module ActiveRecord::SpawnMethods

This is mainly intended for sharing common conditions between multiple associations.

# => Post.where(published: true).joins(:comments)
Post.where(published: true).merge(-> { joins(:comments) })

Procs will be evaluated by merge:

# (This is just an example. You'd probably want to do this with a single query!)
# Returns the intersection of all published posts with the 5 most recently created posts.
Post.where(published: true).merge(recent_posts)
recent_posts = Post.order('created_at DESC').first(5)

# Performs a single join query with both where conditions.
Post.where(published: true).joins(:comments).merge( Comment.where(spam: false) )

Returns an array representing the intersection of the resulting records with other, if other is an array.
Merges in the conditions from other, if other is an ActiveRecord::Relation.

def merge(other, *rest)
  if other.is_a?(Array)
    records & other
  elsif other
    spawn.merge!(other, *rest)
  else
    raise ArgumentError, "invalid argument: #{other.inspect}."
  end
end