assert_db_query_count test helper for ActiveRecord

assert_db_query_count_test_helper_for_active_record.rb

require 'active_record'
ActiveRecord::VERSION::STRING # => "5.1.6"

ActiveRecord::Base.establish_connection adapter: 'sqlite3', database: ':memory:'
ActiveRecord::Schema.define do
  self.verbose = false
  create_table(:users) do |t|
    t.string :name
    t.boolean :activated, default: false
  end
end

class User < ActiveRecord::Base
  # Both of these update the records correctly, but the update_all does it in one query
  # and the each { |u| u.update ... } does it in 7 queries! (swap the bool to see what the failure looks like)
  if true
    def self.activate
      update_all activated: true
    end
  else
    def self.activate
      all.each { |u| u.update activated: true }
    end
  end
end


require 'rspec/autorun'

RSpec.describe 'User.activate' do
  def assert_db_query_count(expected, &block)
    queries = []
    record  = proc { |*, query| queries << query }
    result  = ActiveSupport::Notifications.subscribed record, "sql.active_record", &block
    expect(queries.size).to(
      eq(expected),
      "Expected #{expected} quer#{expected == 1 ? 'y' : 'ies'}, got #{queries.size}:\n" \
        << queries.map(&:inspect).join("\n")
    )
    result
  end

  it 'updates efficiently updates activated to true' do
    %w[yes no yes].map { |n| User.create! name: n }
    assert_db_query_count(1) { User.where(name: 'yes').activate }
    expect(User.order(:id).pluck(:name, :activated)).to eq [
      ['yes', true],
      ['no',  false],
      ['yes', true],
    ]
  end
end

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