Avoid scopes that use find_by

post.rb

# BAD
# This scope will return an ActiveRecord::Relation if the first query returns nil
class Post < ApplicationRecord
  scope :featured, -> { find_by(featured: true) }
end

# => Post.featured
# => Post Load (0.1ms)  SELECT "posts".* FROM "posts" WHERE "posts"."featured" = ? LIMIT ?  [["featured", 1], ["LIMIT", 1]]
# => Post Load (0.1ms)  SELECT "posts".* FROM "posts" /* loading for inspect */ LIMIT ?  [["LIMIT", 11]]
# => #<ActiveRecord::Relation [...]>

# GOOD
# This class method will respect the find_by query
class Post < ApplicationRecord    
  def self.featured 
    find_by(featured: true)
  end
end

# => Post.featured
# => Post Load (0.1ms)  SELECT "posts".* FROM "posts" WHERE "posts"."featured" = ? LIMIT ?  [["featured", 1], ["LIMIT", 1]]
# => nil

https://stackoverflow.com/a/31329956