Active Record eager loading strategies

rails_eager_load.md

N+1 query problem

• ORMs make it easy to a query per loop iteration, which we want to avoid

eager_load

• single query (left outer join)
• can reference the other table's columns in where

preload

• a few queries (one per table)
• typically faster

includes

• acts like preload by default
• falls back on eager_load if you reference the associated tables

joins

• inner join
• precise; allows you to avoid loading extra data
• can use aliases to add new methods to left table's records

Other AR optimizations

• queries can be built without being fired (calling e.g. all, first, count, or each does that)
• results of recent queries are cached
• these don't help with the N+1 problem

Tips

• index on foreign keys
• benchmark with a remote db if that's what prod uses