module ActiveRecord
  module Associations
    # Implements the details of eager loading of Active Record associations.
    #
    # Note that 'eager loading' and 'preloading' are actually the same thing.
    # However, there are two different eager loading strategies.
    #
    # The first one is by using table joins. This was only strategy available
    # prior to Rails 2.1. Suppose that you have an Author model with columns
    # 'name' and 'age', and a Book model with columns 'name' and 'sales'. Using
    # this strategy, Active Record would try to retrieve all data for an author
    # and all of its books via a single query:
    #
    #   SELECT * FROM authors
    #   LEFT OUTER JOIN books ON authors.id = books.id
    #   WHERE authors.name = 'Ken Akamatsu'
    #
    # However, this could result in many rows that contain redundant data. After
    # having received the first row, we already have enough data to instantiate
    # the Author object. In all subsequent rows, only the data for the joined
    # 'books' table is useful; the joined 'authors' data is just redundant, and
    # processing this redundant data takes memory and CPU time. The problem
    # quickly becomes worse and worse as the level of eager loading increases
    # (i.e. if Active Record is to eager load the associations' associations as
    # well).
    #
    # The second strategy is to use multiple database queries, one for each
    # level of association. Since Rails 2.1, this is the default strategy. In
    # situations where a table join is necessary (e.g. when the +:conditions+
    # option references an association's column), it will fallback to the table
    # join strategy.
    class Preloader #:nodoc:
      extend ActiveSupport::Autoload

      eager_autoload do
        autoload :Association,           'active_record/associations/preloader/association'
        autoload :SingularAssociation,   'active_record/associations/preloader/singular_association'
        autoload :CollectionAssociation, 'active_record/associations/preloader/collection_association'
        autoload :ThroughAssociation,    'active_record/associations/preloader/through_association'

        autoload :HasMany,             'active_record/associations/preloader/has_many'
        autoload :HasManyThrough,      'active_record/associations/preloader/has_many_through'
        autoload :HasOne,              'active_record/associations/preloader/has_one'
        autoload :HasOneThrough,       'active_record/associations/preloader/has_one_through'
        autoload :HasAndBelongsToMany, 'active_record/associations/preloader/has_and_belongs_to_many'
        autoload :BelongsTo,           'active_record/associations/preloader/belongs_to'
      end

      attr_reader :records, :associations, :options, :model

      # Eager loads the named associations for the given Active Record record(s).
      #
      # In this description, 'association name' shall refer to the name passed
      # to an association creation method. For example, a model that specifies
      # <tt>belongs_to :author</tt>, <tt>has_many :buyers</tt> has association
      # names +:author+ and +:buyers+.
      #
      # == Parameters
      # +records+ is an array of ActiveRecord::Base. This array needs not be flat,
      # i.e. +records+ itself may also contain arrays of records. In any case,
      # +preload_associations+ will preload the all associations records by
      # flattening +records+.
      #
      # +associations+ specifies one or more associations that you want to
      # preload. It may be:
      # - a Symbol or a String which specifies a single association name. For
      #   example, specifying +:books+ allows this method to preload all books
      #   for an Author.
      # - an Array which specifies multiple association names. This array
      #   is processed recursively. For example, specifying <tt>[:avatar, :books]</tt>
      #   allows this method to preload an author's avatar as well as all of his
      #   books.
      # - a Hash which specifies multiple association names, as well as
      #   association names for the to-be-preloaded association objects. For
      #   example, specifying <tt>{ :author => :avatar }</tt> will preload a
      #   book's author, as well as that author's avatar.
      #
      # +:associations+ has the same format as the +:include+ option for
      # <tt>ActiveRecord::Base.find</tt>. So +associations+ could look like this:
      #
      #   :books
      #   [ :books, :author ]
      #   { :author => :avatar }
      #   [ :books, { :author => :avatar } ]
      #
      # +options+ contains options that will be passed to ActiveRecord::Base#find
      # (which is called under the hood for preloading records). But it is passed
      # only one level deep in the +associations+ argument, i.e. it's not passed
      # to the child associations when +associations+ is a Hash.
      def initialize(records, associations, options = {})
        @records      = Array.wrap(records).compact.uniq
        @associations = Array.wrap(associations)
        @options      = options
      end

      def run
        unless records.empty?
          associations.each { |association| preload(association) }
        end
      end

      private

      def preload(association)
        case association
        when Hash
          preload_hash(association)
        when String, Symbol
          preload_one(association.to_sym)
        else
          raise ArgumentError, "#{association.inspect} was not recognised for preload"
        end
      end

      def preload_hash(association)
        association.each do |parent, child|
          Preloader.new(records, parent, options).run
          Preloader.new(records.map { |record| record.send(parent) }.flatten, child).run
        end
      end

      # Not all records have the same class, so group then preload group on the reflection
      # itself so that if various subclass share the same association then we do not split
      # them unnecessarily
      #
      # Additionally, polymorphic belongs_to associations can have multiple associated
      # classes, depending on the polymorphic_type field. So we group by the classes as
      # well.
      def preload_one(association)
        grouped_records(association).each do |reflection, klasses|
          klasses.each do |klass, records|
            preloader_for(reflection).new(klass, records, reflection, options).run
          end
        end
      end

      def grouped_records(association)
        Hash[
          records_by_reflection(association).map do |reflection, records|
            [reflection, records.group_by { |record| association_klass(reflection, record) }]
          end
        ]
      end

      def records_by_reflection(association)
        records.group_by do |record|
          reflection = record.class.reflections[association]

          unless reflection
            raise ActiveRecord::ConfigurationError, "Association named '#{association}' was not found; " \
                                                    "perhaps you misspelled it?"
          end

          reflection
        end
      end

      def association_klass(reflection, record)
        if reflection.macro == :belongs_to && reflection.options[:polymorphic]
          klass = record.send(reflection.foreign_type)
          klass && klass.constantize
        else
          reflection.klass
        end
      end

      def preloader_for(reflection)
        case reflection.macro
        when :has_many
          reflection.options[:through] ? HasManyThrough : HasMany
        when :has_one
          reflection.options[:through] ? HasOneThrough : HasOne
        when :has_and_belongs_to_many
          HasAndBelongsToMany
        when :belongs_to
          BelongsTo
        end
      end
    end
  end
end
