activerecord - Rails: uniq vs. distinct

Question

Can someone briefly explain to me the difference in use between the methods uniq and distinct?

I've seen both used in similar context, but the difference isnt quite clear to me.

Answer 1

Rails queries acts like arrays, thus .uniq produces the same result as .distinct, but

• .distinct is sql query method
• .uniq is array method

Note: In Rails 5+ Relation#uniq is deprecated and recommended to use Relation#distinct instead. See http://edgeguides.rubyonrails.org/5_0_release_notes.html#active-record-deprecations

Hint:

Using .includes before calling .uniq/.distinct can slow or speed up your app, because

• uniq won't spawn additional sql query
• distinct will do

But both results will be the same

Example:

users = User.includes(:posts)
puts users
# First sql query for includes

users.uniq
# No sql query! (here you speed up you app)
users.distinct
# Second distinct sql query! (here you slow down your app)

This can be useful to make performant application

Hint:

Same works for

• .size vs .count;
• present? vs .exists?
• map vs pluck