如何以最有效的方式获取所有嵌套记录。 轨道

Question

Say a user has many trees and trees have many apples . Say a method takes in a user's id. I eventually want the data to look something like this:

[
   { tree_id1: [apple1, apple2, apple3] },
   { tree_id2: [apple4, apple5, apple6] },
   { tree_id3: [apple9, apple8, apple7] }
]

So I'll eventually have to iterate through the user's trees and associate the apples with it. But I don't want to have to hit the database to retrieve the tree's apples every time. That seems like it'd be an N+1 (get the user's trees, then all of the tree's apples).

What can I do to retrieve the necessary records efficiently so that I can organize the records in the data format that I need?

When you make N+1 queries... why is it so bad? Where is the database in relation to app code when you say are on heroku? Is it a network call?

Answer 1

You can make use of eager loading

users = User.includes(trees: :apples)

Instead of firing N + 1 queries it will fire only three queries

1) Select * from users
2) Select * from trees where user_id IN [ids_of_users_fetch_from_above_query]
2) Select * from apples where apple_id IN [ids_of_trees_fetch_from_above_query]

So now when you write

users.first.apples

No query is fired as the records are already eager loaded when you fetched users

And It is the feature of Rails

Answer 2

You can overcome the N + 1 by applying eager loading

you can do it by using includes like the blow:

User.includes(trees: :apples)