking a significant time to perform.

Comparison(very naive):

1000000.times do
  product.price
end

Before this change takes roughly 1.55 seconds, after it would take 0.99 seconds. In more realistic cases such as validations of large number of objects as part of factories I've seen 15-20% differences

This code should be able to accept aliases and non-aliases and preserve the mapping, and save time on repeated access. It is very possible I've missed some cache busters, please help me identify cases like that, as well as confirm my performance estimations, or if you can help with an even faster solution.