Fix nested filter on self-referencing relationships

src/Core/Models/GraphQLFilterParsers.cs

@@ -419,10 +419,26 @@ private void HandleNestedFilterForSql(
         predicatesForExistsQuery.Push(existsQueryFilterPredicate);
 
         // Add JoinPredicates to the subquery query structure so a predicate connecting
-        // the outer table is added to the where clause of subquery
-        existsQuery.AddJoinPredicatesForRelatedEntity(
-            targetEntityName: queryStructure.EntityName,
-            relatedSourceAlias: queryStructure.SourceAlias,
+        // the outer table is added to the where clause of subquery.
+        // For self-referencing relationships (e.g., parent/child hierarchy), we need to use
+        // the relationship name to look up the correct foreign key definition.
+        // The parent query (queryStructure) calls AddJoinPredicatesForRelationship which adds
+        // predicates to the subquery (existsQuery), connecting queryStructure.SourceAlias to existsQuery.SourceAlias.
+        string relationshipName = filterField.Name;
+        EntityRelationshipKey fkLookupKey = new(queryStructure.EntityName, relationshipName);
+
+        if (queryStructure is not BaseSqlQueryStructure sqlQueryStructure)
+        {
+            throw new DataApiBuilderException(
+                message: "Expected SQL query structure for nested filter processing.",
+                statusCode: HttpStatusCode.InternalServerError,
+                subStatusCode: DataApiBuilderException.SubStatusCodes.UnexpectedError);
+        }
+
+        sqlQueryStructure.AddJoinPredicatesForRelationship(
+            fkLookupKey: fkLookupKey,
+            targetEntityName: nestedFilterEntityName,
+            subqueryTargetTableAlias: existsQuery.SourceAlias,
             subQuery: existsQuery);
 
         // The right operand is the SqlExistsQueryStructure.

src/Service.Tests/SqlTests/GraphQLFilterTests/MsSqlGQLFilterTests.cs

@@ -355,6 +355,51 @@ WHERE [table3].[name] IN ('Aniruddh')
             await TestNestedFilterWithOrAndIN(existsPredicate, roleName: "authenticated");
         }
 
+        /// <summary>
+        /// Test Nested Filter for Self-Referencing relationship
+        /// Tests that nested filters work correctly on self-referencing relationships (e.g., parent/child hierarchy).
+        /// Uses DimAccount table with parent_account relationship.
+        /// </summary>
+        [TestMethod]
+        public async Task TestNestedFilterSelfReferencing()
+        {
+            // This query should find all accounts whose parent account has AccountKey = 1
+            // Expected to return account with AccountKey 2 (direct child of account 1)
+            string existsPredicate = $@"
+                EXISTS( SELECT 1
+                        FROM {GetPreIndentDefaultSchema()}[DimAccount] AS [table1]
+                        WHERE [table1].[AccountKey] = 1
+                        AND [table0].[ParentAccountKey] = [table1].[AccountKey] )";
+
+            string graphQLQueryName = "dbo_DimAccounts";
+            // Gets all the accounts that have a parent account with AccountKey = 1
+            string gqlQuery = @"{
+                dbo_DimAccounts (" + QueryBuilder.FILTER_FIELD_NAME + ": {" +
+                    @"parent_account: { AccountKey: { eq: 1 }}})
+                    {
+                      items {
+                        AccountKey
+                        ParentAccountKey
+                      }
+                    }
+                }";
+
+            string dbQuery = MakeQueryOn(
+                table: "DimAccount",
+                queriedColumns: new List<string> { "AccountKey", "ParentAccountKey" },
+                existsPredicate,
+                GetDefaultSchema(),
+                pkColumns: new List<string> { "AccountKey" });
+
+            JsonElement actual = await ExecuteGraphQLRequestAsync(
+                gqlQuery,
+                graphQLQueryName,
+                isAuthenticated: false);
+
+            string expected = await GetDatabaseResultAsync(dbQuery);
+            SqlTestHelper.PerformTestEqualJsonStrings(expected, actual.ToString());
+        }
+
         /// <summary>
         /// Gets the default schema for
         /// MsSql.