Don't apply sorting to collection until after scoping

[agrobbin]

Consider sorting that requires a JOIN as part of the SQL query (or really anything that was raised in #3085):

ActiveAdmin.register Foo do
  config.order_clause = FooOrderClause

  index do
    column :latest_bar, sortable: 'bars.created_at' do
      ...
    end
  end
end

class FooOrderClause < ActiveAdmin::OrderClause
  def apply(chain)
    case table_column
    when 'bars.created_at'
      chain.joins(:bars).select(table_column)
    else
      super
    end
  end
end

Oftentimes, including that JOIN is not a huge deal in terms of performance, however, there have been situations where that table/column can increase the time it takes for a query to run by a couple hundred milliseconds, for various reasons.

Increasing the time to load the page slightly (0.1s) is not a big deal, in an admin, but imagine if you had 7-10 different scopes on that page as well? Because scopes leverage collection_before_scope, they will proceed to include the JOIN in their query, even though it is only necessary to do sorting of a collection that is not actually being sorted, only counted.

Applying sorting of the collection after scoping, instead of before, should have no bearing on the end-result, while ensuring that any custom sorting logic is not needlessly applied to all scope count queries.

[deivid-rodriguez]

This makes sense to me!

[Fivell]

thanks