Question

I recently stumbled upon this blog post, which states that Array#uniq behaves differently on arrays of ActiveRecord objects, but neither gives a specific example for it, nor does it tell the reason for this.

I would like to see a reproducible example of this the different behavior and an explanation for it.

edit:
I’m talking about Array#uniq, not ActiveRecord::Relation#uniq. If the latter one is what the above mentioned blog post actually means, then my question has been already been answered by loz (see below).

Answer 1

Active record ‘arrays’ are not arrays they are a different. So (asside from .all, which explicitly returns an array) if you use associations or similar you get different behavior, as lazy loading is in operation:

post = Post.find(1)
post.comments
=> Array[Comment(:1), Comment(:2)]... etc

Looks like an array, but

post.comments.class
=> ActiveRecord::Relation

So performing Uniq operates on the relationship, which adds more constraints to the query which will eventually be run on the database when the records are required:

post.comments.uniq

This will perform the uniq by doing a DISTINCT or UNIQUE query clause in SQL, rather than doing uniq in ruby..

Uniq details are found here: rails uniq documentation, and you can see about what returns ActiveRecord::Relation in active record here