Get all associated data for all records of a model type in Rails?

Question

How do I meld together getting all of the associated data for all of the records of a given model?

I have the following models:

User --N:1--> Reservation <--1:N-- Concert

So pseudo-code:

Reservation belongs_to User
Reservation belongs_to Concert
User has_many Reservations
User has_many Concerts through Reservations
Concert has_many Reservations
Concert has_many Users through Reservations

How do I make a single big array of everything?

• I can get all my Reservations via Reservation.all
• I can get the User for a particular Reservation via Reservation.find(25).user
• I can get the Concert for a particular Reservation via Reservation.find(25).concert

But how do I get it for all of them? If I do

Reservation.all.each do |res|
   res.user.name+","+res.concert.name+","+res.concert.date # etc.
end

Then it will do two new database queries for each reservation as it loops through. For 10 records, it might not matter, but for thousands, it can be very painful. Add to it other associations (e.g. Concert belongs_to venue, User has_one email, etc.)...

Is there any way to say, "Get all of the reservations and the following attached info" so it loads in a single SQL query?

Answer 1

What you're trying to accomplish is called eager loading, and can be done using includes in ActiveRecord. See below:

N + 1 queries problem

Active Record lets you specify in advance all the associations that are going to be loaded. This is possible by specifying the includes method of the Model.find call. With includes, Active Record ensures that all of the specified associations are loaded using the minimum possible number of queries.

http://guides.rubyonrails.org/active_record_querying.html#eager-loading-associations

In your example you could use the following:

Reservation.all.includes(:user, :concert)

---

It is also a good idea to specify the :inverse_of option for your :belongs_to relations. This optimizes object loading and makes sure that cross-referencing a model will point back to the same object in memory, i.e.:

@user == @user.reservations.first.user # true

More information available here:

If you are using a belongs_to on the join model, it is a good idea to set the :inverse_of option on the belongs_to ...

http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html

In your example:

# app/models/reservation.rb

belongs_to :user, :inverse_of => :reservations
belongs_to :concert, :inverse_of => :reservations