module ActiveRecord

  # Statement cache is used to cache a single statement in order to avoid creating the AST again.
  # Initializing the cache is done by passing the statement in the initialization block:
  #
  #   cache = ActiveRecord::StatementCache.new do
  #     Book.where(name: "my book").limit(100)
  #   end
  #
  # The cached statement is executed by using the +execute+ method:
  #
  #   cache.execute
  #
  # The relation returned by the block is cached, and for each +execute+ call the cached relation gets duped.
  # Database is queried when +to_a+ is called on the relation.
  class StatementCache
    def initialize(block = Proc.new)
      @mutex    = Mutex.new
      @relation = nil
      @block    = block
    end

    def execute(*vals)
      rel = relation vals
      @mutex.synchronize do
        rel.set_binds vals
        rel.to_a
      end
    end

    private
    def relation(values)
      @relation || @mutex.synchronize {
        @block.call(*values)
      }
    end
  end
end