# encoding: utf-8
module Mongoid
  module Relations
    module Eager
      # Base class for eager load preload functions.
      #
      # @since 4.0.0
      class Base

        # Instantiate the eager load class.
        #
        # @example Create the new belongs to eager load preloader.
        #   BelongsTo.new(relations_metadata, parent_docs)
        #
        # @param [ Array<Metadata> ] Relations to eager load
        # @param [ Array<Document> ] Documents to preload the relations
        #
        # @return [ Base ] The eager load preloader
        #
        # @since 4.0.0
        def initialize(associations, docs)
          @associations = associations
          @docs = docs
          @grouped_docs = {}
        end

        # Shift the current relation metadata
        #
        # @example Shift the current metadata.
        #   loader.shift_metadata
        #
        # @return [ Object ] The relation metadata object.
        #
        # @since 4.0.0
        def shift_metadata
          @metadata = @associations.shift
        end

        # Run the preloader.
        #
        # @example Preload the relations into the documents.
        #   loader.run
        #
        # @return [ Array ] The list of documents given.
        #
        # @since 4.0.0
        def run
          while shift_metadata
            preload
          end
          @docs
        end

        # Preload the current relation.
        #
        # This method should be implemented in the subclass
        #
        # @example Preload the current relation into the documents.
        #   loader.preload
        #
        # @since 4.0.0
        def preload
          raise NotImplementedError
        end

        # Run the preloader.
        #
        # @example Iterate over the documents loaded for the current relation
        #   loader.each_loaded_document { |doc| }
        #
        # @since 4.0.0
        def each_loaded_document
          criteria = @metadata.klass.any_in(key => keys_from_docs)
          criteria.inclusions = criteria.inclusions - [ @metadata ]
          criteria.each do |doc|
            yield doc
          end
        end

        # Set the pre-loaded document into its parent.
        #
        # @example Set docs into parent with pk = "foo"
        #   loader.set_on_parent("foo", docs)
        #
        # @param [ ObjectId ] parent`s id
        # @param [ Document, Array ] element to push into the parent
        #
        # @since 4.0.0
        def set_on_parent(id, element)
          grouped_docs[id].each do |d|
            set_relation(d, element)
          end
        end

        # Return a hash with the current documents grouped by key.
        #
        # @example Return a hash with the current documents grouped by key.
        #   loader.grouped_docs
        #
        # @return [ Hash ] hash with groupd documents.
        #
        # @since 4.0.0
        def grouped_docs
          @grouped_docs[@metadata.name] ||= @docs.group_by do |doc|
            doc.send(group_by_key)
          end
        end

        # Group the documents and return the keys
        #
        # @example
        #   loader.keys_from_docs
        #
        # @return [ Array ] keys, ids
        #
        # @since 4.0.0
        def keys_from_docs
          grouped_docs.keys
        end

        # Return the key to group the current documents.
        #
        # This method should be implemented in the subclass
        #
        # @example Return the key for group
        #   loader.group_by_key
        #
        # @return [ Symbol ] Key to group by the current documents.
        #
        # @since 4.0.0
        def group_by_key
          raise NotImplementedError
        end

        protected
        # Set the pre-loaded document into its parent.
        #
        # @example Set docs into parent using the current relation name.
        #   loader.set_relation(doc, docs)
        #
        # @param [ Document ] document
        # @param [ Document, Array ] element to set into the parent
        #
        # @since 4.0.0
        def set_relation(doc, element)
          doc.set_relation(@metadata.name, element)
        end
      end
    end
  end
end