Postgres uses wrong index

I've got a query:

EXPLAIN ANALYZE
SELECT CAST(DATE(associationtime) AS text) AS date ,
       cast(SUM(extract(epoch
                        FROM disassociationtime) - extract(epoch
                                                           FROM associationtime)) AS bigint) AS sessionduration,
       cast(SUM(tx) AS bigint)AS tx,
       cast(SUM(rx) AS bigint) AS rx,
       cast(SUM(dataRetries) AS bigint) AS DATA,
       cast(SUM(rtsRetries) AS bigint) AS rts,
       count(*)
FROM SESSION
WHERE ssid_id=42
  AND ap_id=1731
  AND DATE(associationtime)>=DATE('Tue Nov 04 00:00:00 MSK 2014')
  AND DATE(associationtime)<=DATE('Thu Nov 20 00:00:00 MSK 2014')
GROUP BY(DATE(associationtime))
ORDER BY DATE(associationtime);

The output is:

 GroupAggregate  (cost=0.44..17710.66 rows=1 width=32) (actual time=4.501..78.880 rows=17 loops=1)
   ->  Index Scan using session_lim_values_idx on session  (cost=0.44..17538.94 rows=6868 width=32) (actual time=0.074..73.266 rows=7869 loops=1)
         Index Cond: ((date(associationtime) >= '2014-11-04'::date) AND (date(associationtime) <= '2014-11-20'::date))
         Filter: ((ssid_id = 42) AND (ap_id = 1731))
         Rows Removed by Filter: 297425
 Total runtime: 78.932 ms

Look at this line:

Index Scan using session_lim_values_idx

As you can see, query uses three fields to scan: ssid_id, ap_id and associationtime. I've got an index fo this:

ssid_pkey                  | btree | {id}
ap_pkey                    | btree | {id}
testingshit_pkey           | btree | {one,two,three}
session_date_ssid_idx      | btree | {ssid_id,date(associationtime),"date_trunc('hour'::text, associationtime)"}
session_pkey               | btree | {associationtime,disassociationtime,sessionduration,clientip,clientmac,devicename,tx,rx,protocol,snr,rssi,dataretries,rtsretries }
session_main_idx           | btree | {ssid_id,ap_id,associationtime,disassociationtime,sessionduration,clientip,clientmac,devicename,tx,rx,protocol,snr,rssi,dataretres,rtsretries}
session_date_idx           | btree | {date(associationtime),"date_trunc('hour'::text, associationtime)"}
session_date_apid_idx      | btree | {ap_id,date(associationtime),"date_trunc('hour'::text, associationtime)"}
session_date_ssid_apid_idx | btree | {ssid_id,ap_id,date(associationtime),"date_trunc('hour'::text, associationtime)"}
ap_apname_idx              | btree | {apname}
users_pkey                 | btree | {username}
user_roles_pkey            | btree | {user_role_id}
session_lim_values_idx     | btree | {date(associationtime)}

It's called session_date_ssid_apid_idx. But why query uses wrong index?

session_date_ssid_apid_idx:

------------+-----------------------------+-------------------------------------------
 ssid_id    | integer                     | ssid_id
 ap_id      | integer                     | ap_id
 date       | date                        | date(associationtime)
 date_trunc | timestamp without time zone | date_trunc('hour'::text, associationtime)

session_lim_values_idx:

date    | date | date(associationtime)

What index would you create?

UPD: \d session

 --------------------+-----------------------------+------------------------------------------------------
 id                 | integer                     | NOT NULL DEFAULT nextval('session_id_seq'::regclass)
 ssid_id            | integer                     | NOT NULL
 ap_id              | integer                     | NOT NULL
 associationtime    | timestamp without time zone | NOT NULL
 disassociationtime | timestamp without time zone | NOT NULL
 sessionduration    | character varying(100)      | NOT NULL
 clientip           | character varying(100)      | NOT NULL
 clientmac          | character varying(100)      | NOT NULL
 devicename         | character varying(100)      | NOT NULL
 tx                 | integer                     | NOT NULL
 rx                 | integer                     | NOT NULL
 protocol           | character varying(100)      | NOT NULL
 snr                | integer                     | NOT NULL
 rssi               | integer                     | NOT NULL
 dataretries        | integer                     | NOT NULL
 rtsretries         | integer                     | NOT NULL
╚эфхъё√:
    "session_pkey" PRIMARY KEY, btree (associationtime, disassociationtime, sessionduration, clientip, clientmac, devicename, tx, rx, protocol, snr, rssi, dataretries, rtsretries)
    "session_date_ap_ssid_idx" btree (ssid_id, ap_id, associationtime)
    "session_date_apid_idx" btree (ap_id, date(associationtime), date_trunc('hour'::text, associationtime))
    "session_date_idx" btree (date(associationtime), date_trunc('hour'::text, associationtime))
    "session_date_ssid_apid_idx" btree (ssid_id, ap_id, associationtime)
    "session_date_ssid_idx" btree (ssid_id, date(associationtime), date_trunc('hour'::text, associationtime))
    "session_lim_values_idx" btree (date(associationtime))
    "session_main_idx" btree (ssid_id, ap_id, associationtime, disassociationtime, sessionduration, clientip, clientmac, devicename, tx, rx, protocol, snr, rssi, dataretries, rtsretries)