Question

I’m currently reading Rails 3 In Action. There is code that I was wondering if someone could explain to me. I’m having a hard time understanding it:

    scope :readable_by, lambda { |user| joins(:permissions).where(permissions: { action: "view", user_id: user.id })}

thanks,
mike

Answer 1

It’s called a Rails scope. It essentially creates a class method called .readable_by(user) that does a SQL join on the permissions table and returns records where the action column value is “view” and the user_id column value equals user.id.

It could be used like so (assuming it’s defined in the Comments model):

readable_comments = Comments.readable_by(current_user)

A simple scope that does nothing is this:

scope :my_scope_name, lambda {}

A scope that accepts a parameter is this:

scope :my_scope_name, lambda { |my_parameter| }

And then the above scope uses some ActiveRecord finder methods, specifically joins and where.