ActiveRecord includes. Specify included columns

Question

I have model Profile. Profile has_one User. User model has field email. When I call

Profile.some_scope.includes(:user) 

it calls

SELECT users.* FROM users WHERE users.id IN (some ids) 

But my User model has many fields that I am not using in rendering. Is it possible to load only emails from users? So, SQL should look like

SELECT users.email FROM users WHERE users.id IN (some ids) 

Answer 1

Rails doesn't have the facility to pass the options for include query. But we can pass these params with the association declaration under the model.

For your scenario, you need to create a new association with users model under the profile model, like below

belongs_to :user_only_fetch_email, :select => "users.id, users.email", :class_name => "User" 

I just created one more association but it points to User model only. So your query will be,

Profile.includes(:user_only_fetch_email) 

or

Profile.includes(:user_only_fetch_email).find(some_profile_ids) 

Answer 2

If you want to select specific attributes, you should use joins rather than includes.

From this asciicast:

the include option doesn’t really work with the select option as we don’t have control over how the first part of the SELECT statement is generated. If you need control over the fields in the SELECT then you should use joins over include.

Using joins:

Profile.some_scope.joins(:users).select("users.email")