Index: ext/session/sqlite3session.c
==================================================================
--- ext/session/sqlite3session.c
+++ ext/session/sqlite3session.c
@@ -665,18 +665,18 @@
     int bNoop = 1;
     int i;
     u8 *pCsr = p->aRecord;
     sessionAppendByte(pBuf, SQLITE_UPDATE, pRc);
     for(i=0; i<sqlite3_column_count(pStmt); i++){
-      int nCopy = 0;
+      int bChanged = 0;
       int nAdvance;
       int eType = *pCsr;
       switch( eType ){
         case SQLITE_NULL:
           nAdvance = 1;
           if( sqlite3_column_type(pStmt, i)!=SQLITE_NULL ){
-            nCopy = 1;
+            bChanged = 1;
           }
           break;
 
         case SQLITE_FLOAT:
         case SQLITE_INTEGER: {
@@ -689,11 +689,11 @@
               double dVal;
               memcpy(&dVal, &iVal, 8);
               if( dVal==sqlite3_column_double(pStmt, i) ) break;
             }
           }
-          nCopy = 9;
+          bChanged = 1;
           break;
         }
 
         case SQLITE_TEXT:
         case SQLITE_BLOB: {
@@ -704,25 +704,27 @@
            && nByte==sqlite3_column_bytes(pStmt, i) 
            && 0==memcmp(&pCsr[nHdr], sqlite3_column_blob(pStmt, i), nByte)
           ){
             break;
           }
-          nCopy = nAdvance;
+          bChanged = 1;
         }
       }
-      if( abPK[i] ){
-        nCopy = nAdvance;
-      }
 
-      if( nCopy==0 ){
+      if( bChanged || abPK[i] ){
+        sessionAppendBlob(pBuf, pCsr, nAdvance, pRc);
+      }else{
         sessionAppendByte(pBuf, 0, pRc);
-        sessionAppendByte(&buf2, 0, pRc);
-      }else{
-        sessionAppendBlob(pBuf, pCsr, nCopy, pRc);
+      }
+
+      if( bChanged ){
         sessionAppendCol(&buf2, pStmt, i, pRc);
         bNoop = 0;
+      }else{
+        sessionAppendByte(&buf2, 0, pRc);
       }
+
       pCsr += nAdvance;
     }
 
     if( bNoop ){
       pBuf->nBuf -= (1 + sqlite3_column_count(pStmt));
@@ -1449,10 +1451,56 @@
     }
     sqlite3_free(buf.aBuf);
   }
   return rc;
 }
+
+static int sessionConstraintConflict(
+  sqlite3 *db,                    /* Database handle */
+  sqlite3_changeset_iter *pIter,  /* Changeset iterator */
+  u8 *abPK,                       /* Primary key flags array */
+  sqlite3_stmt *pSelect,          /* SELECT statement from sessionSelectRow() */
+  int(*xConflict)(void *, int, sqlite3_changeset_iter*),
+  void *pCtx
+){
+  int res;
+  int rc;
+  int i;
+  int nCol;
+  int op;
+  const char *zDummy;
+
+  sqlite3changeset_op(pIter, &zDummy, &nCol, &op);
+  assert( op==SQLITE_UPDATE || op==SQLITE_INSERT );
+
+  /* Bind the new.* PRIMARY KEY values to the SELECT statement. */
+  for(i=0; i<nCol; i++){
+    if( abPK[i] ){
+      sqlite3_value *pVal;
+      if( op==SQLITE_UPDATE ) rc = sqlite3changeset_old(pIter, i, &pVal);
+      else                    rc = sqlite3changeset_new(pIter, i, &pVal);
+      if( rc!=SQLITE_OK ) return rc;
+      sqlite3_bind_value(pSelect, i+1, pVal);
+    }
+  }
+
+  if( SQLITE_ROW==sqlite3_step(pSelect) ){
+    /* There exists another row with the new.* primary key. */
+    pIter->pConflict = pSelect;
+    res = xConflict(pCtx, SQLITE_CHANGESET_CONFLICT, pIter);
+    pIter->pConflict = 0;
+    sqlite3_reset(pSelect);
+  }else{
+    /* No other row with the new.* primary key. */
+    rc = sqlite3_reset(pSelect);
+    if( rc==SQLITE_OK ){
+      res = xConflict(pCtx, SQLITE_CHANGESET_CONSTRAINT, pIter);
+    }
+  }
+
+  return rc;
+}
 
