Rails: uniq vs. distinct

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

Rails 5.1 has removed the uniq method from Activerecord Relation and added distinct method...

• If you use uniq with query it will just convert the Activerecord Relaction to Array class...
• You can not have Query chain if you added uniq there....(i.e you can not do User.active.uniq.subscribed it will throw error undefined method subscribed for Array )
• If your DB is large and you want to fetch only required distinct entries its good to use distinct method with Activerecord Relation query...