Detect n+1 count queries from ActiveRecord::Associations::CollectionProxy#count

[quintinm-dev]

Just gauging interest here, as there may be a lot of considerations that I've missed. Generally people should use size, but if they use count the n+1 goes undetected. If you do want this in, we could backport it to older Rails versions.

Minimal example:

class Blog < ApplicationRecord
  has_many :comments, dependent: :destroy
end

MAX = 5

# currently undetected
Blog.all.filter { |b| b.comments.count < MAX }

# gets detected as counter cache
Blog.all.filter { |b| b.comments.size < MAX }

# gets detected as n+1
Blog.all.filter { |b| b.comments.length < MAX }

# gets detected as n+1
Blog.all.filter { |b| b.comments.to_a.count < MAX }

[flyerhzm]

@quintinm-dev yes, we should detect n+1 for count method, I'll merge it if you can backport it to older Rails versions.

[quintinm-dev]

@flyerhzm done, should be ready for a review now 😎

[flyerhzm]

@quintinm-dev it looks good, thanks