Question

Suppose I have a non-empty array ids of Thing object ids and I want to find the corresponding objects using things = Thing.find_all_by_id(ids). My impression is that things will not necessarily have an ordering analogous to that of ids.

1. Is my impression correct?

2. If so, what can I used instead of find_all_by_id that preserves order and doesn’t hit the database unnecessarily many times?

Answer 1

1. Yes
2. Use Array#sort

Check it out:

Thing.where(:id => ids).sort! {|a, b| ids.index(a.id) <=> ids.index(b.id)}

where(:id => ids) will generate a query using an IN(). Then the sort! method will iterate through the query results and compare the positions of the id’s in the ids array.