case.rb

# CASE implementation for Arel
# Based on: https://github.com/take-five/acts_as_ordered_tree/blob/master/lib/acts_as_ordered_tree/transaction/dsl.rb
# Extended with support for CASE expr WHEN expr[, expr, ...] by Felix Buenemann

module Arel
  module Nodes
    # Case node
    #
    # @example
    #   switch.when(table[:x].gt(1), table[:y]).else(table[:z])
    #   # CASE WHEN "table"."x" > 1 THEN "table"."y" ELSE "table"."z" END
    #   switch.when(table[:x].gt(1)).then(table[:y]).else(table[:z])
    #   # CASE "table"."x" WHEN 1 THEN "table"."y" ELSE "table"."z" END
    #   switch.case(table[:x]).when(1).then(table[:y]).else(table[:z])
    class Case < Arel::Nodes::Node
      include Arel::Expression
      include Arel::Predications
      include Arel::AliasPredication

      attr_reader :expression, :conditions, :default

      def initialize(expression = nil)
        @expression = expression
        @conditions = []
        @default = nil
      end

      def case(expression)
        @expression = expression
        self
      end

      def when(condition, expression = nil)
        @conditions << When.new(condition, expression)
        self
      end

      def then(expression)
        @conditions.last.right = expression
        self
      end

      def else(expression)
        @default = Else.new(expression)
        self
      end
    end

    class When < Arel::Nodes::Binary
    end

    class Else < Arel::Nodes::Unary
    end
  end

  module Visitors
    class ToSql < Arel::Visitors::Visitor
      private
      def visit_Arel_Nodes_Case o, *a
        expression = o.expression && visit(o.expression, *a)
        conditions = o.conditions.map { |x| visit x, *a }.join(' ')
        default = o.default && visit(o.default, *a)

        "CASE #{[expression, conditions, default].compact.join(' ')} END"
      end

      def visit_Arel_Nodes_When o, *a
        "WHEN #{visit o.left} THEN #{visit o.right, *a}"
      end

      def visit_Arel_Nodes_Else o, *a
        "ELSE #{visit o.expr, *a}"
      end
    end

    class DepthFirst < Arel::Visitors::Visitor
      def visit_Arel_Nodes_Case o, *a
        visit o.conditions, *a
        visit o.default, *a
      end
      alias :visit_Arel_Nodes_When :binary
      alias :visit_Arel_Nodes_Else :unary
    end
  end
end