require 'active_support/core_ext/hash/keys'

module ActiveRecord
  class Relation
    class HashMerger # :nodoc:
      attr_reader :relation, :hash

      def initialize(relation, hash)
        hash.assert_valid_keys(*Relation::VALUE_METHODS)

        @relation = relation
        @hash     = hash
      end

      def merge #:nodoc:
        Merger.new(relation, other).merge
      end

      # Applying values to a relation has some side effects. E.g.
      # interpolation might take place for where values. So we should
      # build a relation to merge in rather than directly merging
      # the values.
      def other
        other = Relation.create(relation.klass, relation.table, relation.predicate_builder)
        hash.each { |k, v|
          if k == :joins
            if Hash === v
              other.joins!(v)
            else
              other.joins!(*v)
            end
          elsif k == :select
            other._select!(v)
          else
            other.send("#{k}!", v)
          end
        }
        other
      end
    end

    class Merger # :nodoc:
      attr_reader :relation, :values, :other

      def initialize(relation, other)
        @relation = relation
        @values   = other.values
        @other    = other
      end

      NORMAL_VALUES = Relation::VALUE_METHODS -
                      Relation::CLAUSE_METHODS -
                      [:includes, :preload, :joins, :order, :reverse_order, :lock, :create_with, :reordering] # :nodoc:

      def normal_values
        NORMAL_VALUES
      end

      def merge
        normal_values.each do |name|
          value = values[name]
          # The unless clause is here mostly for performance reasons (since the `send` call might be moderately
          # expensive), most of the time the value is going to be `nil` or `.blank?`, the only catch is that
          # `false.blank?` returns `true`, so there needs to be an extra check so that explicit `false` values
          # don't fall through the cracks.
          unless value.nil? || (value.blank? && false != value)
            if name == :select
              relation._select!(*value)
            else
              relation.send("#{name}!", *value)
            end
          end
        end

        merge_multi_values
        merge_single_values
        merge_clauses
        merge_preloads
        merge_joins

        relation
      end

      private

      def merge_preloads
        return if other.preload_values.empty? && other.includes_values.empty?

        if other.klass == relation.klass
          relation.preload!(*other.preload_values) unless other.preload_values.empty?
          relation.includes!(other.includes_values) unless other.includes_values.empty?
        else
          reflection = relation.klass.reflect_on_all_associations.find do |r|
            r.class_name == other.klass.name
          end || return

          unless other.preload_values.empty?
            relation.preload! reflection.name => other.preload_values
          end

          unless other.includes_values.empty?
            relation.includes! reflection.name => other.includes_values
          end
        end
      end

      def merge_joins
        return if other.joins_values.blank?

        if other.klass == relation.klass
          relation.joins!(*other.joins_values)
        else
          joins_dependency, rest = other.joins_values.partition do |join|
            case join
            when Hash, Symbol, Array
              true
            else
              false
            end
          end

          join_dependency = ActiveRecord::Associations::JoinDependency.new(other.klass,
                                                                           joins_dependency,
                                                                           [])
          relation.joins! rest

          @relation = relation.joins join_dependency
        end
      end

      def merge_multi_values
        if other.reordering_value
          # override any order specified in the original relation
          relation.reorder! other.order_values
        elsif other.order_values
          # merge in order_values from relation
          relation.order! other.order_values
        end

        relation.extend(*other.extending_values) unless other.extending_values.blank?
      end

      def merge_single_values
        relation.lock_value ||= other.lock_value

        unless other.create_with_value.blank?
          relation.create_with_value = (relation.create_with_value || {}).merge(other.create_with_value)
        end
      end

      CLAUSE_METHOD_NAMES = CLAUSE_METHODS.map do |name|
        ["#{name}_clause", "#{name}_clause="]
      end

      def merge_clauses
        CLAUSE_METHOD_NAMES.each do |(reader, writer)|
          clause = relation.send(reader)
          other_clause = other.send(reader)
          relation.send(writer, clause.merge(other_clause))
        end
      end
    end
  end
end
