Allow update mutations to pass null for optional relations (#1002)

This makes it possible to reset an optional relation with an update mutation:

```graphql
mutation resetEvent {
  updateNotification(id: 1, data: {title: "N1-updated", event: null}) {
    id
    event
  }
}
```

crates/postgres-subsystem/postgres-resolver/src/update_data_param_mapper.rs

@@ -100,16 +100,22 @@ fn compute_update_columns<'a>(
                 let self_column = self_column_id.get_column(&subsystem.database);
                 let foreign_type_pk_field_name =
                     &foreign_pk_field_id.resolve(&subsystem.entity_types).name;
-                get_argument_field(argument, &field.name).map(|argument_value| {
-                    match get_argument_field(argument_value, foreign_type_pk_field_name) {
-                        Some(foreign_type_pk_arg) => {
-                            let value_column =
-                                cast::literal_column(foreign_type_pk_arg, self_column);
-                            (self_column_id, value_column.unwrap())
+
+                match get_argument_field(argument, &field.name) {
+                    Some(Val::Null) => Some((self_column_id, Column::Null)), // `{..., foreign_field: null}` means set the column to null
+                    Some(argument_value) => {
+                        // `{..., foreign_field: { id: 1 }}` means set the column to the id of the nested object
+                        match get_argument_field(argument_value, foreign_type_pk_field_name) {
+                            Some(foreign_type_pk_arg) => {
+                                let value_column =
+                                    cast::literal_column(foreign_type_pk_arg, self_column);
+                                Some((self_column_id, value_column.unwrap()))
+                            }
+                            None => unreachable!("Expected pk argument"), // Validation should have caught this
                         }
-                        None => unreachable!("Expected pk argument"), // Validation should have caught this
                     }
-                })
+                    None => None,
+                }
             }
             PostgresRelation::OneToMany { .. } => None,
         })

integration-tests/optional-relation/src/index.exo

@@ -0,0 +1,18 @@
+// Notification has an optional event
+@postgres
+module NotificationDatabase {
+  @access(true)
+  type Notification {
+    @pk id: Int = autoIncrement()
+    title: String
+    description: String
+    event: Event?
+  }
+
+  @access(true)
+  type Event {
+    @pk id: Int = autoIncrement()
+    title: String
+    notifications: Set<Notification>?
+  }
+}

integration-tests/optional-relation/tests/init.gql

@@ -0,0 +1,24 @@
+stages:
+    - operation: |
+        mutation {
+            e1: createEvent(data: {title: "E1"}) {
+                id @bind(name: "e1Id")
+            }
+            e2: createEvent(data: {title: "E2"}) {
+                id @bind(name: "e2Id")
+            }
+        }
+    - operation: |
+        mutation($e1Id: Int!, $e2Id: Int!) {
+            n1: createNotification(data: {title: "N1", description: "N1-desc", event: {id: $e1Id}}) {
+                id @bind(name: "n1id")
+            }
+            n2: createNotification(data: {title: "N2", description: "N2-desc", event: {id: $e2Id}}) {
+                id @bind(name: "n2id")
+            }
+        }  
+      variable: |
+        {
+            "e1Id": $.e1Id,
+            "e2Id": $.e2Id
+        }      

integration-tests/optional-relation/tests/reset-relation.exotest

@@ -0,0 +1,52 @@
+stages:
+  - operation: |
+      mutation resetEvent($eId: Int!) {
+          updateNotification(id: $eId, data: {title: "N1-updated", event: null}) {
+            id
+            event
+          }
+      }
+    variable: |
+      {
+          "eId": $.e1Id,
+      }   
+    response: |
+      {
+        "data": {
+          "updateNotification": {
+            "id": 1,
+            "event": null
+          }
+        }
+      }
+  - operation: |
+      query {
+          notifications @unordered {
+            id
+            title
+            event {
+              id
+              title
+            }
+          }
+      }  
+    response: |
+      {
+        "data": {
+          "notifications": [
+            {
+              "id": $.n1id,
+              "title": "N1-updated",
+              "event": null
+            },
+            {
+              "id": $.n2id,
+              "title": "N2",
+              "event": {
+                "id": $.e2Id,
+                "title": "E2"
+              }
+            }
+          ]
+        }
+      }