 int sqlite3changeset_apply(
   sqlite3 *db,
   int nChangeset,
   void *pChangeset,
@@ -1477,10 +1525,12 @@
   sqlite3_stmt *pUpdate = 0;      /* DELETE statement */
   sqlite3_stmt *pInsert = 0;      /* INSERT statement */
   sqlite3_stmt *pSelect = 0;      /* SELECT statement */
 
   rc = sqlite3_exec(db, "SAVEPOINT changeset_apply", 0, 0, 0);
+  if( rc!=SQLITE_OK ) return rc;
+
   sqlite3changeset_start(&pIter, nChangeset, pChangeset);
   while( SQLITE_ROW==sqlite3changeset_next(pIter) ){
     int op;
     const char *zThis;
     sqlite3changeset_op(pIter, &zThis, &nCol, &op);
@@ -1492,10 +1542,17 @@
       sqlite3_finalize(pDelete);
       sqlite3_finalize(pUpdate);
       sqlite3_finalize(pInsert);
       sqlite3_finalize(pSelect);
       pSelect = pUpdate = pInsert = pDelete = 0;
+
+      if( (rc = sessionSelectRow(db, zTab, nCol, azCol, abPK, &pSelect))
+       || (rc = sessionUpdateRow(db, zTab, nCol, azCol, abPK, &pUpdate))
+       || (rc = sessionDeleteRow(db, zTab, nCol, azCol, abPK, &pDelete))
+      ){
+        break;
+      }
     }
 
     if( op==SQLITE_DELETE ){
       int res;
       int i;
@@ -1557,11 +1614,11 @@
           rc = sqlite3changeset_new(pIter, i, &pNew);
         }
         if( rc==SQLITE_OK ){
           if( pOld ) sqlite3_bind_value(pUpdate, i*3+1, pOld);
           sqlite3_bind_int(pUpdate, i*3+2, !!pNew);
-          if( pNew ) sqlite3_bind_value(pUpdate, i*3+3, pOld);
+          if( pNew ) sqlite3_bind_value(pUpdate, i*3+3, pNew);
         }
       }
       if( rc==SQLITE_OK ) rc = sqlite3_bind_int(pUpdate, nCol*3+1, 0);
       if( rc!=SQLITE_OK ) break;
 
@@ -1590,11 +1647,36 @@
           if( rc==SQLITE_OK ){
             res = xConflict(pCtx, SQLITE_CHANGESET_NOTFOUND, pIter);
           }
         }
       }else if( rc==SQLITE_CONSTRAINT ){
-        assert(0);
+        /* This may be a CONSTRAINT or CONFLICT error. It is a CONFLICT if
+        ** the only problem is a duplicate PRIMARY KEY, or a CONSTRAINT 
+        ** otherwise. */
+        int bPKChange = 0;
+
+        /* Check if the PK has been modified. */
+        rc = SQLITE_OK;
+        for(i=0; i<nCol && rc==SQLITE_OK; i++){
+          if( abPK[i] ){
+            sqlite3_value *pNew;
+            rc = sqlite3changeset_new(pIter, i, &pNew);
+            if( rc==SQLITE_OK && pNew ){
+              bPKChange = 1;
+              break;
+            }
+          }
+        }
+
+        if( bPKChange ){
+          /* See if there exists a row with a duplicate primary key. */
+          rc = sessionConstraintConflict(
+              db, pIter, abPK, pSelect, xConflict, pCtx
+          );
+        }else{
+          res = xConflict(pCtx, SQLITE_CHANGESET_CONSTRAINT, pIter);
+        }
       }
 
