how to retrieve an aray of only specific attributes in rails 2.3

I have a scenario where I need to retrieve an array of specific
attributes from a table. I have a TransportAttendanceBlock table. In the
table I have blocked(boolean) and blocked_date attributes. From a
controller I am fetching this table by passing params of date and
boolean value.

    Controller code:
        @blocked_date =
      named_scope :by_blocked_date_and_blocked, lambda{|date,status|

Here in the controller I am getting the entire objects of
TransportAttendanceBlock table.
But I need to just pass any array of dates only in the @blocked_date
variable as json.
So how do I extract only blocked_date attributes and assign it to
@blocked_date variable. Please help. I am using rails2.3 and ruby 1.8.7