Unexpected behavior of .empty? with ActiveRecord/ActiveSupport and .present?

[csm8118]

Hi,

I ran into unexpected behavior based on the implementation of .empty? as defined in lib/mongoid/findable.rb:

# Returns true if count is zero
#
# @example Are there no saved documents for this model?
#   Person.empty?
#
# @return [ true | false ] If the collection is empty.
def empty?
  count == 0
end

For an example, say you have this class:

class Cat
  include Mongoid::Document
end

I am now able to call Cat.empty? which executes a query against mongodb to check the collection size.

MONGODB | 0.0.0.0:27017 req:218 conn:1:1 sconn:5 | checkr-development.aggregate | STARTED | {"aggregate"=>"cats", "pipeline"=>[{"$match"=>{}}, {"$group"=>{"_id"=>1, "n"=>{"$sum"=>1}}}], "cursor"=>{}, "$db"=>"dev", "lsid"=>{"id"=><BSON::Binary:0x344940 type=uuid data=0xeeb08e2891d34974...>}}

Consider now how ActiveSupport defines the present? method:

def present?
  !blank?
end

And now the blank? method:

def blank?
  respond_to?(:empty?) ? !!empty? : false
end

Calling Cat.present? ultimately calls Cat.empty? and queries mongodb.

Contrast this with a relational database-backed ActiveRecord that does not define empty? at the class level:

class Dog < ActiveRecord::Base
end

> Dog.respond_to?(:empty?)
=> false

When this functionality was introduced in mongodb many years ago someone made a similar observation. It feels very non-obvious that calling present? on a class would make a query to a database. Thoughts on deprecating this functionality?

Thank you for your time and consideration.