implement aggregations on nested fields (#136)

Turns on the nested field aggregation connector capability, and makes it work.

This also touches up the aggregation implementation generally. It uses the updated, unified system for referencing fields with paths. It also changes aggregation result types (besides count) to be nullable which avoids an error when aggregating over an empty document set.

CHANGELOG.md

@@ -4,9 +4,18 @@ This changelog documents the changes between release versions.
 
 ## [Unreleased]
 
+### Added
+
+- You can now aggregate values in nested object fields ([#136](https://github.com/hasura/ndc-mongodb/pull/136))
+
+### Changed
+
+- Result types for aggregation operations other than count are now nullable ([#136](https://github.com/hasura/ndc-mongodb/pull/136))
+
 ### Fixed
 
 - Upgrade dependencies to get fix for RUSTSEC-2024-0421, a vulnerability in domain name comparisons ([#138](https://github.com/hasura/ndc-mongodb/pull/138))
+- Aggregations on empty document sets now produce `null` instead of failing with an error ([#136](https://github.com/hasura/ndc-mongodb/pull/136))
 
 #### Fix for RUSTSEC-2024-0421 / CVE-2024-12224
 

crates/integration-tests/src/tests/aggregation.rs

@@ -101,3 +101,103 @@ async fn aggregates_mixture_of_numeric_and_null_values() -> anyhow::Result<()> {
     );
     Ok(())
 }
+
+#[tokio::test]
+async fn returns_null_when_aggregating_empty_result_set() -> anyhow::Result<()> {
+    assert_yaml_snapshot!(
+        graphql_query(
+            r#"
+            query {
+              moviesAggregate(filter_input: {where: {title: {_eq: "no such movie"}}}) {
+                runtime {
+                  avg
+                }
+              }
+            }
+            "#
+        )
+        .run()
+        .await?
+    );
+    Ok(())
+}
+
+#[tokio::test]
+async fn returns_zero_when_counting_empty_result_set() -> anyhow::Result<()> {
+    assert_yaml_snapshot!(
+        graphql_query(
+            r#"
+            query {
+              moviesAggregate(filter_input: {where: {title: {_eq: "no such movie"}}}) {
+                _count
+                title {
+                  count
+                }
+              }
+            }
+            "#
+        )
+        .run()
+        .await?
+    );
+    Ok(())
+}
+
+#[tokio::test]
+async fn returns_zero_when_counting_nested_fields_in_empty_result_set() -> anyhow::Result<()> {
+    assert_yaml_snapshot!(
+        graphql_query(
+            r#"
+            query {
+              moviesAggregate(filter_input: {where: {title: {_eq: "no such movie"}}}) {
+                awards {
+                  nominations {
+                    count
+                    _count
+                  }
+                }
+              }
+            }
+            "#
+        )
+        .run()
+        .await?
+    );
+    Ok(())
+}
+
+#[tokio::test]
+async fn aggregates_nested_field_values() -> anyhow::Result<()> {
+    assert_yaml_snapshot!(
+        graphql_query(
+            r#"
+            query {
+              moviesAggregate(
+                filter_input: {where: {title: {_in: ["Within Our Gates", "The Ace of Hearts"]}}}
+              ) {
+                tomatoes {
+                  viewer {
+                    rating {
+                      avg
+                    }
+                  }
+                  critic {
+                    rating {
+                      avg
+                    }
+                  }
+                }
+                imdb {
+                  rating {
+                    avg
+                  }
+                }
+              }
+            }
+            "#
+        )
+        .run()
+        .await?
+    );
+    Ok(())
+}

crates/integration-tests/src/tests/snapshots/integration_tests__tests__aggregation__aggregates_nested_field_values.snap

@@ -0,0 +1,17 @@
+---
+source: crates/integration-tests/src/tests/aggregation.rs
+expression: "graphql_query(r#\"\n            query {\n              moviesAggregate(\n                filter_input: {where: {title: {_in: [\"Within Our Gates\", \"The Ace of Hearts\"]}}}\n              ) {\n                tomatoes {\n                  viewer {\n                    rating {\n                      avg\n                    }\n                  }\n                  critic {\n                    rating {\n                      avg\n                    }\n                  }\n                }\n                imdb {\n                  rating {\n                    avg\n                  }\n                }\n              }\n            }\n            \"#).run().await?"
+---
+data:
+  moviesAggregate:
+    tomatoes:
+      viewer:
+        rating:
+          avg: 3.45
+      critic:
+        rating:
+          avg: ~
+    imdb:
+      rating:
+        avg: 6.65
+errors: ~

crates/integration-tests/src/tests/snapshots/integration_tests__tests__aggregation__returns_null_when_aggregating_empty_result_set.snap

