Identify item by key object instead of key string representation

[DouweM]

Imagine I have the following code:

class File < ActiveRecord::Base
  belongs_to :project

  def self.lazy(project, file_name)
    BatchLoader.for(file_name).batch(key: project) do |file_names, loader, args|
      args[:key].files.where(name: file_names).each do |file|
        loader.call(file.name, file)
      end
    end
  end
end

project = Project.first
same_project = Project.find(project.id)

file_a = File.lazy(project, "file_a")
file_b = File.lazy(same_project, "file_b")

Right now, calling file_a.name followed by file_b.name will result in two queries being executed, because the string representations of project and same_project are different (they contain the actual Ruby object_id), which means the block_hash_keys will be different as well.

With the change in this PR, calling file_a.name followed by file_b.name will result in only one batch query being executed, since project and same_project have the same #hash and implement #eql? to regard each other as identical, which means the block_hash_keys will be identical for hash keying purposes.

In this specific example, I could also have used key: project.id and called File.where(project_id: args[:key], name: file_names) instead of args[:key].files.where(name: file_names), but that is not an option in some of the batch loaders in the GitLab code base.

In general, I think it makes sense for BatchLoader to follow the same equality rules here that Ruby usually would if the user has specifically overwritten #hash and #eql?.

[DouweM]

I decided to reverse key and block.source_location, since key is used to subcategorize.

[coveralls]

Coverage increased (+0.03%) to 99.759% when pulling b8e2af3 on DouweM:patch-1 into 1f0048b on exAspArk:master.

[DouweM]

@exAspArk Please have a look at this contribution!

[exAspArk]

@DouweM hey, thank you! I'll take a look tomorrow.

[exAspArk]

@DouweM LGTM! Just one thing, could you please write a test which uses instance objects as a key?

[DouweM]

@exAspArk Test added!

[exAspArk]

@DouweM awesome, thanks a lot for your contribution! 🙌

I released your changes in 1.2.2.