Added two new Order constants: ASC_NULLS_FIRST and DESC_NULLS_LAST.

Only supported on postgres. If needed we could allow for fallbacks
for other databases.

src/main/java/com/samskivert/depot/clause/OrderBy.java

@@ -18,7 +18,26 @@ import com.samskivert.depot.impl.expression.LiteralExp;
 public class OrderBy implements QueryClause
 {
     /** Indicates the order of the clause. */
-    public enum Order { ASC, DESC }
+    public enum Order
+    {
+        /** Ascending (nulls last). */
+        ASC,
+
+        /** Descending (nulls first). */
+        DESC,
+
+        /** Ascending; nulls first (postgres only). */
+        ASC_NULLS_FIRST,
+
+        /** Descending; nulls last (postgres only). */
+        DESC_NULLS_LAST;
+
+        @Override
+        public String toString ()
+        {
+            return name().replace('_', ' ');
+        }
+    }
 
     /**
      * Creates and returns a random order by clause.

src/main/java/com/samskivert/depot/impl/BuildVisitor.java

@@ -272,7 +272,7 @@ public abstract class BuildVisitor implements FragmentVisitor<Void>
                 _builder.append(", ");
             }
             values[ii].accept(this);
-            _builder.append(" ").append(orders[ii]);
+            _builder.append(" ").append(validateOrder(orders[ii]));
         }
         return null;
     }
@@ -974,6 +974,22 @@ public abstract class BuildVisitor implements FragmentVisitor<Void>
         _builder.append(")");
     }
 
+    /**
+     * Validate/massage the Order value as supported by the underlying database.
+     */
+    protected OrderBy.Order validateOrder (OrderBy.Order order)
+    {
+        if ((order == OrderBy.Order.ASC_NULLS_FIRST) || (order == OrderBy.Order.DESC_NULLS_LAST)) {
+            if (false /*Boolean.getBoolean("com.samskivert.depot.massageOrderByNulls")*/) {
+                order = (order == OrderBy.Order.ASC_NULLS_FIRST)
+                    ? OrderBy.Order.ASC : OrderBy.Order.DESC;
+            } else {
+                throw new RuntimeException(order + " is not supported on your database.");
+            }
+        }
+        return order;
+    }
+
     protected BuildVisitor (DepotTypes types, boolean allowComplexIndices)
     {
         _types = types;

src/main/java/com/samskivert/depot/impl/PostgreSQLBuilder.java

@@ -21,6 +21,7 @@ import com.samskivert.depot.Exps;
 import com.samskivert.depot.PersistentRecord;
 import com.samskivert.depot.annotation.FullTextIndex.Configuration;
 import com.samskivert.depot.annotation.FullTextIndex;
+import com.samskivert.depot.clause.OrderBy;
 import com.samskivert.depot.impl.expression.IntervalExp;
 import com.samskivert.depot.impl.expression.DateFun.DatePart;
 import com.samskivert.depot.impl.expression.DateFun.DateTruncate;
@@ -118,6 +119,10 @@ public class PostgreSQLBuilder
             _builder.append("\"").append(field).append("\"");
         }
 
+        @Override protected OrderBy.Order validateOrder (OrderBy.Order order) {
+            return order;
+        }
+
         protected PGBuildVisitor (DepotTypes types)
         {
             super(types, true);