@@ -0,0 +1,9 @@
+---
+source: crates/integration-tests/src/tests/aggregation.rs
+expression: "graphql_query(r#\"\n            query {\n              moviesAggregate(filter_input: {where: {title: {_eq: \"no such movie\"}}}) {\n                runtime {\n                  avg\n                }\n              }\n            }\n            \"#).run().await?"
+---
+data:
+  moviesAggregate:
+    runtime:
+      avg: ~
+errors: ~

crates/integration-tests/src/tests/snapshots/integration_tests__tests__aggregation__returns_zero_when_counting_empty_result_set.snap

@@ -0,0 +1,10 @@
+---
+source: crates/integration-tests/src/tests/aggregation.rs
+expression: "graphql_query(r#\"\n            query {\n              moviesAggregate(filter_input: {where: {title: {_eq: \"no such movie\"}}}) {\n                _count\n                title {\n                  count\n                }\n              }\n            }\n            \"#).run().await?"
+---
+data:
+  moviesAggregate:
+    _count: 0
+    title:
+      count: 0
+errors: ~

crates/integration-tests/src/tests/snapshots/integration_tests__tests__aggregation__returns_zero_when_counting_nested_fields_in_empty_result_set.snap

@@ -0,0 +1,11 @@
+---
+source: crates/integration-tests/src/tests/aggregation.rs
+expression: "graphql_query(r#\"\n            query {\n              moviesAggregate(filter_input: {where: {title: {_eq: \"no such movie\"}}}) {\n                awards {\n                  nominations {\n                    count\n                    _count\n                  }\n                }\n              }\n            }\n            \"#).run().await?"
+---
+data:
+  moviesAggregate:
+    awards:
+      nominations:
+        count: 0
+        _count: 0
+errors: ~

crates/mongodb-agent-common/src/query/mod.rs

@@ -110,11 +110,11 @@ mod tests {
             {
                 "$facet": {
                     "avg": [
-                        { "$match": { "gpa": { "$exists": true, "$ne": null } } },
+                        { "$match": { "gpa": { "$ne": null } } },
                         { "$group": { "_id": null, "result": { "$avg": "$gpa" } } },
                     ],
                     "count": [
-                        { "$match": { "gpa": { "$exists": true, "$ne": null } } },
+                        { "$match": { "gpa": { "$ne": null } } },
                         { "$group": { "_id": "$gpa" } },
                         { "$count": "result" },
                     ],
@@ -123,10 +123,17 @@ mod tests {
             {
                 "$replaceWith": {
                     "aggregates": {
-                        "avg": { "$getField": {
-                            "field": "result",
-                            "input": { "$first": { "$getField": { "$literal": "avg" } } },
-                        } },
+                        "avg": {
+                            "$ifNull": [
+                                {
+                                    "$getField": {
+                                        "field": "result",
+                                        "input": { "$first": { "$getField": { "$literal": "avg" } } },
+                                    }
+                                },
+                                null
+                            ]
+                        },
                         "count": {
                             "$ifNull": [
                                 {
@@ -180,24 +187,31 @@ mod tests {
             { "$match": { "gpa": { "$lt": 4.0 } } },
             {
                 "$facet": {
-                    "avg": [
-                        { "$match": { "gpa": { "$exists": true, "$ne": null } } },
-                        { "$group": { "_id": null, "result": { "$avg": "$gpa" } } },
-                    ],
                     "__ROWS__": [{
                         "$replaceWith": {
                             "student_gpa": { "$ifNull": ["$gpa", null] },
                         },
                     }],
+                    "avg": [
+                        { "$match": { "gpa": { "$ne": null } } },
+                        { "$group": { "_id": null, "result": { "$avg": "$gpa" } } },
+                    ],
                 },
             },
             {
                 "$replaceWith": {
                     "aggregates": {
-                        "avg": { "$getField": {
-                            "field": "result",
-                            "input": { "$first": { "$getField": { "$literal": "avg" } } },
-                        } },
+                        "avg": {
+                            "$ifNull": [
+                                {
+                                    "$getField": {
+                                        "field": "result",
+                                        "input": { "$first": { "$getField": { "$literal": "avg" } } },
+                                    }
+                                },
+                                null
+                            ]
+                        },
                     },
                     "rows": "$__ROWS__",
                 },