CAY-2891 Handle -null expression

cayenne/src/main/java/org/apache/cayenne/access/translator/select/QualifierTranslator.java

@@ -214,6 +214,11 @@ private Node expressionNodeToSqlNode(Expression node, Expression parentNode) {
             case GREATER_THAN_EQUAL_TO:
                 return new OpExpressionNode(expToStr(node.getType()));
             case NEGATIVE:
+                // If operand is a scalar null, produce NULL directly (no unary minus)
+                if (node.getOperandCount() > 0 && node.getOperand(0) == null) {
+                    expressionsToSkip.add(node);
+                    return new ValueNode(null, false, null, false);
+                }
                 // we need to add minus sign as a prefix, not a separator
                 return new FunctionNode(expToStr(node.getType()), null, false);
             case TRUE:

cayenne/src/test/java/org/apache/cayenne/access/translator/select/QualifierTranslatorTest.java

@@ -34,6 +34,7 @@
 import org.junit.Test;
 
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 
 import static org.hamcrest.CoreMatchers.instanceOf;
@@ -158,6 +159,39 @@ public void translateBetween() {
         }
     }
 
+    @Test
+    public void translateNegate_NullScalar() {
+        Node node = translate("-(null)");
+        assertThat(node, instanceOf(ValueNode.class));
+        assertNull(((ValueNode) node).getValue());
+
+        SQLGenerationVisitor visitor = new SQLGenerationVisitor(new StringBuilderAppendable());
+        node.visit(visitor);
+        assertEquals(" NULL", visitor.getSQLString());
+    }
+
+    @Test
+    public void translateNegate_NumberScalar() {
+        Node node = translate("-1");
+        assertThat(node, instanceOf(FunctionNode.class));
+
+        SQLGenerationVisitor visitor = new SQLGenerationVisitor(new StringBuilderAppendable());
+        node.visit(visitor);
+        assertEquals(" - 1", visitor.getSQLString());
+    }
+
+    @Test
+    public void translateNegate_NullParam() {
+        Expression exp = ExpressionFactory.exp("-$v").params(Collections.singletonMap("v", null));
+        Node node = translator.translate(exp);
+        assertThat(node, instanceOf(ValueNode.class));
+        assertNull(((ValueNode) node).getValue());
+
+        SQLGenerationVisitor visitor = new SQLGenerationVisitor(new StringBuilderAppendable());
+        node.visit(visitor);
+        assertEquals(" NULL", visitor.getSQLString());
+    }
+
     @Test
     public void translateNot() {
         Node not = translate("not true");
@@ -587,4 +621,4 @@ private Node translate(String s) {
         return translator.translate(ExpressionFactory.exp(s));
     }
 
-}
+}

cayenne/src/test/java/org/apache/cayenne/exp/parser/ASTNegateTest.java

@@ -38,4 +38,12 @@ public void testParse() {
         assertEquals("-1", exp.toString());
     }
 
-}
+    @Test
+    public void testEvaluate_null() {
+        ASTNegate negate = new ASTNegate();
+        negate.setOperand(0, new ASTScalar(null));
+
+        Object result = negate.evaluate(null);
+        assertNull(result);
+    }
+}