Trouble on eager loading and using the 'where' method

问题

I am running Ruby on Rails 3.1. I am using the includes method and I would like to understand why when I call the where method on an eager loaded collection it reloads associated objects instead of just finding that object in the eager loaded collection. Since all related objects are already been eager loaded I expect that the where method does not hit the database to reload those objects!

That is, I have the following:

article_categories =
  article
    .categories
    .where(:user_id => @current_user.id)
    .includes(:comments)

If I run

# CASE 1:

article_categories.each do |article_category|
  article_category.comments.map(&:title)
end

eager loading works as expected: the "N + 1 query problem" is avoided. However, the above code returns also comment titles that have not ":user_id == @current_user.id", but I do not want to retrieve those ones at all.

So, since I thought that by using the eager loading I already get all comments, I used a further where(:user_id => @current_user.id) statement like in the following code:

# CASE 2:

article_categories.each do |article_category|
  article_category.comments.where(:user_id => @current_user.id).map(&:title)
end

However, in this case eager loading does not work as expected: the "N + 1 query problem" is not avoided... but comments have ":user_id == @current_user.id"!

I would like to eager loading comments with ":user_id == @current_user.id" (how can I make that by taking advantage from the eager loading?) and I would like to understand why the where statement cancels eager loading effects.

回答1:

In case 1 the where clause only applies to categories. If you also want it to apply to comments you can do this:

article_categories =
  article
    .categories
    .includes(:comments)
    .where('categories.user_id = ? AND comments.user_id = ?', @current_user.id, @current_user.id)

来源：https://stackoverflow.com/questions/9629046/trouble-on-eager-loading-and-using-the-where-method