Changes to enhance existing length op

document-store/src/integrationTest/java/org/hypertrace/core/documentstore/DocStoreQueryV1Test.java

@@ -919,6 +919,58 @@ public void testAggregateWithMultipleGroupingLevels(String dataStoreName) throws
     testCountApi(dataStoreName, query, "query/multi_level_grouping_response.json");
   }
 
+  @ParameterizedTest
+  @ArgumentsSource(MongoProvider.class)
+  public void testSortByListSizeWithMissingField(String dataStoreName) throws IOException {
+    Datastore datastore = datastoreMap.get(dataStoreName);
+    String collectionName = "list_size_sort_collection";
+    datastore.deleteCollection(collectionName);
+    datastore.createCollection(collectionName, null);
+    Collection collection = datastore.getCollection(collectionName);
+
+    try {
+      collection.upsert(
+          new SingleValueKey(TENANT_ID, "three"),
+          new JSONDocument("{\"item\":\"three\",\"tags\":[\"a\",\"b\",\"c\"]}"));
+      collection.upsert(
+          new SingleValueKey(TENANT_ID, "one"),
+          new JSONDocument("{\"item\":\"one\",\"tags\":[\"x\"]}"));
+      // Document intentionally missing the "tags" field; LENGTH must resolve to 0 instead of
+      // failing
+      collection.upsert(
+          new SingleValueKey(TENANT_ID, "none"), new JSONDocument("{\"item\":\"none\"}"));
+
+      Query query =
+          Query.builder()
+              .addSelection(IdentifierExpression.of("item"))
+              .addSelection(
+                  FunctionExpression.builder()
+                      .operator(LENGTH)
+                      .operand(IdentifierExpression.of("tags"))
+                      .build(),
+                  "tag_count")
+              .addSort(IdentifierExpression.of("tag_count"), ASC)
+              .build();
+
+      Iterator<Document> resultDocs = collection.aggregate(query);
+      List<Map<String, Object>> results = new ArrayList<>();
+      while (resultDocs.hasNext()) {
+        results.add(Utils.convertDocumentToMap(resultDocs.next()));
+      }
+
+      assertEquals(3, results.size());
+      // Document without the "tags" field counts as 0 and sorts first in ascending order
+      assertEquals("none", results.get(0).get("item"));
+      assertEquals(0, ((Number) results.get(0).get("tag_count")).intValue());
+      assertEquals("one", results.get(1).get("item"));
+      assertEquals(1, ((Number) results.get(1).get("tag_count")).intValue());
+      assertEquals("three", results.get(2).get("item"));
+      assertEquals(3, ((Number) results.get(2).get("tag_count")).intValue());
+    } finally {
+      datastore.deleteCollection(collectionName);
+    }
+  }
+
   @ParameterizedTest
   @ArgumentsSource(AllProvider.class)
   public void testAggregateWithFunctionalLeftHandSideFilter(final String dataStoreName)

document-store/src/main/java/org/hypertrace/core/documentstore/mongo/query/parser/MongoFunctionExpressionParser.java

@@ -69,6 +69,12 @@ Map<String, Object> parse(final FunctionExpression expression) {
 
     if (numArgs == 1) {
       Object value = expression.getOperands().get(0).accept(parser);
+      // $size fails when the operand resolves to a missing/absent (or null) field. Default such a
+      // value to an empty array so that LENGTH of an absent field is 0 instead of throwing an
+      // error.
+      if (operator == LENGTH) {
+        value = Map.of("$ifNull", List.of(value, List.of()));
+      }
       return Map.of(key, value);
     }
 

document-store/src/test/resources/mongo/pipeline/distinct_count.json

@@ -19,7 +19,12 @@
   {
     "$project": {
       "section_count": {
-        "$size": "$section_count"
+        "$size": {
+          "$ifNull": [
+            "$section_count",
+            []
+          ]
+        }
       }
     }
   }

document-store/src/test/resources/mongo/pipeline/field_count.json

@@ -10,7 +10,12 @@
   {
     "$project": {
       "total": {
-        "$size": "$total"
+        "$size": {
+          "$ifNull": [
+            "$total",
+            []
+          ]
+        }
       }
     }
   }

document-store/src/test/resources/mongo/pipeline/optimize_sorts_simple_sort_with_aggregation_selection.json

@@ -10,7 +10,12 @@
   {
     "$project": {
       "total": {
-        "$size": "$total"
+        "$size": {
+          "$ifNull": [
+            "$total",
+            []
+          ]
+        }
       }
     }
   },

document-store/src/test/resources/mongo/pipeline/simple.json

@@ -10,7 +10,12 @@
   {
     "$project": {
       "total": {
-        "$size": "$total"
+        "$size": {
+          "$ifNull": [
+            "$total",
+            []
+          ]
+        }
       }
     }
   }

document-store/src/test/resources/mongo/pipeline/with_projections.json

@@ -11,7 +11,12 @@
     "$project": {
       "name": 1,
       "total": {
-        "$size": "$total"
+        "$size": {
+          "$ifNull": [
+            "$total",
+            []
+          ]
+        }
       }
     }
   }