has_many association. Is this possible??

I have the following schema.

table: user
  t.column :id, :integer

table: friends
  t.column :user_id, :integer #Owner of this friendship
  t.column :friend_id, :integer #FK to User table

table: activity
  t.column :user_id

Basically I want to display all activity from friends that i have

I currently have the following has_many finder...

has_many :friend_activities,
    :class_name => 'Activity',
    :finder_sql => 'select a.* from activities a, friend f where
a.user_id = f.friend_id and f.user_id = #{id}'

Is it even possible to change the above has_many to not user
the :finder_sql but using some combination
of :through, :source … ??? If so can someone please help.