This error is raised when trying to eager load a polymorphic association using a JOIN. Eager loading polymorphic associations is only possible with ActiveRecord::Relation#preload.

Methods
N
Class Public methods
new(reflection = nil)
# File activerecord/lib/active_record/associations.rb, line 181
def initialize(reflection = nil)
  if reflection
    super("Cannot eagerly load the polymorphic association #{reflection.name.inspect}")
  else
    super("Eager load polymorphic error.")
  end
end