Why does single `=` work in `if` statement?

Question

This code is provided as an example in for use with devise and OmniAuth, it works in my project.

class User < ActiveRecord::Base
  def self.new_with_session(params, session)
    super.tap do |user|
      if data = session["devise.facebook_data"] && session["devise.facebook_data"]["extra"]["raw_info"]
        user.email = data["email"] if user.email.blank?
      end
    end
  end
end

I don't know why it's a single equals sign as apposed to a double equals sign, which I thought was necessary for if-statements. My IDE "intelliJ IDEA" agrees with my concerns.

Answer 1

The only necessary thing for an if statement to be valid is a boolean expression. In this case, since = returns the result of the assignment, what's actually being tested is the falsiness of session["devise.facebook_data"].

IntelliJ has a good point to lodge a complaint about code like this, as it's difficult to read without knowing a thing or two about Ruby. A recommendation would be to move that to an explicit assignment statement instead. This has the added benefit of DRYing up a reference to it twice.

class User < ActiveRecord::Base
  def self.new_with_session(params, session)
    super.tap do |user|
      data = session["devise.facebook_data"]
      if data && data["extra"]["raw_info"]
        user.email = data["email"] if user.email.blank?
      end
    end
  end
end