hack: filter null values from joined JSON arrays

query-engine/connector-test-kit-rs/query-engine-tests/tests/queries/simple/m2m.rs

@@ -69,6 +69,49 @@ mod m2m {
         Ok(())
     }
 
+    fn schema() -> String {
+        let schema = indoc! {
+            r#"model Item {
+                id         Int        @id @default(autoincrement())
+                categories Category[]
+                createdAt  DateTime   @default(now())
+                updatedAt  DateTime?  @updatedAt
+              }
+              
+              model Category {
+                id        Int       @id @default(autoincrement())
+                items     Item[]
+                createdAt DateTime  @default(now())
+                updatedAt DateTime? @updatedAt
+              }"#
+        };
+
+        schema.to_owned()
+    }
+
+    // https://github.com/prisma/prisma/issues/16390
+    #[connector_test(schema(schema), relation_mode = "prisma", only(Postgres))]
+    async fn repro_16390(runner: Runner) -> TestResult<()> {
+        run_query!(&runner, r#"mutation { createOneCategory(data: {}) { id } }"#);
+        run_query!(
+            &runner,
+            r#"mutation { createOneItem(data: { categories: { connect: { id: 1 } } }) { id } }"#
+        );
+        run_query!(&runner, r#"mutation { deleteOneItem(where: { id: 1 }) { id } }"#);
+
+        insta::assert_snapshot!(
+          run_query!(&runner, r#"{ findUniqueItem(where: { id: 1 }) { id categories { id } } }"#),
+          @r###"{"data":{"findUniqueItem":null}}"###
+        );
+
+        insta::assert_snapshot!(
+          run_query!(&runner, r#"{ findUniqueCategory(where: { id: 1 }) { id items { id } } }"#),
+          @r###"{"data":{"findUniqueCategory":{"id":1,"items":[]}}}"###
+        );
+
+        Ok(())
+    }
+
     async fn test_data(runner: &Runner) -> TestResult<()> {
         runner
             .query(

query-engine/connectors/sql-query-connector/src/database/operations/coerce.rs

@@ -29,7 +29,18 @@ fn coerce_json_relation_to_pv(value: serde_json::Value, rs: &RelationSelection)
     match value {
         // one-to-many
         serde_json::Value::Array(values) if rs.field.is_list() => {
-            let iter = values.into_iter().map(|value| coerce_json_relation_to_pv(value, rs));
+            let iter = values.into_iter().filter_map(|value| {
+                // FIXME: In the case of m2m relations, the aggregation produces null values if the B side of the m2m table points to a record that doesn't exist.
+                // FIXME: This only seems to happen because of a bug with `relationMode=prisma`` which doesn't cleanup the relation table properly when deleting records that belongs to a m2m relation.
+                // FIXME: This hack filters down the null values from the array, but we should fix the root cause instead, if possible.
+                // FIXME: In theory, the aggregated array should only contain objects, which are the joined rows.
+                // FIXME: See m2m.rs::repro_16390 for a reproduction.
+                if value.is_null() && rs.field.relation().is_many_to_many() {
+                    None
+                } else {
+                    Some(coerce_json_relation_to_pv(value, rs))
+                }
+            });
 
             // Reverses order when using negative take.
             let iter = match rs.args.needs_reversed_order() {
@@ -75,7 +86,7 @@ fn coerce_json_relation_to_pv(value: serde_json::Value, rs: &RelationSelection)
 
             Ok(PrismaValue::Object(map))
         }
-        _ => unreachable!(),
+        x => unreachable!("Unexpected value when deserializing JSON relation data: {x:?}"),
     }
 }