order_query finds next / previous Active Record(s) in one query

order_query

order_query gives you next or previous records relative to the current one efficiently.

For example, you have a list of items, sorted by priority. You have 10,000 items! If you are showing the user a single item, how do you provide buttons for the user to see the previous item or the next item?

You could pass the item's position to the item page and use OFFSET in your SQL query. The downside of this, apart from having to pass a number that may change, is that the database cannot jump to the offset; it has to read every record until it reaches, say, the 9001st record. This is slow. Here is where order_query comes in!

order_query uses the same ORDER BY query, but also includes a WHERE clause that excludes records before (for next) or after (for prev) the current one.

    class Post < ActiveRecord::Base       include OrderQuery       order_query :order_list, [         [:pinned, [true, false]],         [:published_at, :desc],         [:id, :desc]       ]     end

Now to use it:

    # get the order object, scope default: Post.all     p = Post.find(31).order_list(scope) #=> OrderQuery::RelativeOrder<...>     p.before #=> ActiveRecord::Relation<...>     p.previous #=> Post<...>     p.position #=> 5     p.next #=> Post<...>     p.after #=> ActiveRecord::Relation<...>

The magic is in the generated WHERE clause that excludes all the records after / before the current one, such as:

    -- post: pinned=true published_at='2014-03-21 15:01:35.064096' id=9     SELECT "posts".* FROM "posts" WHERE       ("posts"."pinned" = 'f' OR        "posts"."pinned" = 't' AND (           "posts"."published_at" < '2014-03-21 15:01:35.064096' OR           "posts"."published_at" = '2014-03-21 15:01:35.064096' AND "posts"."id" < 9))     ORDER BY       "posts"."pinned"='t' DESC,       "posts"."pinned"='f' DESC, "posts"."published_at" DESC, "posts"."id" DESC     LIMIT 1

See more on Github: https://github.com/glebm/order_query