class ActiveRecord::Associations::Preloader::Branch

def build_children(children)
  Array.wrap(children).flat_map { |association|
    Array(association).flat_map { |parent, child|
      Branch.new(
        parent: self,
        association: parent,
        children: child,
        associate_by_default: associate_by_default,
        scope: scope
      )
    }
  }
end

def done?
  root? || (@loaders && @loaders.all?(&:run?))
end

def future_classes
  (immediate_future_classes + children.flat_map(&:future_classes)).uniq
end

def grouped_records
  h = {}
  polymorphic_parent = !root? && parent.polymorphic?
  source_records.each do |record|
    reflection = record.class._reflect_on_association(association)
    next if polymorphic_parent && !reflection || !record.association(association).klass
    (h[reflection] ||= []) << record
  end
  h
end

def immediate_future_classes
  if parent.done?
    loaders.flat_map(&:future_classes).uniq
  else
    likely_reflections.reject(&:polymorphic?).flat_map do |reflection|
      reflection.
        chain.
        map(&:klass)
    end.uniq
  end
end

def initialize(association:, children:, parent:, associate_by_default:, scope:)
  @association = if association
    begin
      @association = association.to_sym
    rescue NoMethodError
      raise ArgumentError, "Association names must be Symbol or String, got: #{association.class.name}"
    end
  end
  @parent = parent
  @scope = scope
  @associate_by_default = associate_by_default
  @children = build_children(children)
  @loaders = nil
end

def likely_reflections
  parent_classes = parent.target_classes
  parent_classes.filter_map do |parent_klass|
    parent_klass._reflect_on_association(@association)
  end
end

def loaders
  @loaders ||=
    grouped_records.flat_map do |reflection, reflection_records|
      preloaders_for_reflection(reflection, reflection_records)
    end
end

def polymorphic?
  return false if root?
  return @polymorphic if defined?(@polymorphic)
  @polymorphic = source_records.any? do |record|
    reflection = record.class._reflect_on_association(association)
    reflection && reflection.options[:polymorphic]
  end
end

def preloaded_records
  @preloaded_records ||= loaders.flat_map(&:preloaded_records)
end

def preloader_for(reflection)
  if reflection.options[:through]
    ThroughAssociation
  else
    Association
  end
end

def preloaders_for_reflection(reflection, reflection_records)
  reflection_records.group_by do |record|
    klass = record.association(association).klass
    if reflection.scope && reflection.scope.arity != 0
      # For instance dependent scopes, the scope is potentially
      # different for each record. To allow this we'll group each
      # object separately into its own preloader
      reflection_scope = reflection.join_scopes(klass.arel_table, klass.predicate_builder, klass, record).inject(&:merge!)
    end
    [klass, reflection_scope]
  end.map do |(rhs_klass, reflection_scope), rs|
    preloader_for(reflection).new(rhs_klass, rs, reflection, scope, reflection_scope, associate_by_default)
  end
end

def root?
  parent.nil?
end

def runnable_loaders
  loaders.flat_map(&:runnable_loaders).reject(&:run?)
end

def source_records
  @parent.preloaded_records
end

def target_classes
  if done?
    preloaded_records.map(&:klass).uniq
  elsif parent.done?
    loaders.map(&:klass).uniq
  else
    likely_reflections.reject(&:polymorphic?).map(&:klass).uniq
  end
end