fix(rvlite): CypherEngine MATCH returns all matched rows with properties

[dmoellenbeck]

Summary

Fixes #269 — CypherEngine.execute() now correctly returns multiple rows from MATCH ... RETURN queries, with all node properties properly serialized to JavaScript.

Problem

Any MATCH (n:Label) RETURN n.prop query returned at most 1 row with empty properties {}, regardless of how many nodes matched:

cypher.execute('CREATE (:Person {name: "Alice", age: 30})');
cypher.execute('CREATE (:Person {name: "Bob", age: 25})');

cypher.execute('MATCH (p:Person) RETURN p.name, p.age');
// Before: { columns: ["?column?","?column?"], rows: [{}] }
// After:  { columns: ["p.name","p.age"], rows: [{p.name:"Alice",p.age:30}, {p.name:"Bob",p.age:25}] }

Root Cause

Two separate bugs in the Cypher executor:

1. Context merge in execute_match() (executor.rs): All matched node contexts were merged into a single ExecutionContext via context.bind(). Each call overwrote the previous binding, so only the last matched node survived into the RETURN phase.

2. WASM serialization in CypherEngine::execute() (mod.rs): serde_wasm_bindgen::to_value() silently dropped ContextValue enum variants during serialization to JavaScript, producing empty {} objects.

Solution

executor.rs

• execute_match() returns Vec<ExecutionContext> — one context per matched row, instead of merging into one
• execute_return() iterates all contexts, producing one result row per match
• Added expression_to_column_name() — generates readable column names ("p.name") instead of "?column?"
• Added ContextValue::to_json_value() and value_to_json() — flattens ContextValue enums to plain serde_json::Value for reliable serialization

mod.rs

• CypherEngine::execute() now serializes via serde_json::to_string() + js_sys::JSON::parse() instead of serde_wasm_bindgen::to_value(), avoiding the silent enum serialization failure

Testing

• All 24 existing cypher unit tests pass (cargo test -p rvlite -- cypher)
• 3 new integration tests added in tests/cypher_integration_test.rs:
  • test_match_return_multiple_rows — Creates 3 nodes, verifies 3 rows with correct column names
  • test_match_where_return_multiple_rows — WHERE filter returns subset of nodes
  • test_match_return_variable_node — RETURN n returns full node objects with properties
• WASM build verified with wasm-pack build on macOS Apple Silicon
• Tested end-to-end via MCP server (rvlite_cypher tool)

Breaking Changes

None. The fix only changes what was previously incorrect behavior (empty results). Existing queries that returned 0-1 rows will now correctly return all matching rows.

Related

• Issue Bug: GraphDatabase.open() doesn't load persisted data; Cypher relationship traversals return empty results #64 fixed a similar problem for ruvector-graph-node (NAPI bindings), but the fix was not applied to the WASM rvlite crate
• The graph_store.rs correctly stores and retrieves node properties — the storage layer was not affected

Additional Note for MCP Server

The rvlite_cypher handler in bin/mcp-server.js currently uses require('rvlite') (CJS), but rvlite v0.2.x is ESM-only — the CJS entry point exports an empty object. This means the MCP tool always falls through to "rvlite package not installed". A separate fix is needed to switch the handler to await import('rvlite/wasm') with WASM init. Happy to open a follow-up issue/PR for this if helpful.