How does ActiveRecord::Relation get added to Rails' models and why does each model have individual Relation class?

Question

Suppose I have a Rails model: class Project < ActiveRecord::Base

In the Rails console:

> Project.all
=> #

        

Answer 1

There are actually two questions you are asking:

1. How does it work?
2. Why is it like that? (What for?)

@arieljuod has already given you some explanations and a link.

However the second question is still unanswered.

There is another similar question exists which I hope will help you find all the answers:

How can an ActiveRecord::Relation object call class methods

It looks like the two questions (by the link and yours one) answer each other )

Take a look at @nikita-shilnikov's answer. Good luck in your investigation!

Answer 2

Check this line of code from ActiveRecord.

https://github.com/rails/rails/blob/f40860800c231ecd1daef6cf6b5a8a8eda76478d/activerecord/lib/active_record/relation/delegation.rb#L23

mangled_name = klass.name.gsub("::", "_")

So, for your questions:

• it get's added on activerecord's base when it extendes the delegation module https://github.com/rails/rails/blob/f40860800c231ecd1daef6cf6b5a8a8eda76478d/activerecord/lib/active_record/base.rb#L290

• it's actually the same class, just something like an alias (not actually an alias, it's a constant with the class as value)

• the class is actually an ActiveRecord::Relation, it's just that the name was changed