fix(mongodb): support legacy Agent findOne queries

Closes #5300

crates/dbx-core/src/db/agent_driver.rs

@@ -1272,6 +1272,7 @@ pub enum MongoAgentMethod {
     ListDatabases,
     ListCollections,
     FindDocuments,
+    FindOne,
     FindDocumentsExtendedJson,
     CountDocuments,
     ServerVersion,
@@ -1287,10 +1288,11 @@ pub enum MongoAgentMethod {
 }
 
 impl MongoAgentMethod {
-    pub const ALL: [Self; 15] = [
+    pub const ALL: [Self; 16] = [
         Self::ListDatabases,
         Self::ListCollections,
         Self::FindDocuments,
+        Self::FindOne,
         Self::FindDocumentsExtendedJson,
         Self::CountDocuments,
         Self::ServerVersion,
@@ -1310,6 +1312,7 @@ impl MongoAgentMethod {
             Self::ListDatabases => "list_databases",
             Self::ListCollections => "list_collections",
             Self::FindDocuments => "find_documents",
+            Self::FindOne => "find_one",
             Self::FindDocumentsExtendedJson => "find_documents_extended_json",
             Self::CountDocuments => "count_documents",
             Self::ServerVersion => "server_version",
@@ -2464,6 +2467,10 @@ impl AgentDriverClient {
         self.call_mongo_method(MongoAgentMethod::FindDocuments, params).await
     }
 
+    pub async fn mongo_find_one<T: DeserializeOwned + Send + 'static>(&mut self, params: Value) -> Result<T, String> {
+        self.call_mongo_method(MongoAgentMethod::FindOne, params).await
+    }
+
     /// Calls the Mongo agent read method that returns MongoDB relaxed Extended JSON.
     pub async fn mongo_find_documents_extended_json<T: DeserializeOwned + Send + 'static>(
         &mut self,
@@ -4314,6 +4321,7 @@ for line in sys.stdin:
         assert_eq!(MongoAgentMethod::ListDatabases.as_str(), "list_databases");
         assert_eq!(MongoAgentMethod::ListCollections.as_str(), "list_collections");
         assert_eq!(MongoAgentMethod::FindDocuments.as_str(), "find_documents");
+        assert_eq!(MongoAgentMethod::FindOne.as_str(), "find_one");
         assert_eq!(MongoAgentMethod::FindDocumentsExtendedJson.as_str(), "find_documents_extended_json");
         assert_eq!(MongoAgentMethod::CountDocuments.as_str(), "count_documents");
         assert_eq!(MongoAgentMethod::ServerVersion.as_str(), "server_version");
@@ -4381,6 +4389,7 @@ for line in sys.stdin:
         let _mongo_list_collections = AgentDriverClient::mongo_list_collections::<serde_json::Value>;
         let _mongo_list_collection_specs = AgentDriverClient::mongo_list_collection_specs::<serde_json::Value>;
         let _mongo_find_documents = AgentDriverClient::mongo_find_documents::<serde_json::Value>;
+        let _mongo_find_one = AgentDriverClient::mongo_find_one::<serde_json::Value>;
         let _mongo_find_documents_extended_json =
             AgentDriverClient::mongo_find_documents_extended_json::<serde_json::Value>;
         let _mongo_server_version = AgentDriverClient::mongo_server_version::<serde_json::Value>;

crates/dbx-core/src/mongo_ops.rs

@@ -170,8 +170,18 @@ pub async fn mongo_find_one_core(
         PoolKind::MongoDb(client) => {
             mongo_driver::find_one(client, database, collection, filter, projection, options).await
         }
-        // The legacy agent only exposes paginated find, which also performs a count.
-        PoolKind::Agent(_) => Err("MongoDB legacy agent does not support the bounded findOne path".to_string()),
+        PoolKind::Agent(client) => {
+            let mut client = client.lock().await;
+            client
+                .mongo_find_one(serde_json::json!({
+                    "database": database,
+                    "collection": collection,
+                    "filter": filter,
+                    "projection": projection,
+                    "options": options,
+                }))
+                .await
+        }
         _ => Err("Not a MongoDB connection".to_string()),
     }
 }