Change NIN filter to use "not in" operator in Milvus

- Milvus requires "not in" operator syntax instead of "nin" for filtering.
- See https://milvus.io/docs/boolean.md
- Updated test and added additional test coverage

Author: leevh0908

vector-stores/spring-ai-milvus-store/src/main/java/org/springframework/ai/vectorstore/MilvusFilterExpressionConverter.java

@@ -58,7 +58,7 @@ private String getOperationSymbol(Expression exp) {
 			case IN:
 				return " in ";
 			case NIN:
-				return " nin ";
+				return " not in ";
 			default:
 				throw new RuntimeException("Not supported expression type:" + exp.type());
 		}

vector-stores/spring-ai-milvus-store/src/test/java/org/springframework/ai/vectorstore/MilvusFilterExpressionConverterTests.java

@@ -29,8 +29,10 @@
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.springframework.ai.vectorstore.filter.Filter.ExpressionType.AND;
 import static org.springframework.ai.vectorstore.filter.Filter.ExpressionType.EQ;
+import static org.springframework.ai.vectorstore.filter.Filter.ExpressionType.GT;
 import static org.springframework.ai.vectorstore.filter.Filter.ExpressionType.GTE;
 import static org.springframework.ai.vectorstore.filter.Filter.ExpressionType.IN;
+import static org.springframework.ai.vectorstore.filter.Filter.ExpressionType.LT;
 import static org.springframework.ai.vectorstore.filter.Filter.ExpressionType.LTE;
 import static org.springframework.ai.vectorstore.filter.Filter.ExpressionType.NE;
 import static org.springframework.ai.vectorstore.filter.Filter.ExpressionType.NIN;
@@ -86,11 +88,11 @@ public void testGroup() {
 						new Expression(EQ, new Key("country"), new Value("BG")))),
 				new Expression(NIN, new Key("city"), new Value(List.of("Sofia", "Plovdiv")))));
 		assertThat(vectorExpr).isEqualTo(
-				"metadata[\"year\"] >= 2020 || metadata[\"country\"] == \"BG\" && metadata[\"year\"] >= 2020 || metadata[\"country\"] == \"BG\" && metadata[\"city\"] nin [\"Sofia\",\"Plovdiv\"]");
+				"metadata[\"year\"] >= 2020 || metadata[\"country\"] == \"BG\" && metadata[\"year\"] >= 2020 || metadata[\"country\"] == \"BG\" && metadata[\"city\"] not in [\"Sofia\",\"Plovdiv\"]");
 	}
 
 	@Test
-	public void tesBoolean() {
+	public void testBoolean() {
 		// isOpen == true AND year >= 2020 AND country IN ["BG", "NL", "US"]
 		String vectorExpr = this.converter.convertExpression(new Expression(AND,
 				new Expression(AND, new Expression(EQ, new Key("isOpen"), new Value(true)),
@@ -121,4 +123,36 @@ public void testComplexIdentifiers() {
 		assertThat(vectorExpr).isEqualTo("metadata[\"country 1 2 3\"] == \"BG\"");
 	}
 
+	@Test
+	public void testLt() {
+		// temperature < 0
+		String vectorExpr = this.converter.convertExpression(new Expression(LT, new Key("temperature"), new Value(0)));
+		assertThat(vectorExpr).isEqualTo("metadata[\"temperature\"] < 0");
+	}
+
+	@Test
+	public void testLte() {
+		// humidity <= 100
+		String vectorExpr = this.converter.convertExpression(new Expression(LTE, new Key("humidity"), new Value(100)));
+		assertThat(vectorExpr).isEqualTo("metadata[\"humidity\"] <= 100");
+	}
+
+	@Test
+	public void testGt() {
+		// price > 1000
+		String vectorExpr = this.converter.convertExpression(new Expression(GT, new Key("price"), new Value(1000)));
+		assertThat(vectorExpr).isEqualTo("metadata[\"price\"] > 1000");
+	}
+
+	@Test
+	public void testCombinedComparisons() {
+		// price > 1000 && temperature < 25 && humidity <= 80
+		String vectorExpr = this.converter.convertExpression(new Expression(AND,
+				new Expression(AND, new Expression(GT, new Key("price"), new Value(1000)),
+						new Expression(LT, new Key("temperature"), new Value(25))),
+				new Expression(LTE, new Key("humidity"), new Value(80))));
+		assertThat(vectorExpr)
+			.isEqualTo("metadata[\"price\"] > 1000 && metadata[\"temperature\"] < 25 && metadata[\"humidity\"] <= 80");
+	}
+
 }