     }else{
       int i;
       assert( op==SQLITE_INSERT );
@@ -1622,33 +1704,13 @@
       if( rc!=SQLITE_OK ) break;
 
       sqlite3_step(pInsert);
       rc = sqlite3_reset(pInsert);
       if( rc==SQLITE_CONSTRAINT && xConflict ){
-        int res;
-
-        /* Figure out if this is a primary key or other constraint. */
-        rc = sessionSelectRow(db, zTab, nCol, azCol, abPK, &pSelect);
-        for(i=0; rc==SQLITE_OK && i<nCol; i++){
-          if( abPK[i] ){
-            sqlite3_value *pVal;
-            rc = sqlite3changeset_new(pIter, i, &pVal);
-            if( rc==SQLITE_OK ) sqlite3_bind_value(pSelect, i+1, pVal);
-          }
-        }
-        if( rc!=SQLITE_OK ) break;
-        if( SQLITE_ROW==sqlite3_step(pSelect) ){
-          pIter->pConflict = pSelect;
-          res = xConflict(pCtx, SQLITE_CHANGESET_CONFLICT, pIter);
-          pIter->pConflict = 0;
-          sqlite3_reset(pSelect);
-        }else{
-          rc = sqlite3_reset(pSelect);
-          if( rc==SQLITE_OK ){
-            res = xConflict(pCtx, SQLITE_CHANGESET_CONSTRAINT, pIter);
-          }
-        }
+        rc = sessionConstraintConflict(
+              db, pIter, abPK, pSelect, xConflict, pCtx
+        );
       }
     }
   }
   rc2 = sqlite3changeset_finalize(pIter);
   if( rc==SQLITE_DONE ) rc = rc2;

Index: test/session1.test
==================================================================
--- test/session1.test
+++ test/session1.test
@@ -257,26 +257,33 @@
 #
 do_execsql_test 3.3.1 {
   CREATE TABLE t4(a, b, c, PRIMARY KEY(b, c));
   INSERT INTO t4 VALUES(1, 2, 3);
   INSERT INTO t4 VALUES(4, 5, 6);
+  INSERT INTO t4 VALUES(7, 8, 9);
+  INSERT INTO t4 VALUES(10, 11, 12);
 }
 do_db2_test 3.3.2 {
-  CREATE TABLE t4(a, b, c, PRIMARY KEY(b, c));
+  CREATE TABLE t4(a NOT NULL, b, c, PRIMARY KEY(b, c));
   INSERT INTO t4 VALUES(0, 2, 3);
   INSERT INTO t4 VALUES(4, 5, 7);
+  INSERT INTO t4 VALUES(7, 8, 9);
+  INSERT INTO t4 VALUES(10, 11, 12);
 }
-
-do_conflict_test 3.2.3 -tables t4 -sql {
+do_conflict_test 3.3.3 -tables t4 -sql {
   UPDATE t4 SET a = -1 WHERE b = 2;
+  UPDATE t4 SET a = -1 WHERE b = 5;
+  UPDATE t4 SET a = NULL WHERE c = 9;
+  UPDATE t4 SET a = 'x' WHERE b = 11;
 } -conflicts {
-  {UPDATE t4 DATA {i 1 i 2 i 3} {i -1 i 2 i 3} {i 0 i 2 i 3}}
+  {UPDATE t4 DATA {i 1 i 2 i 3} {i -1 {} {} {} {}} {i 0 i 2 i 3}}
+  {UPDATE t4 NOTFOUND {i 4 i 5 i 6} {i -1 {} {} {} {}}}
+  {UPDATE t4 CONSTRAINT {i 7 i 8 i 9} {n {} {} {} {} {}}}
 }
 
-do_db2_test 3.3.4 {
-  SELECT * FROM t4
-} {0 2 3 4 5 7}
+do_db2_test     3.3.4 { SELECT * FROM t4 } {0 2 3 4 5 7 7 8 9 x 11 12}
+do_execsql_test 3.3.5 { SELECT * FROM t4 } {-1 2 3 -1 5 6 {} 8 9 x 11 12}
 
 
 catch { db2 close }
 finish_test