auto-explain-on-activerecord.rb

ActiveSupport::Notifications.subscribe("sql.active_record") do |name, start, finish, id, payload|
  if payload[:sql] =~ /\A(?!EXPLAIN).*SELECT.+FROM/
    ActiveRecord::Base.logger.warn(
      ActiveRecord::Base.connection.explain(payload[:sql], payload[:binds])
    )
  end
end

preview.png

For instance, you can inspect SQL consistency with testing like this:

explain_enabled = false

ActiveSupport::Notifications.subscribe("sql.active_record") do |name, start, finish, id, payload|
  if explain_enabled && payload[:sql] =~ /\A(?!EXPLAIN).*SELECT.+FROM/
    ActiveRecord::Base.logger.warn(
      ActiveRecord::Base.connection.explain(payload[:sql], payload[:binds])
    )
  end
end

shared_context 'with explain', :explain do
  before do
    explain_enabled = true
  end

  after do
    explain_enabled = false
  end
end