Difference between checking presence of child association between active record relation and ransack

relationship_example

# code set up
class Big < ApplicationRecord
  has_many :littles
end

class Little < ApplicationRecord
  belongs_to :big # foreign_key is the default littles.big_id
end

# in console
Big.count # => 2
Little.count # => 1

# Using rails active relationship queries
## 1a) count of bigs that don't have a little
Big.left_outer_joins(:little).where(littles: { big_id: nil }).count
# => 1
Big.left_outer_joins(:little).where(littles: { big_id: nil }).count.to_sql
# => SELECT bigs.* FROM bigs LEFT OUTER JOIN little ON littles.big_id = bigs.id WHERE littles.big_id IS NULL

## 1b) count of bigs that do have a little
Big.left_outer_joins(:little).where.not(littles: { big_id: nil }).count
# => 1
Big.left_outer_joins(:little).where.not(littles: { big_id: nil }).to_sql
# => SELECT bigs.* FROM bigs LEFT OUTER JOIN little ON littles.big_id = bigs.id WHERE littles.big_id IS NOT NULL


# Using ransack present predicate
## 2a) count of bigs that don't have a little
Big.ransack(little_big_id_present: false).result.count
# => 1
Big.ransack(little_big_id_present: false).result.sql
# => SELECT bigs.* FROM bigs LEFT OUTER JOIN littles ON littles.big_id = big.id WHERE (littles.big_id IS NULL OR littles.big_id != NULL)
# N.B this SQL evaluates to the same as 1a due to 'OR' in where clause.

## 2b) (attempting to query) count of bigs that have a little
Big.ransack(little_big_id_present: true).result.count
# => 0 
Big.ransack(little_big_id_present: true).result.to_sql
# => SELECT bigs.* FROM bigs LEFT OUTER JOIN littles ON littles.big_id = big.id WHERE (littles.big_id IS NOT NULL AND littles.big_id != NULL)